agricultural meteorological variables and their observations

CHAPTER 2AGRICULTURAL METEOROLOGICAL VARIABLES ANDTHEIR OBSERVATIONS2.1 BASIC ASPECTS OF AGRICULTURALMETEOROLOGICAL OBSERVATIONSObservations of the physical and biological variablesin the environment are essential in agriculturalmeteorology. Meteorological considerations enterinto assessing the performance of plants andanimals because their growth is a result of thecombined effect of genetic characteristics (nature)and their response to the environment (nurture).Without quantitative data, agrometeorologicalplanning, forecasting, research and services byagrometeorologists cannot properly assist agriculturalproducers to survive and to meet theever-increasing demands for food and agriculturalby-products. Such data are also needed to assess theimpacts of agricultural activities and processes onthe environment and climate. The followingsections provide guidance on the types of observationsrequired, their extent, organization andaccuracy, as well as on the instruments needed toobtain the data, with an emphasis on those foroperational and long-term stations. Older books onmeasurements are generally available to the public,but more recently, the number of books withcomponents useful to agricultural meteorology hasdiminished. Reference can be made here, for example,to books that have become more widely usedsince the previous edition of this Guide wascompiled, such as Fritschen and Gay (1979),Greacen (1981), Meteorological Office (1981),Woodward and Sheehy (1983), Russell et al. (1989),Pearcy et al. (1989), Goel and Norman (1990),Kaimal and Finnigan (1994), Smith and Mullins(2001), Strangeways (2003) and WMO (1984, 1994b,2008a, 2008b). In relation to operational agrometeorology,reference can be made to certain chaptersin Rosenberg et al. (1983), Griffiths (1994), Baldyand Stigter (1997), and WMO (2001b).The observations required depend on the purposefor which they will be used. For the characterizationof agroclimate, for climate monitoring andprediction, and for the management of naturalresources, national coverage over periods of manyyears is required. These data also provide thebackground for the shorter-term decision-makinginvolved in activities such as response farming,monitoring of, and preparedness and early warningfor, natural disasters, along with forecasts for pestsand diseases. For these activities, additionalobservations are needed. The preparation ofadvisories and services on farming methods,including irrigation and microclimate managementand manipulation, also requires specialized data.Finally, the needs of research call for detailed andprecise data according to each research topic. Thereare too many specialized methods to be included inthis review, but almost all research projects requireinformation on the background climatology thatmay be derived from the outputs of the long-termtypes of stations listed below.2.1.1 Data as a support system foragrometeorological servicesIn section 1.4.1 of Chapter 1, data are consideredparts of support systems for agrometeorologicalservices. This applies to assessments as well aspredictions. It should be stressed that this refers toreal data, that is, observed parameters, or “groundtruth”. As already mentioned in Chapter 1, collectionof good observations has gone out of fashionin many countries because of the illusion thatcomputer-modelled estimates can replace them.Models can be useful only if they get real input dataand if additional real observations are available tocheck the validity of model output.When the data are to be related to agricultural operations,agricultural data are also essential, includingthe state of the crops and of animals. These complementarydata are often collected bynon-meteorological personnel. For all agrometeorologicalapplications, in order to make informationavailable to assist farmers all the time at the fieldlevel, to prepare advisories, and to allow forlonger‐term planning, it is necessary to combinethe agricultural and the meteorological data. Tomake better use of the agrometeorological data insupporting agrometeorological services and toprovide for effective transfer of the knowledge ofagrometeorology to farmers at farm level, thescience of information technology is also veryuseful (see also Chapter 17 of this Guide).2.1.2 Physical climatic variablesAgricultural meteorology is concerned with everyaspect of local and regional climates and the causesof their variations, which makes standardobservation of climatic variables a fundamental

2–2GUIDE TO AGRICULTURAL METEOROLOGICAL PRACTICESnecessity (for instance, Hubbard, 1994). It is alsoconcerned with any climatic modifications, whichmay be introduced by human management ofagriculture, animal husbandry or forestry operations(for example, Stigter, 1994a). Physical variables ofclimate are observed to assist the management ofagricultural activities. Such management includesdetermining the time, extent and manner ofcultivation and other agricultural operations(sowing; harvesting; planting; application ofbiocides and herbicides; ploughing; harrowing;rolling; irrigation; suppression of evaporation;design, construction and repair of buildings forstorage, animal husbandry, and so on) and differentmethods of conservation, industrial use andtransport of agricultural products.Indispensable climatic parameters in the developmentof agricultural meteorology include, more orless, all those pertaining to geographical climatology,especially those that allow interpretation ofphysical processes in the lowest atmosphere andupper soil layers, which are the climatic determinantsfor the local or regional biosphere (Monteithand Unsworth, 2007). Parameters pertaining toenergy and water balance are thus very important,such as precipitation, humidity, temperature, solarradiation and air motion. Further, certain physicaland chemical characteristics of the atmosphere,precipitation and soil are also important in agriculturalmeteorology. These characteristics can includeCO 2 and SO 2 ; dissolved and suspended matter inprecipitation; and soil temperature, moisture andsalinity. Such measurements require specializedequipment, which is available only at a few selectedstations. Non-routine physical (and biological, seebelow) observations, such as those required forresearch, surveys and special services (as discussedin Baldy and Stigter, 1997, for example, andAppendix II to this Guide), are usually more detailedthan standard observations and thus need to bemore accurate whenever processes must be studiedinstead of phenomena.2.1.3 Biological variablesBesides scientific observation of the physical environment,the simultaneous evaluation of its effecton the objects of agriculture, namely, plants,animals and trees, both individually and as communities,is also a prerequisite of agriculturalmeteorology. The routine observations provided byclimatological and agrometeorological stationsshould be accompanied by routine biological observations.In order to obtain the best results, theseobservations should be comparable with those ofthe physical environment in extent, standard andaccuracy. Biological observations generally arephenological or phenometric in nature or both.Phenological observations are made to evaluatepossible relations between the physical environmentand the development of plants and animals,while the phenometric types are made to relate thephysical environment with biomass changes. TheManual on the Global Observing System (WMO-No.544) and some of the WMO Technical Notes 1include certain details about observations of thistype. Literature covering this topic is given in 2.3.2and biological measurements are provided in 2.4.2.Important observations include assessments ofdamage caused by weather, diseases and parasites,as well as measurements of growth and yield.2.1.4 Scale of observationsIn agricultural meteorology, observations arerequired on the macro-, meso- and microscales. Onthe larger scales it should make use of all availablelocal observations of environmental physicalparameters made by the international synopticnetwork of stations (see also 2.1.5). In practice,observations can be used in real time in agriculture.For parameters with very little spatial variation(such as sunshine duration), low-density observationnetworks normally suffice for agriculturalpurposes. Most of the planning activities in theagricultural realm, however, require higher-densitydata. These can sometimes be obtained from synopticstation observations through the use ofappropriate interpolations (Wieringa, 1998; WMO,2001b). For biometeorological research, microscaleobservations are often required.New typical characteristic distances of these climaticscales are referred to in Chapter 1 of WMO (2008b).In this publication the mesoscale is defined as 3 kmto 100 km, the toposcale or local scale as 100 m to3 km and the microscale as less than 100 m (in thelast case with the notation “for agricultural meteorology”).Indeed, a mesoscale of 100 km does notfeel right in agricultural production and toposcaleis also not the right term for a farm. In WMO(2008b), however, it is also stated in particular thatapplications have their own preferred time andspace scales for averaging, station density and resolutionof phenomena: small for agriculturalmeteorology and large for global long-range forecasting.With respect to agricultural meteorology1Please note that the following WMO Technical Notes arelisted for further reading on subjects relevant to this chapter:Nos. 11, 21, 26, 55, 56, 83, 86, 97, 101, 125, 126, 133, 161,168, 179, 192 and 315. They can be found in Appendix I.Bof this Guide.

CHAPTER 2. AGRICULTURAL METEOROLOGICAL VARIABLES AND THEIR OBSERVATIONS 2–3this is discussed (differently) in WMO (2001b) andin Keane (2001), partly after Guyot (1998).It follows from the above that it is desirable that usecan be made of observations from agricultural meteorologicalstations. Such stations are equipped toperform general meteorological and biologicalobservations and are usually located at experimentalstations or research institutes of agriculture,horticulture, animal husbandry, forestry and soilsciences. Frequencies of observation, the timescaleto be applied for measurements, and their averagingdepend on the phenomena and processes understudy, their scales, and rates of change. In WMO(2008b) this is discussed under “representativeness”(see also 2.2.2.1).For research work in agricultural meteorology,standard instrumentation under standard environmentalconditions is often useful, but in many casesspecial stations, with special equipment and nonstandardexposure conditions, are required (forexample, WMO, 1994a). For biometeorologicalresearch and for many agrometeorological problems,additional observations in confined areas,such as within crops, woods, agricultural buildingsor containers for conservation or transport ofproduce, are often required.2.1.5 Extent of observationsAgricultural meteorology can and should make useof all available local observations of environmentalphysical parameters from fixed points in the synoptic,climatological or hydrological networks,including a broad range of area and point dataderived from numerical weather analysis and predictions.This includes certain upper-air data (at least inthe lower layers up to 3 000 m), for instance, upperwinds (aerobiology) and temperature and humidityprofiles (for energy budgets). In fact, it is desirablethat at selected stations additional observations ofmore specific interest to agriculture be made.Climatological and hydrological stations, which areoften more representative of agricultural areas thansynoptic stations, provide information (dailyprecipitation amounts, extreme temperatures, andso on) that is useful for operational agrometeorologicalpurposes and in the management of risks anduncertainties. Since these networks of synoptic,climatological and hydrological stations arerestricted in density or in kind of observation, it isdesirable that they be supplemented by agriculturalmeteorological stations. The complete networkshould include all aspects of climatic and soilvariations and each type of agricultural, horticultural,animal husbandry, hydrobiological, and forestryoperations that exist in the country.New possibilities for agricultural meteorology areoffered by the availability of remote-sensing techniques(for example, Milford, 1994), which allowfor the evaluation of some variables of the physicalenvironment and the biomass over extended areasand help to guide interpolation. These types of dataare useful to supplement agrometeorological informationand to aid in providing forecasting andwarning services to agriculture.2.1.6 Data without metadata areunreliableMeteorological observations do not provide reliableinformation about the state of the local atmosphereunless one knows how the observations were made,including the instrument, its installation heightand exposure, sampling modalities and averagingtimes, and the way in which the measurementswere processed. Specifications of all these links ofthe ob ser vation chain are called metadata, and theiravailability determines the value of mea surements.Average wind speed observed at 2 m height will beabout two thirds of the wind speed at 10 m height.A maximum temperature observed with a fastthermo meter above dry sand can be many degreeshigher than the maximum observed nearby on thesame day above wet clay with a slow thermograph.To judge the content and quali ty of observations, itis essential to know their metadata (WMO, 2002).Traditionally, for synoptic stations this issue was dealtwith by WMO rules that specified standard instrumentexposures and comparable observationprocedures. The required very open terrain is notalways available (even at airports), however, andmany observation budgets are insufficient to meet therules. Around 1990, climate investigations showedthe great importance of knowing the actual stationexposures and the like, even for officially standardizedsynoptic stations. For agrometeorological stations,which make varying types of observations in varyingterrain, metadata have always been important, butgenerally were referred to as “station history”.Therefore, it is more important than ever that recordsare made and kept at agrometeorological stations ofthe instrumentation (type, calibration, maintenance),instrument exposures (mounting, siting,surroundings at toposcale), and observationprocedures (sampling, averaging, frequency ofmeasurements, recording, archiving). Fullerspecification of necessary metadata is given in 2.2.5below.

2–4GUIDE TO AGRICULTURAL METEOROLOGICAL PRACTICES2.2 AGRICULTURAL METEOROLOGICALSTATIONS2.2.1 ClassificationReference should be made to Linacre (1992). Accordingto WMO (2003b), each agricultural meteorologicalstation belongs to one of the following categories:(a) A principal agricultural meteorologicalstation provides detailed simultaneous meteorologicaland biological information and itis where research in agricultural meteorologyis carried out. The instrumental facilities,range and frequency of observations,in both meteorological and biological fields,and the professional personnel are such thatfundamental investigations into agriculturalmeteorological questions of interest to thecountries or regions concerned can be carriedout.(b) An ordinary agricultural meteorologicalstation provides, on a routine basis, simultaneousmeteorological and biological informationand may be equipped to assist in research intospecific problems; in general, the programmeof biological or phenological observations forresearch will be related to the local climaticregime of the station and to local agriculture.(c) An auxiliary agricultural meteorologicalstation provides meteorological and biologicalinformation. The meteorological informationmay include such items as soil temperature,soil moisture, potential evapotranspiration,duration of vegetative wetting, and detailedmeasurements in the very lowest layer of theatmosphere. The biological information maycover phenology, onset and spread of plantdiseases, and so forth.(d) An agricultural meteorological station forspecific purposes is a station set up temporarilyor permanently for the observation of one orseveral variables and/or specified phenomena.Stations corresponding to (a) are not commonbecause of their requirements for trained professionals,technical personnel and equipment. In mostcountries the majority of agricultural meteorologicalstations belong to categories (b), (c) and (d).2.2.2 Selection and layout of a stationsite2.2.2.1 Selection of a representative sitelocationThe accuracy of observations at a given time is adeterminable fixed quality, but their representativityvaries with their application. Representativity of ameasu rement is the degree to which it describesreliably the value of some parameter (for instance,humidity or wind speed) at a specified space scalefor a specified purpose (WMO, 2001b).Instrumentation, exposure and observationprocedures must be matched to achieve usefulrepresentation – for example, local 2-minuteaverages for aviation, or hourly mesoscale averagesfor synoptic forecasts.Therefore, when selecting a site for a station, thepurpose of its observations must be decided first –should it be regionally representative, then evenin a woody region an open location is preferable,because the station’s observation must relate tothe lower atmosphere of the region. If the purposeof establishing a station is monito ring or operationalsupport of some local agricultural situation,then it can be repre sen tative when its location istypical for that application, maybe in a forest, in avery humid area (for disease protection purposes),or at the bottom of a valley (for studying frostprotection). Even so, locations should be avoidedthat are on or near steeply sloping ground, or nearlakes, swamps or areas with frequent sprinkling orflooding.The site of a weather station should be fairlylevel and under no circumstances should it lieon concrete, asphalt, or crushed rock. Whereverthe local climate and soil do not permit a grasscover, the ground should have natural covercommon to the area, to the extent possible.Obstructions such as trees, shrubs and buildingsshould not be too close to the instruments.Sunshine and radiation measurements can betaken only in the absence of shadow during thegreater part of the day; brief periods of shadowsnear sunrise and/or sunset may be unavoidable.Wind should not be measured at a proximity toobstructions that is less than ten times theirheight. Tree drip into raingauges should not beallowed to occur.Accessibility to the weather station and the possibilityof recruiting good observers locally shouldalso be criteria for selection of a site. Finally, formajor stations, the likelihood that the conditionsof the location will remain the same over anextended length of time with little change in thesurroundings should be investigated.2.2.2.2 Layout of station instrumentsTo minimize tampering by animals and people,it is desirable to fence the weather station

CHAPTER 2. AGRICULTURAL METEOROLOGICAL VARIABLES AND THEIR OBSERVATIONS 2–5enclosure. A sample layout is shown in Figure2.1. This layout is designed to eliminate as far aspossible mutual interference of instru ments orshadowing of instruments by fence posts. Thedoor of the thermometer screen must open awayfrom the sun, to ensure that direct sunlight doesnot enter the screen during observations. Atequatorial and tropical stations, the screen willhave doors opening to both the north and thesouth. A larger enclosure is recommended whensmall plants are used for phenologicalobservations. A rather sheltered enclosure is nota good place for measuring wind; a nearbylocation with better exposure may be preferablefor the wind mast.2.2.3 Primary handling of dataIf the weather station is part of a network, anotherfactor to be considered is the use of the data:whether they will be used for climatological orreal‐time information purposes. If the data are usedfor the latter, a rapid communication system isnecessary for data transmission, whether by landline,radio or satellite. The issues of using data forclimatological purposes were discussed underagenda point 10.3 of the fourteenth session of theCommission for Agricultural Meteorology (CAgM),held in New Delhi, India, in 2006, on the “Expertteam on database management, validation andapplication of models, research methods at theNorth1.5 m1.5 m1.5 m2 mThermometerscreen100 cmSoil thermometerCup-counteranemometeron slender2 m pole1.5 mRaingauge 11.5 m 1.5 mWestRaingauge 23 m 1.5 mSoil thermometer 30 cm60 cmRecordingraingaugeEastConcrete slab1.4 mMin.therm1 m75 mGrass minimum thermometer2 m1.25 m5 mSoil thermometers20 cm 10 cm 5cm1.5 mSunshinerecorderon 2 mpillarBare-soil minimumthermometerBare patch to bekept weeded2 m1 m1.5 m1 mSouthFigure 2.1. Layout of an observing station in the northern hemisphere showingminimum distances between installations (from WMO, 2000b)

2–6GUIDE TO AGRICULTURAL METEOROLOGICAL PRACTICESeco-regional level”. This discussion emphasizedthat data should be entered locally as they arecollected (either on an hourly or daily basis) andthat the data should be entered only once into aDatabase Management System (DBMS) and be madeavailable to all portions of the NationalMeteorological and Hydrological Services (NMHSs).The DBMS system used should be capable ofhandling climatic and other types of data, such asecological, hydrological, agricultural and geo-referenceddata; it should also be able to easily importdata from a variety of formats. Also, all data shouldbe input directly into a DBMS and then used byvarious software application packages. Some qualitycontrol (QC) of the data can be conducted locallyas the data are being entered. Other QC such asspatial quality checks can be undertaken at thecentral database. It is important that all data, bothraw and those processed for the long-term archive,be backed up securely at every stage.2.2.4 NetworksWhen agricultural meteorological stations are beingestablished or reorganized, the number of stationswithin each region should depend on its extent,climatic types and sub-types, and the spatial variationsof such factors as the natural vegetation, maincrops and agricultural methods. As far as possible,each large homogeneous phyto-geographical regionshould be represented by at least one principal agriculturalmeteorological station.Similarly, each characteristic area devoted to aparticular aspect of agriculture, animal husbandry,hydrobiology or forestry should, wherever possible,be represented by an ordinary agricultural meteorologicalstation. Sufficient auxiliary agriculturalmeteorological stations should be installed toensure adequate spatial density of the observationsof the meteorological and biological variables ofmajor agrometeorological concern to the country.From another point of view, marginal areas of agricultureand silviculture will often deserve specialattention. One main object of observations made insuch areas would be to determine the boundary ofthe region where an individual crop could be grownsuccessfully or a specific agricultural or silviculturalprocedure might be profitable; another would be toascertain the frequency and the typical geographicaldistributions of the main weather hazards, witha view to reducing their adverse effects as far aspossible by means of protective measures.Areas where agricultural production is markedlyexposed to losses through plant and animal diseasesare of special interest, as meteorological factors canbe important in the development of these diseases.National parks and nature reserves, although usuallynot representative of the areas that are of majoreconomic importance in agriculture, may providegood locations for reference stations where observationscan be made over long periods underpractically identical conditions.The selection of these stations, whether principal,ordinary, auxiliary or for specific purposes, will varyfrom one country to another, but some generalguidance may be given. The first consideration isthat all agrometeorological stations should belocated in regions of agricultural, silvicultural,pastoral or other forms of production. For informationon representativity, see 2.2.2.1. In thisconnection, the following locations will often besuitable for principal (and ordinary) stations:(a) Experimental stations or research institutes foragriculture, horticulture, animal husbandry,forestry, hydrobiology and soil sciences;(b) Agricultural and allied colleges;(c) Areas of importance for agriculture and animalhusbandry;(d) Forest areas;(e) National parks and reserves.In the case of auxiliary stations and stations establishedfor specific purposes, selected farms shouldalso be considered. Experience has shown, however,that if the observations are made by alternatinggroups of students who may be insufficientlytrained for this purpose, as in the case of observatorieslocated at higher education institutions, verycareful supervision will be needed to ensure observationsof acceptable quality. In general, theobservational accuracy should be a major consideration;quality must not be sacrificed for quantity.No difficulties should normally arise in locatingbasic equipment in areas devoted to agriculture,horticulture and animal husbandry, since theterrain is usually relatively level and open, satisfyingthe general standards for locatingagrometeorological and climatological stations.Stations located in forested or silvicultural regionsrequire special consideration. They should be representativeof the general climate in the forest, andshould reflect the effects of tree development withinthe forest. The exposure conditions and instrumentalrequirements of these stations are described inChapter 11.At the fourteenth session of CAgM it was restatedthat adequate density of (agrometeorological)

CHAPTER 2. AGRICULTURAL METEOROLOGICAL VARIABLES AND THEIR OBSERVATIONS 2–7stations and intra- and extrapolation of routinestation data to agricultural field conditions remainof great concern, particularly in developingcountries. Automatic weather stations can assist insolving some of the related problems, butinstrument coordination, calibration andmaintenance are serious issues to be consideredwith great attention, and even more so withautomatic weather stations, again particularly indeveloping countries.Special equipment required for non-routine observations,such as that needed for experiments,research and special agrometeorological services, isgenerally installed outside standard enclosures, forinstance, within crops, above crop canopies or inareas under cultivation.2.2.5 Documentation of agriculturalmeteorological stationsThe metadata information that is necessary insupport of reliable observations is described atlength in WMO (2003a) and more briefly in WMO(2008b). Its acquisition is summarized below.Full information on all of the agricultural meteorologicalstations in the country should be availablein the NMHSs. For this purpose an up-to-date directoryof these stations, whether controlled by theNMHS or by other services or agencies, should bemaintained. In countries where there are manyregional agrometeorological services or wherenetworks are managed by farmers and commercialenterprises, constant updating of this general directoryat the national level will be needed. Thedirectory should archive for each station:(a) Station identification: name, network codenumber(s), category of station;(b) Geographical location: latitude and longitude(accurate in units of a few hundred metres, forexample, 0.001 degree), mapping of mesoscaleregion (≈1:100 000) with major terrain elevationchanges; physical constants and profileof local soil;(c) Observing programme specification andhistory: for each parameter, the dates onwhich records begin and end and the dateson which instruments, observation heightor site are changed. Archive of all updates ofstation mappings as described in (e) through(h) below. Description of observation routineprocedures and basic data processing. Unitsin use. Routine transformations of observedparameters to archived data;(d) Station information contact: name of stationsupervisingorganization or institution,identification (name, address, telephone orfax, or e-mail) of observer(s) or other person(s)responsible for local measurements and/ortheir archiving.To support and complement this national documentation,the station observer(s) at individualstations should maintain local documentation onthe following metadata:(e) Toposcale map of surroundings (with a scaleof ≈1:5 000), as specified by the Commissionfor Instruments and Methods of Observation(CIMO) (WMO, 2008b), including locationand size of obstacles, surrounding vegetation,and significant terrain features (such as hillsand hollows, lakes, built-up areas, roads). Thismap should be updated at least yearly;(f) Microscale map of the station enclosure withan indication of the location of instrumentsand their height above the ground, updatedupon changes. Description of the instrumentshelter;(g) Photos of the enclosure and all instrumentpositions outside the enclosure, showingthem in their surroundings (that is, fromsufficient distance, 20 m or more), taken fromall directions (at least six or eight, with thedirections identified on the photo print),updated upon significant changes;(h) Regularly updated horizon mapping of solarradiation observation (see WMO, 2008b);(i) Specification of all instruments: manufacturerand model, serial number, output type andsensitivity, recording or frequency of observation,beginning and end of use;(j) Regularly used logbook with history of stationactivities: calibrations and other control activities,maintenance, all interruptions and missingobservations, significant developments(for example, nearby building activities,growth of vegetation).For some parameters, “particular” metadata requirementsare mentioned in 2.4. As the above representsonly a summary of the requirements, it is advisableto consult WMO (2003a) for a more detaileddescription.2.2.6 Inspection and supervision ofstationsAgricultural meteorological stations maintained bythe National Meteorological Service should beinspected at least once a year to determine whetherthe exposure has changed significantly and toensure that observations conform to the appropriatestandards and that the instruments are

2–8GUIDE TO AGRICULTURAL METEOROLOGICAL PRACTICESfunctioning correctly and are calibrated at therequired times. The time interval between successiveinspections of an individual station will dependupon the programme of the station and the qualificationsof the local personnel responsible for theprogramme.If other authorities make agricultural meteorologicalobservations, they should enter into cooperativearrangements or special agreements with theNational Meteorological Service to ensure adequatesupervision and maintenance of the network,including calibration of equipment.2.2.7 Fixed agrometeorological stationsThese stations are foreseen as operating for anextended period at a fixed place, and may be:(a) Minimum equipment stations, consistingof a small portable screen, minimum andmaximum thermometers, dry and wet bulbthermometers, totalizing anemometer at aconvenient height, and raingauge. For screensthat are not standard, the radiation errorshould be determined;(b) Standard equipment stations, consisting ofstandard screen instruments and raingauge asin (a) above, thermohygrograph, wind vane,and wind-run and sunshine recorders. Theseallow one to determine evaporation usingempirical methods;(c) Semi-automatic stations with an uninterruptiblepower supply, which are requiredto provide the measurements when trainedpersonnel are not available. There is no automaticdata communication;(d) Automatic stations, which require lesssupervision, but installation, calibrationand inspection must be of a high standard.An uninterruptible power supply is requiredand data from these stations can be usedfor direct computer processing. Initial andmaintenance costs, as well as proper calibrations,may be limiting factors. Data shouldpreferably also be communicated automatically.2.2.8 Mobile stationsMobile stations are used for surveys and research.Some mobile stations move continuously andothers need equilibrium of sensors or certain periodsfor measurements, such as for local windobservations. When an extended but superficialsurvey of air temperature and humidity is required,vehicles usually carry the instruments. In thesecircumstances, use is made of thermocouples andthermistors that have a rapid response (low “timeconstants”) and high sensitivity.When using motor vehicles, all mechanical instrumentsshould have anti-shock mounts and shouldbe mounted so that the recording movement isperpendicular to the direction of the most frequentvibrations, in order to reduce the effect of thesevibrations on the instruments.2.2.9 Agricultural mesoclimatologicalsurveysThe objective of agricultural mesoclimatologicalsurveys is to determine meteorological variablesor local special factors affecting agriculturalproduction on a local mesoscale that are notrepresentative of the general climate of the region.The surface relief (topoclimatology) and character(landscape), regional wind circulations, waterbodies, forests, urban areas and like characteristicscome under these categories. Reference maybe made to An Introduction to Agrotopoclimatology(WMO-No. 378). These surveys are particularlyuseful where high measurement densities areneeded and in developing countries or sparselypopulated regions, where network sites are widelyseparated. Additional data from temporarystations that function from one to five years areuseful for comparison with data from the basicnetwork and for evaluation of interpolation ofdata between temporary and basic networkstations. Observations with special instruments,from fixed or mobile stations, may serve tocomplete the general pattern.In the older literature, mesoclimatology and topoclimatologywere seen as studying the influence of theearth’s actual surface on climate and of the climateon that surface. Many important factors that influencethe local exchanges of energy and moisturewere noted: configuration and roughness of theearth’s surface; colour, density, thermal capacity,moisture content and permeability of the soil; propertiesof the vegetation covering it; albedo (thereflection coefficient of a surface); and so on. Morerecent literature still uses the same approach (forinstance, Geiger et al., 1995), adding exchanges ofgases other than water vapour, liquids, particles,and the like. The fourteenth session of CAgM agreedthat special attention should be paid to peak valuesof rain, wind, and flows of water, sediment andother materials carried, because they were locally ofgreat importance to agriculture.The series of publications issued jointly within theframework of the FAO/UNESCO/WMO Interagency

CHAPTER 2. AGRICULTURAL METEOROLOGICAL VARIABLES AND THEIR OBSERVATIONS 2–9Project on Agroclimatology between 1963 and1982 2 , which present agroclimatological studies inseveral developing regions, contain various aspectsof mesoclimatological surveys. The start of a neweragroecological approach, where mesoclimatologicalsurveys are incorporated into wider productionevaluations, can be found in Bunting (1987).Modern quantitative approaches in agroclimatologyat the mesoscale using remote-sensing andGeographical Information System (GIS) technologiesare reviewed in the present Guide asagrometeorological services (Chapters 4 and 6).They often have to be combined with classicalmeasurements, such as ground truth or farm-scaledetails (Salinger et al., 2000). These classical measurementsmay be from fixed or mobile, standardor automatic equipment, while complementaryobservations to describe the special mesoclimaticprocesses may sometimes be used (for instance,WMO, 2008b).2.2.10 Complementary observations todescribe special mesoclimaticprocessesThe spatial characterization, including the verticaldimension, of mesoclimatic patterns of temperature,humidity, pressure and wind in the lowertroposphere for research purposes is determined asfollows:(a) Aircraft meteorograph soundings are performedon days presenting typical air masses for eachseason. It may be advantageous to carry outsoundings at hours of minimum and maximumsurface temperatures. The soundings that aremade should be selected for the problem understudy, vertically spaced every 100–150 m up to800–1 000 m, and then every 300–500 m up to3 000 m;(b) Soundings up to 300–500 m are carried outwith a fixed meteorograph or radiosondesuspended from an anchored balloon. Toavoid wind motion, in the past balloonswere usually fixed with three bracing lines;however, modern instruments compensatefor the movement if required;(c) For the study of wind structure upto300 m, anchored directional balloonsand/or sodars pointing into the direction ofthe wind may be used. For greater heights,2 Une étude d’agroclimatologie dans les zones arides et semi-aridesdu Proche-Orient (WMO-No. 141), An Agroclimatology Survey ofa Semi-arid Area in Africa South of the Sahara (WMO-No. 210),A Study of the Agroclimatology of the Highlands of Eastern Africa(WMO-No. 339), Estudio agroclimatológico de la zona andina(WMO-No. 506) and A Study of the Agroclimatology of theHumid Tropics of South-East Asia (WMO-No. 597).pilot balloons with a low rate of ascent areused; their flights are followed from theground with two theodolites. At night theballoons must be battery-illuminated. Smokebombs may be useful to show wind directionas well as turbulence up to a limited height.2.2.11 Detailed physical observationsof a non-routine or nonpermanentcharacter (agriculturalmicrometeorological research)Detailed accurate observations that are neitherroutine nor permanent are needed for fundamentalresearch, and are usually carried outindependently of conventional agroclimatologicalobservations. Phenomena and processes concernedare, for example, listed and explained in Stigter(1994b). Such observations are made to a highdegree of accuracy by skilled, scientifically trainedstaff and mostly include micrometeorologicalmeasurements made with specially designedinstruments. For observations as highly specific asthese, no general method can be formulated (seefor example Woodward and Sheehy, 1983; Pearcyet al., 1989).2.3 OBSERVATIONS TO BE CARRIED OUTAT AGRICULTURALMETEOROLOGICAL STATIONS2.3.1 Observations of the physicalenvironmentThe observing programme at agricultural meteorologicalstations should include observations of someor all of the following variables characterizing thephysical environment: solar radiation, sunshine andcloudiness, air and soil temperature, air pressure,wind speed and direction, air humidity and soilmoisture, evaporation and precipitation (includingobservations of hail, dew and fog). The water balance,evapotranspiration and other fluxes may be deducedfrom these and other measurements. Minimumaccuracy for the different variables is recommendedin WMO (2008b) as given in Table 2.1.These measurements refer to the programme thatshould be followed for permanent or routinenationwide observations. Nevertheless, the needsof agricultural meteorology frequently require additionaland special information, mainly at principaland ordinary stations, such as the following:(a) Results of agricultural mesometeorologicalsurveys;

2–10GUIDE TO AGRICULTURAL METEOROLOGICAL PRACTICES(b)(c)Table 2.1. Minimum accuracy recommended byWMO for some variablesVariableTemperature, including max/min, wet and dry bulb, soilRainfallSolar radiationincluding sunshineEvaporationData derived from remote-sensing;Accurate physical observations on a nonroutinebasis (for agricultural micrometeorologicalresearch).Some general comments concerning each of thesevariables or groups of variables are offered in thefollowing publications: the Guide to ClimatologicalPractices (WMO-No. 100) gives detailed guidanceon climatological observations in general andconsiders aspects that apply equally to the observationof climatic variables for routine climatologicalpurposes or to the programme of an agriculturalmeteorological station. The Guide to MeteorologicalInstruments and Methods of Measurement (WMO-No. 8) discusses extensively the instruments to beused and observing practices to be followed inmeteorology. It must be stressed that the materialcontained in these publications and in the presentGuide refer to the ultimate aims of an agriculturalmeteorological service. The initial steps taken byany such service can obviously be of a simpler character,but should be such that further expansioncan be made along the lines indicated. Normally,only principal agricultural meteorological stationswould attempt to conduct all the observationsdescribed in the present publication.2.3.1.1 Radiation and sunshineAccuracy required indaily values< ±0.5ºC±1 mm10% (±1h)±1 mmRelative humidity ±5%Photoperiod10% (±1h)Wind speed ±0.5 ms –1Air pressure±0.1 hPaReference may be made to Coulson (1975), Fritschenand Gay (1979), Iqbal (1983), WMO (1984, 2001b,2008b), Goel and Norman (1990), Strangeways(2003), and WMO Technical Note No. 172. In addition,the duration of day length, which influencesthe flowering and growth of shoots of crop plants,should be recorded or obtained at all agriculturalmeteorological stations. This information shouldbe supplemented wherever possible by data obtainedfrom radiation instruments. Principal stationsshould make detailed observations of radiation,including global solar radiation, photosyntheticallyactive radiation (PAR) and net all-wave radiation.The spectral distribution of solar radiation influencesthe growth and development of plants andefforts should be made to include it in the observingprogramme. Important components areultraviolet, PAR and near-infrared radiation.Most commonly, a solarimeter (pyranometer) ismounted horizontally and measures the total solarirradiance on a horizontal surface. In addition, ashade ring (or occulting disk) may be used to cast ashadow on the sensitive area, eliminating the directbeam. The instrument then indicates only thediffuse (sky) radiation. The power of the direct beammay be calculated by subtracting the diffuse readingfrom the total radiation. Beam fraction sensorswithout moving parts are also now available.Solarimeters can be used to measure the short-waveradiation reflected from a crop surface as well. Anadditional sensor is inverted, fitted with a shield toeliminate diffuse sky radiation, and mounted highenough over the surface so that the shadow it castsis a very small part of the surface area (crop canopy)being investigated. A pair of upward and downwardfacing solarimeters forms an albedometer.Research results show that shade influencesphotosynthesis and temperature. At the macrolevel, shade occurs due to clouds, mountain slopes,and so on. At the micro level, shade varies due tothe plant canopy itself, intercropping choices,surrounding trees, and the like. Photosynthesis isthe major metabolic process in agriculture thatdepends on solar radiation. As a result, occurrencesof shade and its distribution, duration and intensityinfluence photosynthesis and therefore theproduction processes. Shade and light also causemany morphological processes in plants andbehavioural changes in animals. Though moreshade reduces agricultural production in many fieldcrops, it improves quality in many cases. In manyfruit crops, fruit quality is improved with partialshade treatments. The effect of shade on crops canbe measured using cloths that reduce insolation bya required percentage over plots of the experimentalfield. Tube solarimeters (made in tubular form foreasy insertion horizontally under a crop canopy)are vital for measuring solar radiation and shadeinfluence in crop growth, agroforestry and mulch

CHAPTER 2. AGRICULTURAL METEOROLOGICAL VARIABLES AND THEIR OBSERVATIONS 2–11studies. For further details see 2.4.1.1, which alsodiscusses infrared thermometers and pyrgeometersfor long-wave radiation from sky and earth.2.3.1.2 Air temperatureReference should be made to Fritschen and Gay(1979), WMO (1984, 2001b, 2008b), Goel andNorman (1990) and Strangeways (2003). Thetemperature of the air should be measured inrepresentative places, at different levels in thelayer adjacent to the soil. Measurements should bemade at principal agricultural meteorologicalstations from ground level up to about 10 m abovethe upper limit of the prevailing vegetation becauseair temperature affects leaf production, expansionand flowering. At ordinary or auxiliary stations,however, the measurements will usually berestricted to the lowest few metres above thesurface, which are the most significant layers forstudying climatic conditions affecting agriculturalcrops, their growth and development; these arealso the layers with the largest gradients and mostrapid fluctuations. To study the vertical distributionof temperature within the lowest two metresof the atmosphere, measurements should be madeat three levels at the least, selected from the followingheights: 5, 10, 20, 50, 100, 150 and 200 cm.Observations taken for special research projectsvary with the needs of the problems underinvestigation.In order to study the diurnal variations of temperature,recording instruments should be used at least atone level. Where a continuous record of temperatureis not possible, the maximum and minimum valuesshould be recorded at two or three levels. Such measurementsshould generally be made under standardconditions, namely, over a short grass cover maintainedas far as possible unchanged throughout theyear or, if this is impossible, over bare soil.Observations should be made, as far as possible, inthe middle of a fairly large representative area (20 to50 m in diameter) containing level ground with soilor vegetation cover. At principal agricultural meteorologicalstations, the measurements should besupplemented by similar ones taken in variousregional crops during the growing season. Thesesupplementary observations should be carried out atthe same levels as the observations over bare soil orgrass, and also at levels immediately below and abovethe upper limit of the vegetation.Exposure to radiation is a serious source of error inmeasuring atmospheric temperature. Probably thebest method of measuring air temperature is byusing freely exposed electrical equipment (resistanceor thermocouple thermometers) having thin orreflective sensitive elements with a very lowabsorption of radiation. Where such instrumentsare not available, shade screens or ventilatedthermometers may be used for levels at least 50 cmabove bare soil or dense vegetation. Non-standardscreens generally used at other meteorologicalstations run a risk of hampering the flow of air pastthe thermometers and, in bright sunshine and lightwinds, they may be heated to a temperature abovethat of the ambient air. The disadvantage isespecially marked for measurements below thestandard level of 1.25 to 2 m. Thermometer screensare therefore not recommended when the verticaldistribution of temperature up to 2 or 3 m is desired,although small open reflective screens have beenused with some success. It is necessary to protectthermometers in the open from precipitation bysmall roof-shaped shelters.2.3.1.3 Temperature of soilReference should be made to Rosenberg et al. (1983)and WMO (1984, 2001b, 2008b). Soil temperaturedirectly influences crop growth because the sownseeds, plant roots and micro-organisms live in thesoil. The physicochemical as well as life processes inagriculture are also directly affected by the temperatureof the soil. Under low soil temperatureconditions, nitrification is inhibited and the intakeof water by roots is reduced. Extreme soil temperaturesinjure plants and thereby affect growth.The observing programme at all categories of agriculturalmeteorological stations should thereforealso include soil temperature measurements. Thelevels at which soil temperatures are observedshould include the following depths: 5, 10, 20, 50and 100 cm. At the deeper levels (50 and 100 cm),where temperature changes are slow, daily readingsare generally sufficient. At shallower depths theobservations may comprise, in order of preference,either continuous values, daily maximum andminimum temperatures, or readings at fixed hours(preferably not more than six hours apart).When soil temperature data are published, informationshould be given on the way the plot is maintained.The depths of the thermometers at 5, 10 and 20 cmshould be checked periodically and maintained.Efforts should be made to ensure that good contact ismaintained between the thermometer and the soil.Regarding the surface of the plot where soil temperatureis measured, two types of standard cover areused – bare soil and short grass. Wherever possible,simultaneous readings should be made under both

2–12GUIDE TO AGRICULTURAL METEOROLOGICAL PRACTICESstandards for comparison. In many places, however,it may be difficult or even impossible to maintainplots conforming to both standards. Hence the onemost suited to the region should be used. Also, whereverthe standard surface is not representative of thesurroundings, the instruments should be placed nearthe centre of a large plot (for bare soil, the Guide toMeteorological Instruments and Methods of Measurement(WMO-No. 8) recommends 2 m × 2 m). A comparisonof soil temperature observations under a standardcover and under crops shows the modifications ofthe temperature regime due to the principal regionalcrops and their cultivation, depending on soil modification,soil shading and suppression of airmovement over the soil (Mungai et al., 2000).When soil temperatures are measured in a forest, thereference level for the depths of measurement shouldbe clearly indicated: whether the upper surface of thelitter, humus or mass layer is considered to be at 0 cm;or whether the soil–litter interface is taken as zeroreference. These details and any seasonal variations inthem should be quoted when the data are published(for further details, see Chapter 11 of this Guide).Whenever the ground is frozen or covered withsnow, it is of special interest to know the soiltemperature under the undisturbed snow, the depthof the snow and the depth of frost in the soil.Measurement of the thermal properties of the soil(such as specific heat and thermal conductivity),temperature profiles, and changes in these profilesshould be included.2.3.1.4 Atmospheric pressureReference should be made to Murthy (1995, 2002).The lower pressures experienced as altitude increaseshave important consequences for plant life at highaltitude. At high altitudes and low atmospheric pressuresthe solubility of carbon dioxide and oxygenin water is reduced. Some plants show stuntedgrowth at higher altitudes as concentrations ofoxygen and carbon dioxide reach low levels. Plantswith strong root systems and tough stems can liveunder increased wind speeds at low pressures inhigh-altitude areas. It is usually adequate to knowthe altitude at which an event takes place, but insome cases pressure variations have to be taken intoaccount. Usually, a station will record pressure aspart of the data for climatological work.2.3.1.5 WindReference should be made to Mazzarella (1972),Wieringa (1980), Kaimal and Finnigan (1994) andWMO (1984, 1998, 2001b, 2008b). Wind transportsheat in either sensible or latent form between lowerand higher layers of the atmosphere and from lowerto higher latitudes. Moderate turbulence promotesthe consumption of CO 2 by crops during photosynthesis.Wind prevents frost by disrupting atemperature inversion. Wind dispersal of pollen andseeds is natural and necessary for certain agriculturalcrops, natural vegetation, and so on. As far as theaction of wind on soil is concerned, it causes soilerosion and transport of particles and dust. Extremewinds cause mechanical damage to crops (for example,lodging or leaf damage) and forests (windthrow).Knowledge of the wind is also necessary for environmentallysensitive spray application and for thedesign of wind protection. For the main regionalcrops, it may be useful to make observations of windprofiles inside and above the crop canopies for abetter understanding of exchange properties.Agricultural meteorological stations need toposcalereference observations of both wind speed anddirection, preferably at 10 m height, but at least atthree times the height of any nearby vegetation (forinstance, crops) and any nearby obstacles, in orderto be above significant flow interference. Lowerlevelwind measurements are not representative attoposcale and cannot be properly corrected either,so they cannot be used as local reference or forcomparison with other stations (WMO, 2001b).Horizontal distance to obstacles should be at least10 times their height. When possible, the windspeed gustiness should be obtained along with averagewind, for instance by recording the largestthree-second gust in each averaging period.This basic programme may be supplemented, wherecircumstances permit, by measurements of windspeed at one or more levels between the surface and10 m; wind direction varies little in that layer.Except for layers rather close to the ground, this canbe done by means of sensitive cup anemometers orpropeller vanes, which tend to lose accuracy,however, because of the need for them to rotateinto the wind (WMO, 2008b). Any more ambitiousprogramme should be carried out at principal agriculturalmeteorological stations or operationallywith mobile stations, as recently done in Africa(WMO, 2005). Wind speed and gustiness are measuredat various levels right down to the ground bymeans of anemometers of high sensitivity, withparallel temperature measurements at those levels.2.3.1.6 Air humidity and soil moisture(including leaf wetness)Reference should be made to WMO (1984, 2001b,2008b).

CHAPTER 2. AGRICULTURAL METEOROLOGICAL VARIABLES AND THEIR OBSERVATIONS 2–132.3.1.6.1 HumidityHumidity is closely related to rainfall, wind andtemperature. Different humidity-related parameterssuch as relative humidity, vapour pressure,dewpoint and other derived characteristics areexplained in many textbooks. They play a significantrole in crop production and strongly determinethe crops grown in a region. Internal water potentials,transpiration and water requirements ofplants are dependent on humidity. Extremely highhumidity is harmful as it enhances the growth ofsome saprophytic and parasitic fungi, bacteria andpests, the growth of which causes extensive damageto crop plants. Extremely low humidity reducesthe yield of crops.Like temperature and for the same reasons, thehumidity of the air should be measured in representativeplaces, at different levels in the layeradjacent to the soil at principal agricultural meteorologicaland other category stations. The proceduresfor air temperature should also be followed for thisweather variable, including taking measurementsabove and within vegetation.2.3.1.6.2 Soil moistureReference may be made to Greacen (1981), Viningand Sharma (1994), Smith and Mullins (2001) andWMO (2001b, 2008b). In scheduling irrigation, theestimation of moisture content is the basic requirement.The soil water content can be determined bydirect methods, such as gravimetric and volumetricdeterminations, and indirect methods, which mayinclude the use of devices such as tensiometers,resistance blocks, neutron moisture meters andtime domain reflectometry (see 2.4.1.6.2).Soil moisture should be measured at all principalstations and, wherever possible, at other agriculturalmeteorological stations. Although rigidstandardization is neither necessary nor, perhaps,even desirable, these measurements should, whereverpossible, be made from the surface to depths of10, 20, 30, 40, 50, 60, 70, 80, 90 and 100 cm. Indeep soils, with a high rate of infiltration, measurementsshould be extended to greater depths. Oftenlevels will be selected in relation to the effectiverooting depths of the plants. Until it is possible tomake reliable continuous recordings at some ofthese levels, it is recommended that observationsbe made at regular intervals of about 10 days; forthe shallower depths, shorter intervals (seven orfive days) will be necessary. In areas with snowcover, more frequent observations are requiredwhen the snow is melting.Standard soil moisture observations should bemade below a natural surface representative of theuncultivated regional environment. Simultaneousobservations in areas devoted to principal regionalcrops and covering all cultural operations willshow modifications introduced by agriculturalprocesses. These soil moisture measurements areparticularly useful in verifying soil moisture valuesestimated from meteorological measurements.Further discussion of soil moisture problems maybe found in Technical Note Nos. 21 and 97. Inoperational agrometeorology, the problem of onfarmmeasuring density was dealt with by Ibrahimet al. (1999), who subsequently accurately determinedwater waste in irrigated groundnut andsorghum (Ibrahim et al., 2002). Oluwasemire et al.(2002) discussed a sampling method in intercroppingconditions, while infiltration of rainwaterand use of this soil moisture could be followedsimply in the field by Mungai et al. (2000).The following additional parameters will contributeto a better understanding of soil moistureconditions:(a) Field capacity and other hydrological constantsof the soil;(b) Permanent wilting point;(c) Depth of groundwater.2.3.1.6.3 Leaf wetness and dewThe weather provides liquid water not only in theform of precipitation, but also in the form of dew,which is not the same as leaf wetness but is one ofits possible causes. Dew (fall) occurs in a humidatmosphere when temperature falls and wind isweak, resulting in condensation both on the vegetationand on the soil. Dew often occurs due todistillation of water from (wet) soil (dew rise).Guttation occurs on vegetation when its internalwater pressure is excessive. In some very dryregions dew may well be a significant source ofmoisture in maintaining plant life (Acosta Baladon,1995).Leaf wetness can result from precipitation, from dewor from guttation. Knowledge of leaf wetness durationis vital information for the protection of cropsagainst fungi and diseases (Technical Note No. 192),and it cannot be deduced usefully with rules ofthumb such as RH > 90 per cent. Actual monitoringhas so far been carried out only in a few countries ona routine basis with specific agrometeorologicalrequirements in mind. Studies and recordings of leafwetness duration (LWD) also help in developingearly warning systems and plant protection, inunderstanding soil evaporation and in improving

2–14GUIDE TO AGRICULTURAL METEOROLOGICAL PRACTICESleaf-surface water evaporation modelling. Calibrationof sensors is also enhanced in this way.2.3.1.7 Precipitation (clouds andhydrometeors)Reference should be made to Meteorological Office(1981), Murthy (1995), Baldy and Stigter (1997), forforms in which rainwater reaches the soil, Murthy(2002) and WMO (2008b). WMO Technical NoteNo. 55 may also be useful.At an agricultural meteorological station, visualobservations and automatic instrumentation tomeasure total cloud coverage, that is, all sky cameraobservations, may be made at regular intervals tomeasure the total amount of cloud. In addition,cloud type and height of cloud base are required forstudies of the radiation balance. Observations ofhydrometeors are useful for many agriculturalpurposes. They include rain and drizzle (includingintensity), snow (including thickness and density ofsnow cover, and water equivalent), hail (includingwater equivalent and size of hailstones), dew (amountand duration), hoar frost, rime fog, and so forth.The amount of precipitation should be measured inthe morning and evening as at synoptic stations.Additional measurements are desirable and theintensity of precipitation could be obtained bymeans of a recording raingauge. Hail is the precipitationof solid ice that is formed inside cumulonimbusclouds, the thunderstorm-producing clouds. It ismeasured according to individual hail stone size orits liquid equivalency. Advanced techniques suchas remote-sensing provide a quick and clear illustrationof hailstorm patterns. Data obtained bymeteorological radar can be useful in supplementingrainfall measurements and may make it possibleto identify and locate hydrometeors that are particularlyharmful to agriculture (hail, very heavyshowers), with a view to taking appropriate action(Wieringa and Holleman, 2006).There are still examples of volunteers who dovaluable work by simply increasing rainfallmeasurement densities for disaster detection, andin more developed countries today they make useof the newly available means of communication(for example, Walsh, 2006). O’Driscoll (2006)described another network of this kind in whichsimple rainfall measurements are made incombination with the reporting of agriculturallyimportant hail and snow. The importance of suchsimple rainfall data for modern farming can also beunderstood by viewing Websites such as http://www.agweb.com.The extent and depth of snow cover should beobserved regularly where appropriate; it may bedesirable to give information about water equivalentand consistency of the snow cover, for instance,once or twice per week.Especially in dry climates with large daily fluctuationsin temperature, the amount of water depositedin the form of dew (or rime) may be of great importancein the water balance of the biosphere. Inaddition, the duration and amount of dew areimportant in connection with certain plant diseases(see also 2.3.1.6 above).2.3.1.8 Evaporation and water balancemeasurementsReference should be made to Rosenberg et al. (1983),WMO (1984, 1994b, 2001b, 2008a, 2008b) andWMO Technical Note Nos. 11, 21, 26, 83, 97 and126. Measurement of evaporation from free watersurfaces and from the soil, and of transpiration fromvegetation, remains of great importance in agriculturalmeteorology. Potential evapotranspiration isdefined as the amount of water that evaporates fromthe soil–air interface and from plants when the soilis at field capacity. Actual evapotranspiration isdefined as the evaporation at the soil–air interface,plus the transpiration of plants, under the existingconditions of soil moisture.Several publications explain the updated, internationallyagreed energy balance calculations of cropevaporation (for example, Hough et al., 1996; FAO,1998; Monteith and Unsworth, 2007), while theirapplications under on-farm tropical field conditionsare now also reported (for example, Ibrahimet al., 2002). Particular attention is drawn to thedifficulty of measuring potential evapotranspirationfor a small wet surface within a large dry area(oasis effect). Observations of the following parameters,which contribute to knowledge of the waterbalance, should be made whenever possible:(a) Evaporation from a free water surface;(b) Height of the water table;(c) Irrigation water applied.Generally, water is applied in fields by different irrigationmethods depending upon the crop and soilcondition: surface irrigation, subsurface irrigation,sprinkler irrigation and drip irrigation. Surfaceirrigation includes flooding, check-basin, basin,border‐strip and furrow irrigation. Flooding is usedexclusively for lowland rice. The check-basinmethod is adopted when the field is quite large andcannot easily be levelled in its entirety. The field isdivided into small plots surrounded by small bunds

CHAPTER 2. AGRICULTURAL METEOROLOGICAL VARIABLES AND THEIR OBSERVATIONS 2–15on all four sides. In the basin method, which is suitablefor fruit crops, only the basin around the treesis irrigated. In the border-strip method the field isdivided into a number of strips by bunds of around15 cm in height. The area between two borders isthe border strip. In furrow irrigation, differentmethods are included, such as deep-furrow, corrugation,alternate-furrow or skip irrigation,wide-spaced furrow irrigation, and within-rowirrigation.Surface irrigation methods are the most economicaland are the easiest to operate. There is no requirementfor additional input, but the method requireslevelling of the field and entails high labour costs. Italso provides for low water distribution efficiency(for example, Ibrahim et al., 2000). In the case ofsubsurface methods, where water gradually wetsthe root zone through capillary movement, weedsare less of a problem due to dry surface soil.Evaporation losses are minimized, but the maintenanceof pipelines is problematic in this type ofirrigation. Microirrigation, adopted where water isscarce, includes drip and sprinkler irrigation and issuitable for horticultural crops.Because of the great importance of water resourcesfor agriculture, detailed knowledge of the factorsaffecting the different terms of local soil waterbudgets and larger-scale water balances is highlydesirable. The programme of observations for agriculturalmeteorology must therefore includehydrological observations, such as lake and watershedwater balances and the water stages in anadjacent lake or river, which are important inconnection with river floods that are significant foragriculture. Cooperation with NationalMeteorological and Hydrological Services is necessaryin this regard (WMO, 1994b, 2008a).2.3.1.9 Fluxes of weather variables (derivedfrom measured quantities)The term “flux” means the rate of flow of fluid,particles or energy through a given surface. Thebasis of modern micrometeorology is the “energybudget”. This may be formed over a surface (suchas a lake or large crop field) or a volume (an individualtree). The key to the energy budget is thepartitioning of the types or forms of energy at asurface. A surface cannot store any of the heat itreceives from net radiation. Therefore, the netall-wave radiation must be partitioned into otherforms of energy, which include storage of energyby the soil (ground heat flux) or body of water,energy used in evaporation or gained fromcondensation (latent heat flux), energy used toheat the air or gained from cooling the air (sensibleheat flux), and the energy associated withbiological processes such as photosynthesis, respiration,and so on. Net radiation is the differencebetween total incoming and outgoing radiationof all wavelengths, and is a measure of the energyavailable at the surface that drives the above processes.Knowledge of the energy budget is alsouseful in devising frost protection methods basedon an alteration of any of the fluxes (ground heat,sensible heat and latent heat). Some excellenttextbooks, for example, Monteith and Unsworth(2007), give mathematical details and applicationsfor these variables.2.3.1.10 Remote-sensing and GISReference should be made to Goel and Norman(1990), Milford (1994) and WMO (2004a, 2008b).Remote-sensing data provide in many ways anenhanced and highly feasible areal supplement tomanual local observations, with a very short timedelay between data collection and transmission.These data can improve information on crop conditionsfor an early warning system. Due to theavailability of new tools, such as GeographicalInformation Systems, management of vast quantitiesof remarkably high-quality data, such astraditional digital maps, databases, models, and soforth, is now possible. GIS refers to tools used in theorganization and management of geographicaldata, and it is a rapid means for combining variousmaps and satellite information sources in modelsthat simulate the interactions of complex naturalsystems. Remote-sensing and GIS in combinationwill continue to revolutionize the inventory, monitoringand measurement of natural resources on aday-to-day basis. Likewise, these technologies areassisting in modelling and understanding biophysicalprocesses at all scales of inquiry (Holden, 2001;WMO, 2004). Chapter 4 of this Guide providesfurther details and examples in this connection.2.3.1.11 Recorders and integratorsReference should be made to Woodward and Sheehy(1983). Recorders and integrators (for totals orperiod averages) are the devices that lie between thesensing of an environmental variable and the finalsite of computations on that variable. A variety oftechniques are available for the use of transducersin this process (between sensors and display orrecording). To convert transducer output into astate suitable for human vision or for recording,translation devices are useful. Electrical translators,amplifiers, and the like, belong to this category. Thetechniques of interfacing between the transducer

2–16GUIDE TO AGRICULTURAL METEOROLOGICAL PRACTICESand the human eye are the most rapidly evolving.Interfacing devices are generally classified into twocategories, that is, analog and digital. Analog devicesprovide a visual representation of transduceroutputs marked on scales. Digital devices provide anumerical readout of the transducer output.A tube is the characteristic link for devices thatmeasure pressure (for example, the manometer,Bourdon tube, and the like). Mechanical linkagesare used between temperature sensors, constructedas bimetal strips or helices, and a display, such as ameter or a chart. Hair hygrometers and barometersalso rely on mechanical linkages. The problems ofelectrical interfaces can be virtually eliminated byreplacing electrical conductors with fibre-opticlinks. Transmission of data from a remote locationto a convenient receiving station is readily achievedby way of a radio or telemetric linkage.Choosing the appropriate set of instruments is acomplex procedure when a considerable range ofinstrumentation is available for displaying andrecording the output from transducers. Smallnumbers of transducers can be efficiently interfacedwith visual display and manual recording. Achart recorder is useful if automatic recording isrequired. Where large numbers of transducers arerequired, automatic recording and display are anecessity. Care should be taken to provide sufficientvisual displays of current measurements toallow check‐ups and control of the observationchain. If all the channels are of similar voltage,with similar response times, a complex data loggeris probably not required; a data acquisition unitmay suffice. When data computation is required,for example for linearization of thermocouplevoltages and conversion of voltages to environmentalunits, then a data logger with programmingsteps contained in a read-only memory may besufficient. Particular metadata for the recording ofsensor signals are signal trans mission data, such ascable length (for signal loss estimation), and amplificationor modification of the signal.2.3.2 Observations of a biologicalnatureReference should be made to Slatyer and McIlroy(1961), WMO (1982), Woodward and Sheehy(1983), Russell et al. (1989), Pearcy et al. (1989),Baker and Bland (1994), and Lowry and Lowry(2001). Biological observations (physical, physiologicaland phenological measurements of canopies,leaves, roots, growth and yields; see Baker andBland, 1994) are needed before relationshipsbetween weather and various aspects of agricultureare explained. Such observations give at least aqualitative, but preferably also a quantitative measureof the response of a plant or animal to weatherconditions. Biological observations assist agrometeorologistsin solving the problems that arise fromthe relations among plants, animals and pests, onthe one hand, and the connections between weatherand the growth and yield of plants and animals, onthe other. It should never be forgotten that suchobservations are made on living organisms with aninherent variability that should be taken intoaccount in sampling methods.As a working method for bioclimatic investigations,phenology should lay down standards forthe observation of those periodic processes thatare of the greatest importance for agriculturalcrops. In the case of annual crops, a wide varietyof bioclimatic factors must be taken into consideration.They include whether the crops arewinter, summer or mid-season crops, how sensitivethey are to low and high air and soiltemperatures, the requirement of growingdegree-days and other heat units in use, irrigation,and other agronomic management.Observations of the start, climax and end of eachphase are carried out: first for those crops thatcover the entire ground (difficult to observe),and second for the crops planted in rows (easy toobserve). In the case of perennial plants, theobservations are carried out on individuals, eachof which, when taken in isolation, represents arepetitive sample. Three to five fruit trees, foresttrees or shrubs of the same age and planted inrepresentative locations in an orchard, wood orplantation are sufficient to give accurate phenologicalaverages.In explaining the effects of the annual weathercycle on the growth and development of livingorganisms, it is necessary to record:(a) Whether the phenological process followsa pattern adjusted to the meteorologicalpattern; only the representative moments ofphases will be observed;(b) Whether the phenological pattern and itsphases are interrupted by weather phenomena;it is now essential to carry out simultaneousobservations of the stage of developmentof all the visible phases of the individualplant.Therefore, each user of biological observationsmust remember their limitations as to generalapplicability; often the best methods for recordingthese observations differ from country to country.As a general principle, it is essential that the

CHAPTER 2. AGRICULTURAL METEOROLOGICAL VARIABLES AND THEIR OBSERVATIONS 2–17accuracy and extent of biological observationsmatch those of the meteorological observationswith which they are to be associated.Biological observations can be conveniently dividedinto six broad categories:(a) Network observations of natural phenomenataken over a large geographical area, dealingwith wild plants, animals, birds and insects;(b) Network observations (similar to (a)) andquantitative measurements on the periodicalgrowth and yields of cultivated plants and farmanimals. They should include observations ondates of certain events in animal and plant life,as well as cultural operations: dates of ploughing,sowing/planting, weeding, spraying, irrigation(including quantities) and harvesting (includingquantities) in the case of plants; calving, milkproduction, and so forth, in animals. Thesedata are required for the objective study of therelationship between environmental factors andagricultural production;(c) Observations of damage to cultivated crops,weeds, animals, and so forth, caused by meteorologicalfactors; occurrence of certain pestsand diseases in plants and animals, their severityand areas in which centres of infectionare situated; damage caused by atmosphericevents, such as hail, drought, frost, stormsand their accompanying phenomena;(d) Detailed observations, of high accuracy orconsiderable complexity, required during aspecific experiment at a research station orexperimental site (see for example Baker andBland, 1994);(e) Network observations of a less complex characterthan in (d) above, taken over a muchgreater geographical area and at a large numberof sites, which are required for operational useor administrative action shortly after they aretaken, that is, for immediate use;(f) Global biological observations for assessingthe areal extent of specific biological events.These observations are reviewed in the followingparagraphs.2.3.2.1 Observations of natural phenomenaThese observations concern weather effects onwild plants and animals that are, for the mostpart, free from deliberate human interference.Because of this relative freedom, these data areregarded as providing a form of integration oflocal climate, and as such may prove suitable foruse as an operational parameter. Wild flowers,trees and shrubs are suitable for these observations,as are also migrating birds and hibernatinganimals.The organization of these observations should besimilar to that for cultivated plants and may oftenbe identical in extent and procedure. Some countriesmake much use of volunteer non-scientificobservers for this work and many countries conductphenological investigations of the types describedhere.2.3.2.2 Observations for agroclimatologicaluseIn this category are phenological observations ofcultivated crops and trees, farm animals, andgeneral activities on the land, all of which arerequired to form an accurate picture of the agriculturalyear. They differ from those in theoperational-use category in that the observationsare made on a wider selection of phenomena at apermanent network of reporting stations. Theobservations are subsequently analysed, publishedor otherwise permanently recorded by a centralauthority, but without any degree of operationalurgency.The network can be less dense than required foroperational purposes but should cover the entirecountry and not be confined to any smaller area ofspecialized agricultural production. The observationscan be simpler in character than thosespecified in some sections of this chapter, but anagreed and fully understood standard of observationis essential. Observations normally consist ofthe recording of measurements of growth andyields and the dates on which certain events takeplace.Each country should select its own standardprogramme of observations, then draw up a standardset of instructions for reporting and recording them,bearing in mind that the items contained in such aprogramme will serve as a basis for introducing anoperational system in the future, as the need arises.The necessity for continuity, reliability anduniformity must be impressed upon the observers,who may be volunteers with little scientific training.Each should be given a recording notebook ofpocket size, which not only has space for theappropriate entries but also contains the necessaryinstructions and illustrations. Notebooks shouldbe retained at the observing site; entries should betranscribed into standard forms for transmissionto the central authority at convenient regularintervals.

2–18GUIDE TO AGRICULTURAL METEOROLOGICAL PRACTICES2.3.2.3 Observations of direct and indirectdamage owing to weather2.3.2.3.1 General weather hazardsWeather hazards that may cause loss or damage tosoils, plants and animals are usually snow, ice, frost,hail, heavy rain, weather conditions leading to highair pollution, unseasonable heat or cold, drought,strong winds, floods, sand- and duststorms, highandlow-level (crop-level) ozone (see WMO, 2008b),and the like. The secondary effects of weather likelyto have adverse effects on agricultural productioninclude forest and grass fires and the incidence ofpests and diseases.In some cases, an adequate observational systemwill have been included, particularly in relationto pests and diseases, forest fires, or the effects ofany regularly occurring hazard, such as snow orfrost. Regular systems for observing weatherdamage can also be incorporated into the categoriesdescribed above in this chapter. Where suchsystems do not exist, however, special arrangementsshould be made to accurately assess theextent of damage.The nature of the observations varies with the typeof hazard and can be selected only by each individualcountry, or by a group of countries with similarclimates. They must, however, be clearly specifiedto eliminate the risk of inaccurate assessments.Furthermore, observation systems must be devisedin anticipation of damage, so that selected observerscan take action immediately after the unusualweather has occurred. It is also important that goodscientific information be available on the causes ofthe damage. A good recent example involves hail(Wieringa and Holleman, 2006).2.3.2.3.2 Greenhouse gasesSimilarly, contemporary agrometeorologists mustunderstand that climate change due to greenhousegases related to agricultural productionindirectly causes damage to the environment andalso to agriculture. Over recent decades, the earthhas become warmer due to the increased presencein the atmosphere of gases such as carbon dioxide(CO 2 ), chlorofluorocarbons (CFCs), methane(CH 4 ) and nitrous oxide (N 2 O). CH 4 and N 2 O arethe gases mainly responsible for global warmingas a consequence of agriculture, while deforestationleads to less absorption of CO 2 from theatmosphere. An increase in these gases in theatmosphere enhances retention of the re-radiatedheat and thus adds to the warming of the earth’ssurface and lower atmosphere. Observations onthese gases and the production-related processesbehind their rates of release are therefore necessaryto understand these processes. Researchresults of agronomists and soil scientists relevantto these problems proved, for example, that thesoil texture has a significant role in the magnitudeof CH 4 emission from rice fields. This isbecause percolating irrigation water removesorganic acids through aeration and high percolationrates in light soils.2.3.2.3.3 Soil erosionSoil erosion and related phenomena are other majorforms of damage caused directly or indirectly byweather. Soil erosion is the process of detachment ofsoil particles from the parent body and transportationof the detached soil particles by wind and water.These particles cause biological damage to crops andfurther problems for water provision; in operationalagrometeorology observations are therefore necessarywherever this is or becomes a major problem(for instance, Mohammed et al., 1995).The detaching agents are falling raindrops, channelflow and wind. The transporting agents areflowing water, rain splash and wind. Dependingon the agents of erosion, it is called water erosionor wind or wave erosion. There are three stages ofsand movement by wind, all of which usuallyoccur simultaneously. The first one is “suspension”(the movement of fine dust particles smallerthan 0.1 mm in diameter by floating in the air).Wind velocities above 3.0 km/h –1 (0.8 ms –1 ) arecapable of lifting silt and very fine sand particlesto heights greater than 3 to 4.5 km. Soil particlescarried in suspension are deposited when thesedimentation force is greater than the forceholding the particles in suspension. Suspensionusually does not account for more than 15 percent of total movement.The second is “saltation”, which is the movementof soil particles by a short series of bounces alongthe ground surface. It is due to the direct pressureof wind on soil particles and their collision withother particles. Particles less than 0.5 mm in diameterare usually moved by saltation. This processmay account for 50–70 per cent of total movement.The third is surface creep, the rolling andsliding of soil particles along the ground surfaceowing to the impact of particles descending andhitting during saltation. Movement of particles bysurface creep causes abrasion of the soil surface,leading to the breakdown of non-erodible soilaggregates due to the impact of moving particles.

CHAPTER 2. AGRICULTURAL METEOROLOGICAL VARIABLES AND THEIR OBSERVATIONS 2–19Surface creep moves coarse particles larger than0.5–2.0 mm in diameter. This process may accountfor 5–25 per cent of the total movement.Measurements of dust, saltating and creeping sand,as well as related soil and crop hazards and defencemechanisms, are essential in operational agrometeorologyof affected areas (for instance,Mohammed et al., 1996; Sterk, 1997).Soil losses by sheet and rill water erosions are mostcritical in sub-humid and humid areas, whereaswind erosion exacts a higher toll in semi-arid andarid areas. For both types of soil erosion, the maintenanceof soil cover at or near the soil surfaceoffers the most effective means of controlling soiland water loss and can be easily quantified (Kinamaet al., 2007). Conservation tillage systems areundoubtedly among the most significant soil andwater conservation practices developed in moderntimes but are, in some places, traditional farmingpractices (Reijntjes et al., 1992). Quantification oftheir impacts should be improved.2.3.2.3.4 Water runoff and soil lossThe portion of precipitation that is not absorbed bythe soil but finds its way into streams after meetingthe persisting demands of evaporation, interceptionand other losses is termed runoff. In some humidregions, the loss may be as high as 50–60 per cent ofthe annual precipitation. In arid sections, it is usuallylower unless the rainfall is of the torrential type.Although the loss of water itself is a negative factor,the soil erosion that accompanies it is usually moreserious. The surface soil is gradually taken away andthis means a loss not only of the natural fertility, butalso of the nutrients that have been artificially added.Also, it is the finer portion of this soil that is alwaysremoved first, and this fraction, as already emphasized,is highest in fertility. A recent example inoperational agrometeorology of measuring soil lossand water runoff from sloping agricultural land isgiven by Kinama et al. (2007).2.3.2.4 Detailed biological observationsAs in the case of physical observations mentionedabove, detailed accurate biological observations areneeded for fundamental research. Such observationsare made by scientifically trained personnel toensure great accuracy. The WMO TechnicalRegulations list the types of biological observationsthat may be required. It must be stressed, however,that these observations require high precision sincethey have been especially selected for researchpurposes. Because observations of this kind areneither routine nor permanent, it is impossible torecommend general methods suitable for allpurposes. This work may be carried out either undernatural conditions in the field or in a laboratoryenvironment, which may often involve the use ofclimate-control chambers, wind tunnels,microscopes and other experimental tools to studythe reactions of both plants and animals to singleor complex meteorological factors.It is always important to measure both the physicaland physiological responses of living organisms,such as the carbon dioxide intake, osmotic pressure,chemical constitution, leaf area, dry matter index,and growth rate in plants; and the basal metabolism,pulse and respiration rate, rectal temperature, bloodvolume and composition, blood pressure, compositionof food, and so on, in animals.Care must be taken in control-chamber experimentsbecause the climate simulation often doesnot represent open-field or natural conditionsquantitatively or qualitatively. In order to achievereliable results, it may be necessary to select a soundstatistical experimental design under natural conditions.In this kind of research, teamwork is highlyrecommended.2.3.2.5 Observations for operational useIn general, the mean data provide a strong basis forcomparison with current data, since departure fromnormal provides the most useful information foroperational use. These are observations needed byregional or central authorities to assist them intaking administrative action, making functionalforecasts or giving technical advice. Although theymust be standard in nature, so that observationsfrom different sources can be compared, such yardsticksas 30-year averages start to become somewhatmeaningless in the light of a rapidly changingclimate. Other approaches need to be developedthat take time trends and increasing variabilitysimultaneously into account.Such observations will be needed from a largenumber of sites that form a national network, andwill be made by skilled or semi-skilled observerswho have received adequate training to meet thedesired observational standards. Arrangementsmust be made to communicate these observationsas quickly as possible to the regional or centralauthorities. Postal services may be adequate butmore rapid means such as the Internet, fax, radio,and the like may often be needed.The density of the network may be limited by theavailability of efficient observing staff. All areas of

2–20GUIDE TO AGRICULTURAL METEOROLOGICAL PRACTICESthe country concerned with one type of operationalrequirement should be adequately covered. Ideally,the network density will depend on the type ofproblem, the crop types and distributions, the soilvariations, the climate, and the population densityin the region, which determine the general andrepetitive sampling rates.The authorities must strictly specify the exact natureof the biological observations in an agreed pattern,preferably accompanied by good illustrations.Observations on yield, which may concern smallexperimental plots or regional or national areas ofproduction, fall under this heading. In planningthese observations, the agrometeorologist shouldcollaborate with statisticians and agricultural experts.Wherever necessary, he/she should encourage theagricultural authorities to obtain the data in a formsuitable for establishing weather–yield relationships.For regional or national yields, he/she should payattention to the accuracy of the yieldmeasurements.The meteorological and biological data are analysedsimultaneously by regional and central authorities,which take operational decisions after proper analyses.A summary of the season’s work should beprepared and either published or permanentlyretained for reference, so that the experience ofeach year is always available for subsequentconsideration.Some examples of information required for specificoperational use are:(a) Forest fires: the state of the forest litterand its susceptibility to burning (see Chapter 8);(b) Diseases: the state of the plant, the presence andrelease of spores, the incidence and spread ofinfection;(c) Pests: the hatching of harmful insects, thebuild-up of insect populations, or their invasionfrom other territories;(d) Weather hazards: the state of crops andwhether they are at a stage particularly susceptibleto weather hazards; animals under stressdue to unseasonal climate or other severeweather conditions;(e) Farming operations: the progress made inthe farming year, in order to make weatherforecasters aware of the operational implicationsof forthcoming weather.2.3.2.6 Global biological observationsBesides the local observations described above, thereare now modern methods for globally evaluating thedistributions of biological phenomena, such as:(a) Aerophotogrammetry (conventional photography).This is for the mapping of reliefand for determining the types of naturalvegetation and crops, their phenologicalstate, the soil type, cattle distribution, andso on. The altitude of the aircraft duringobservation flights must correspond tothe desired photographic resolution of thephenomenon under study (through use ofmultispectral photography). Although thistype of photography is diminishing in thedeveloped world, in the developing world itcould be still important if enough funds andhardware are available.(b) Aerial photography (particular wavelengths).Remote-sensing with special film, sensitizedto a region of the visible spectrum, or toinfrared radiation, gives valuable informationon albedo, intensity and ground emissionactive in the energy balance. Scannershave also become available. Informationcan be obtained on soil moisture deficit,drought stress in vegetation, composition ofthe plant community and its phenologicalcondition, and the state of crops and cattle.(c) Satellite observations. Satellite imagesare useful, especially for extended areas(Chapter 4). Estimates of rainfall and of vegetationindices are routinely available, althoughtheir accuracy varies widely depending onlatitude, observing system, and the like.2.4 INSTRUMENTS USED ATAGRICULTURAL METEOROLOGICALSTATIONSMost of the instruments included as basic equipmentat an agrometeorological station are describedin WMO (2008b). Short descriptions of some agrometeorologicalinstruments generally used forspecific applications are given below. There is a clearneed for frequent recalibration of all instruments.2.4.1 Measurement of the physicalenvironment2.4.1.1 Radiation and sunshineReference may again be made to Coulson (1975),Fritschen and Gay (1979), Iqbal (1983), Goel andNorman (1990), Strangeways (2003), WMO (1984,2001b, 2008b) and Technical Note No. 172. Somebasic remarks on mounting instrumentation havebeen given in 2.3.1.1. Global solar radiation (directand diffuse solar radiation) is measured with

CHAPTER 2. AGRICULTURAL METEOROLOGICAL VARIABLES AND THEIR OBSERVATIONS 2–21pyranometers containing thermocouple junctionsin series as sensors. The sensors are coated black tohave uniform thermal response at all spectralwavelengths. With filters, non-PAR radiation can bemeasured, and the difference between solar imeteroutputs with and without filters gives PAR data.Stigter and Musabilha (1982) did this for the firsttime elaborately in the tropics. Solid state sensors(photoelectric solar cells, photoemissive elements,photoresistors, and so on) may be used whereradiation can be assumed to have constant spectraldistribution (for example, solar radiation withinlimits). Different types of photometers andultraviolet illuminometers, which are adaptationsof these instruments, are used in agrometeorologicalresearch.Light, which is indispensable for photosynthesis, isone of the major components of short-wave radiation.What is measured with a lux meter is not lightintensity, but luminance, which is defined as luminousflux density intercepted per unit area.Quantum sensors that measure the PAR directly inthe range between 0.4 and 0.7 micrometers areavailable. Ideally, crop profile measurements withquantum sensors should be taken on perfectly clearor uniformly overcast days. If this is not possible,however, the problem is partially overcome byexpressing the values at each level relative to theincident radiation. These profiles are comparedwith leaf-area profiles when the light requirementsof crops are being studied.Tube radiometers for use in crops and agroforestryare inherently less accurate than instrumentswith a hemispherical dome, but can be of greatuse in estimating the average radiation below acrop canopy or mulch relative to the radiationabove it. When mounted north to south, thesensitivity varies with the angle of the solar beamto the axis, particularly in the tropics (Mungai etal., 1997). This adds to errors that are the result ofhigh ambient temperatures under low windspeeds, as well as condensation inside the tubes.Calibrations as a function of time and ambientconditions can largely cope with such errors, butfiltered tubes for photosynthetically active radiationappeared unreliable in the tropics (Mungaiet al., 1997). To measure the fractional transmissionof solar radiation through a crop canopy, anumber of tubes are placed beneath the canopy.Their numbers and arrangement depend on theuniformity of the crop stands (Mungai et al.,2000). A reference measure of incident solar radiationabove the canopy is needed. For cropstudies, the output for each tube is usually integratedover periods of a day or longer during thegrowing season. Integrators or loggers are idealfor this purpose. The values of fractional interceptionare subsequently calculated from theintegrals (for example, Mungai et al., 2000).Surface temperature radiometers are used for measurementsof infrared radiation emitted from near orremote surfaces. They are mainly used as hand-heldremote sensors to measure temperatures of radiatingirregular surfaces such as soil, plant cover andanimal skin, and require knowledge of the emissivitycoefficient of the observed surface (WMO,2001b). Operational precautions are given by Stigteret al. (1982).Pyrgeometers are used for the measurement of longwaveradiation from the sky (when facing upward)or from the earth (facing downward).Net all-wave radiometers (measuring net flux ofdownward and upward total radiation, namely,solar, terrestrial and atmospheric radiation) containblack-coated heat-flux plate sensors, in whichthermo couples are embedded to measure thetemperature difference between the two sides of athin uniform plate with well-known thermal properties.Errors due to convection and platetemperature are avoided by using forced ventilation,appropriate shields, and built-in temperaturecompensation circuits. Net radiometers, net pyrradiometers,net exchange radiometers or balancemeters may have a standard diameter (about 6 cm)for regular use or a miniature diameter (about 1 cm)for special work on radiation exchange from plantorgans or small animals.Standard meteorological stations usually measureonly sunshine duration. The traditional instrumentto observe this is the Campbell–Stokes sunshinemeter. WMO abolished the world standard status ofthis sunshine meter in 1989, as the process of evaluatingthe burns on its daily cards was bothcumbersome and arbitrary. Instead, sunshine durationhas been defined as the time during whichdirect radiation (on a plane perpendicular to thesun’s beam) is greater than 120 Wm –2 . This definitionmakes it possible now to use automaticsunshine recorders (for instance, WMO, 2001b,2008b).Particular metadata of radiation measurementsinclude the wavelength trans mis sion spectralwindow of a pyranometer dome, the sunshinerecorder threshold radiation value, horizonmapping for each instrument measuring radiationor sunshine, and procedures or means to keep radiometerdomes clean and clear.

2–22GUIDE TO AGRICULTURAL METEOROLOGICAL PRACTICES2.4.1.2 Air temperatureReference should again be made to Fritschen andGay (1979), Goel and Norman (1990), Strangeways(2003) and WMO (1984, 2001b, 2008b). WMOTechnical Note No. 315 is also useful. General issueswere discussed in 2.3.1.2. Besides the standardinstruments, several others are used in agrometeorologicalsurveys and research.Small and simple radiation screens, some of whichare aspirated when this does not destroy temperatureprofiles, are useful for special fieldwork. Highoutside reflectivity, low heat conductivity, highinside absorption and good ventilation are desirablerequirements in the construction materials anddesign. An idea of the radiation errors can, forexample, be determined by simultaneous, replicatedobservations with the ventilated Assmannpsychrometer at the hours of maximum and minimumtemperature.The most common thermometers for standardobservations in air are those generally calleddifferential expansion thermometers, which includeliquid-in-glass, liquid-in-metal and bimetallicsensors. Because of their sizes and characteristics,many of these instruments are of limited use forother than conventional observations. Spiritin-glass,mercury-in-glass, and bimetallic sensors,however, make useful maximum and minimumtemperature measurements. When temperatureobservations are required in undisturbed and ratherlimited spaces, the most suitable sensors areelectrical and electronic thermometers, whichpermit remote readings to be made.Resistance thermometers are metallic annealedelements, generally of nickel or platinum, whoseelectrical resistance increases with temperature;readings are made with appropriately scaled meters,such as power bridges.Thermocouples are convenient temperature sensorsbecause they are inexpensive and easy to make.Those most frequently used in the environmentaltemperature range are copper–constantan thermocouples,which have a thermal electromotive forceresponse of about 40 µV°C –1 . This relatively weakresponse can be increased by connecting severalthermocouples in series or using stable solid state,direct current amplifiers. Thermocouples are excellentfor measuring temperature differences betweenthe two junctions, for instance, dry and wet bulbtemperatures, or gradients. When they are used tomeasure single temperatures or spatial averagetemperatures (such as surface temperatures, usingthermocouples in parallel), one junction alwaysneeds to be at a known steady referencetemperature.Thermistors are temperature sensors that are seeingincreasing use in agricultural and animal micrometeorology.They are solid semiconductors withlarge temperature coefficients and are produced invarious small shapes, such as beads, rods and flakes.Their small size, high sensitivity and rapid responseare valuable characteristics, which are offset,however, by their lack of linear response (less thanmetallic resistances) in the resistance–temperaturerelationship. Additional components are thereforerequired to achieve linear output.Diodes and transistors with a constant currentsupply that provide outputs much higher than1 mV°C –1 have been used to construct sensitive andaccurate thermometers for application in plantenvironments.Infrared thermometers were discussed in 2.4.1.1. Ablack globe thermometer is a blackened coppersphere commonly 15 cm in diameter, with a thermometeror thermocouple inserted. When a blackglobe thermometer is exposed in the open or undera ventilated shelter, the effects of different radiationfluxes are integrated with convective heat(wind and air temperature) effects. Installed insideclosed barns or stables, under still air conditions,this type of thermometer gives the average radianttemperature of soil, roof and walls at equilibrium.Particular metadata for temperature measurementare the height of the sensor and a description of thescreens employed (dimensions, material andventilation).2.4.1.3 Temperature of soil and other bodies2.4.1.3.1 SoilReference should again be made to Rosenberg et al.(1983) and WMO (1984, 2001b, 2008b). All sensorsmentioned in 2.4.1.2 may be used, although thethermocouple must be of a sturdy construction,provided that presence of the sensor does not affectthe temperature being measured. Soil thermometersof the mercury-in-glass type are frequently used.For measurements of the soil temperature at shallowdepths, these thermometers are bent at anglesbetween 60° and 120° for convenience. At greaterdepths lagged thermometers are lowered into tubes.Care should be taken to prevent water from enteringthe tubes. Alternatively, shielded thermocouplesor thermistors can be used. The temperature of

CHAPTER 2. AGRICULTURAL METEOROLOGICAL VARIABLES AND THEIR OBSERVATIONS 2–23deeper soil layers can be measured with glass thermometers,thermistors, thermocouples, diodes andplatinum resistance thermometers when goodcontact is made with the soil.In cold and temperate climates where the soil isoften deeply frozen and covered with snow, whencontinuous soil temperature records are not availableor when many observing points are needed,different types of snow cover and soil frost depthgauges can be used. These instruments generallyconsist of a water-filled transparent tube, encasedin a plastic cylinder that is fixed in the soil. Thetube is periodically removed from its plastic casingto determine the depth to which the entrappedwater is frozen. If the fixed cylinder extends sufficientlyfar above the soil surface, it can be used as asnow cover scale, provided that the exposed part isgraduated.For measuring the soil surface temperature, noncontactinfrared thermometers are preferable, as longas emissivity is known and again, the presence of thesensor does not affect the temperature being measuredby shading or otherwise influencing the naturalradiation balance (Stigter et al., 1982).Particular metadata for soil temperature profilemeasurements are instrument depths and regularspecifications of the actual state of thesurface.2.4.1.3.2 Other bodiesLike the soil, plant parts such as leaves, stems, rootsand fruits have mass and heat capacity. The temperatureof all these organs can be measured withplatinum resistance thermometers, thermistors,thermocouples, infrared thermometers, diodes, andso forth, if the instruments do not influence theenergy balance of those bodies. To measure theirsurface temperatures and those at the outsidesurface of animals, one should use small contactsensors such as thermocouples and thermistors, ornon-contact methods.In animal micrometeorology special and relativelysimple instruments have been used to simulate thecooling power of the air or the heat load over thehomeothermic animal body. Kata thermometersare spirit-in-glass thermometers with a rather largebulb of accurately determined area. They are usedto measure the time required for a fixed amount ofcooling to occur after the thermometer has beenwarmed to a point above body temperature. Such areading is an index that integrates the cooling effectof temperature and wind.The heated-globe anemometer, which provides areasonable value of the cooling power of airmotions in climatic chambers and other indoorenvironments, is a practical thermo-anemometer.It is constructed with a chrome-plated sphere thatis 15 cm in diameter and heated by a nichromewire that can receive a variable power input.Several thermocouples in parallel with one junctionfixed internally to the globe wall measure thetemperature of the globe wall. The voltage of theheater is regulated to give a differential air–globetemperature of 15°C. The power needed to maintaina steady temperature is a function of theventilation. A correction factor for thermal radiationof walls, ground and roof may be required,however, if these are significantly hotter than theair.2.4.1.4 Atmospheric pressureReference should be made to WMO (2008b).Analysed pressure fields are useful in agriculturalmeteorology. These pressure fields must be accuratelydefined because all the subsequent predictionsof the state of the atmosphere depend to a greatextent on these fields. In mercury barometers thepressure of the atmosphere is balanced against theweight of the column of mercury, whose length ismeasured using a scale graduated in units of pressure.Of the several types of mercury barometers,fixed cistern and Fortin barometers are the mostcommon. For the purpose of comparison, pressurereadings may need to be corrected for ambient airtemperature.In electronic barometers, transducers transform thesensor response into a pressure-related electricalquantity in the form of either analog or digitalsignals. Aneroid displacement transducers, digitalpiezoresistive barometers and cylindrical resonatorbarometers fall into this category. Calibration driftis one of the key sources of error with electronicbarometers. Therefore, the ongoing cost of calibrationmust be taken into consideration whenplanning to replace mercury rometers with electronicones.The advantage of aneroid barometers over conventionalmercury barometers is that they are compactand portable. Another important pressure measuringdevice is the Bourdon tube barometer. It consistsof a sensor element (aneroid capsule), whichchanges its shape under the influence of pressure,and transducers, which transform the change into aform directly usable by the observer, such as on abarograph. The display may be remote from thesensor.

2–24GUIDE TO AGRICULTURAL METEOROLOGICAL PRACTICES2.4.1.5 WindReference should again be made to Mazzarella(1972), Wieringa (1980), Kaimal and Finnigan(1994) and WMO (1984, 1998, 2001b, 2008b). Windspeed and direction measured with standardinstruments under standard exposure arefundamental requirements of the science ofagricultural meteorology. The most commonroutine observation is the wind run, providing anaverage over the measuring period. That periodshould be at least ten minutes for smoothing outtypical gustiness, and at most an hour becausesurface wind has a very pronounced diurnal course.Different instruments are used when it is necessaryto observe the more detailed structure of air motion,however, for instance, in agricultural meso- andmicrometeorological studies. In such cases, windspeeds are measured with cup anemometers of highsensitivity at low velocities or with electricalthermo-anemometers or sonic anemometers.Sensitive cup anemometers that measure all windcomponents and have a horizontal angle of attackof less than about 45° are the most common inroutine and research use. The best have a low stallingspeed (threshold of wind speed below whichthe anemometer does not rotate) of about 0.1 ms –1 ,because friction loads have been minimized. Therotation produces an electrical or phototransistorsignal, which is registered by a recorder or counter.Such transducers also allow separate recording ofgustiness.Sensitive propellers, if mounted on a vane, can bean alternative to cups (WMO, 2008b), but thesedays they are mainly used in research instruments(WMO, 2001b). Pressure tube anemometers on avane are reliable, but so unwieldy that they aredisappearing in favour of smaller instruments. Anew instrument for horizontal wind speed anddirection measu re ment is the hot-disk anemometer,which has the advantage that it has no movingparts. For steady wind direction measurement, windvanes must have fins whose height exceeds theirlength.Sonic anemometers, which sense the transportspeed of sound pulses in opposite directions alonga line and are thus totally linear, respond quicklyenough to measure turbulence and have becomeuseful for flux measurements in research. Theycannot be used in small spaces, however, and theircalibration shifts in wet weather.For the study of wind speeds in restricted spaces,such as crop canopies and surfaces, several kinds ofthermo-anemometers are used. The hot-wireanemometer is an electrically heated wire, whoseheat loss is a function of the airspeed at normalincidence to the wire. It is particularly useful forlow-speed winds but very fragile, and in pollutedsurroundings it loses its calibration so it cannot beused opera tionally. Because of the dependence ofwire heat transfer on wind direction, crossed-wiresensors can be used to separate the wind componentsin turbulent motion.Hot-bead anemometers have heated beads, whoseheat transfer is less dependent on wind direction buthas a slower response. Thermocouples or thermistorssense differences in temperature between heated andnon-heated beads; these differences are a function ofthe wind speed. Shaded Piche evaporimeters havealso been used as cheap interpolating and extrapolatingancillary anemometers in agroforestry whenturbulence is not too high and the temperature andhumidity gradients are low (Kainkwa and Stigter,2000; Stigter et al., 2000).Particular metadata for wind measurement areresponse times of instruments; sensor height; exposure,that is, adequate description of surroundingterrain and obstacles; type of anemometer signal,its transmission and its recording; sampling andaveraging procedure; and unit specification (m/s,knots, km/h, or some type of miles per hour).2.4.1.6 Air humidity and soil moisture(including leaf wetness)2.4.1.6.1 HumidityReference should again be made to Griffiths (1994),WMO (1984, 2001b, 2008b) and WMO TechnicalNote No. 21. The most commonly used hairhygrometers and hair hygrographs may giveacceptable values only if great care is taken in theiruse and maintenance. The accuracy of otherequipment has improved. Besides standardpsychrometers equipped with mercury-in-glassthermometers, portable aspirated and shieldedpsychrometers and mechanical hygrometers, manyinstruments have been developed to measuredifferent aspects of air humidity. Since the abovementionedroutine instruments are bulky andinadequate for remote reading, they are unsuitablefor many agrometeorological observations. Forobservations in undisturbed and small spaces,electrical or electronic instruments are used. Thebest method for measuring humidity distributionin the layers near the ground is also to use thermoelectricequipment, and unventilated thermocouplepsychrometers are the most suitable in vegetation

CHAPTER 2. AGRICULTURAL METEOROLOGICAL VARIABLES AND THEIR OBSERVATIONS 2–25(Rosenberg et al., 1983; WMO, 2001b). Ventilatedpsychrometers may be used for levels at least 50 cmabove bare soil or dense vegetation.For measuring relative humidity directly, use hasbeen made of lithium chloride or sulphonated polystyrenelayers, since the electrical resistance of theseelectrolytes changes with relative humidity. Theseelectrolytic sensors become affected by air contaminationand high relative humidity conditions,however, and are therefore to be used with greatcare and frequent recalibration. For example, resistivepolymer film humidity sensors are increasinglyused. Instruments are usually resistant to contaminants,and common solvents, dirt, oil and otherpollutants do not affect the stability or accuracy ofthe sensor.Electrical dewpoint hygrometers indicate dewpointrather than relative humidity. For example, thelithium chloride dewpoint hygrometer measuresthe equilibrium temperature of a heated soft fibreglasswick impregnated with a saturated solution oflithium chloride. This temperature is linearly relatedto atmospheric dewpoint. The response of theinstrument under low relative humidity conditionsis not so good, however.More expensive and complicated, but also more accurate,instruments require that the air be sampled anddelivered, without changing its water vapour content,to a measuring unit. One such instrument, an illuminatedcondensation mirror, is alternately cooled andheated by a circuit energized by a photocell relay,which maintains the mirror at dewpoint temperature.Infrared gas analyser hygrometers (IRGAs) rely on thefact that water vapour absorbs energy at certain wavelengthsand not others. Two sampling tubes are alsoused to measure absolute values of water vapourconcentration at two levels, while at the same timemeasuring the differences in these values.Single- or double-junction Peltier psychrometersare extensively used for accurate measurement ofwater potential values in plant tissues and soilsamples. They are generally based on the Peltiereffect in chromel–constantan junctions, and thewater potentials are derived from measurements ofequilibrium relative humidity in representative air.Particular metadata for any type of hygrometry areregular notes in the station logbook of maintenanceactivities, such as psychrometer wick replacementor cleaning of sensor surfaces. Moreover, whetheror not sensors are ventilated should be recorded.Because so many different humidity parameters arein use, the metadata should specify not only theparameters and units actually used, but they shouldalso contain information on the way in which thearchived humidity data were calculated from originalobservations (for example, in the form ofconversion tables, graphs and small conversionprogrammes).2.4.1.6.2 Soil and grain moistureReference may be made to Greacen (1981), Gardner(1986), Vining and Sharma (1994), Dirksen (1999),Smith and Mullins (2001), and WMO (2001b,2008b). WMO Technical Note No. 97 also describesinstruments used for the measurement of soil moisture.Time and space variation of soil moisturestorage is the most important component of thewater balance for agrometeorology. Several instrumentshave been constructed to measure soilmoisture variations at a single point, but they avoidthe variability of soils in space and depth (for example,Ibrahim et al., 1999, 2002). Gardner (1986) stilldescribed the following as a relevant indirectmethod of obtaining soil water content: “measurementof a property of some object placed in thesoil, usually a porous absorber, which comes towater equilibrium with the soil”. Blotting paper ispopular here and it may also be useful for soilpotential determinations.Subjective methods of estimating soil moisturehave been used with satisfactory results in someregions where regular observations in a densenetwork are necessary and suitable instruments arelacking. Skilled observers, trained to appreciate theplasticity of soil samples with any simple equipment,form the only requirement for this method.Periodic observations and simultaneous determinationsof soil texture at depths, by competenttechnicians, allow approximate charts to beconstructed.The direct methods of soil water measurement facilitateimplementation of easy follow-up methods atoperational levels. Gravimetric observations of soilwater content have been in use for a long time inmany countries. An auger to obtain a soil sample, ascale for weighing it, and an oven for drying it at100°C–105°C are used for the purpose. Comparisonof weights before and after drying permits evaluationof moisture content, which is expressed as apercentage of dry soil or, where possible, by volume(in mm) per metre depth of soil sample. Because oflarge sampling errors and high soil variability, theuse of three or more replicates for each observationaldepth is recommended (see also WMOTechnical Note No. 21). The volumetric method isuseful for measuring the absolute amount of water

2–26GUIDE TO AGRICULTURAL METEOROLOGICAL PRACTICESin a given soil and it has known volumes of soilsampled.Tensiometers measure soil moisture tension, whichis a useful agricultural quantity, especially for lightand irrigated soils. The instrument consists of aporous cup (usually ceramic or sintered glass) filledwith water, buried in the soil and attached to a pressuregauge (for instance, a mercury manometer).The water in the cup is absorbed by the soil throughits pores until the pressure deficiency in the instrumentis equal to the suction pressure exerted by thesurrounding soil. Along with this direct measurement,an indirect measurement of soil moisturetension can be obtained from electrical resistanceblocks.Electrical resistance blocks of porous materials (suchas gypsum) whose electrical resistance changeswhen moistened, without alteration of the chemicalcomposition, can be calibrated as a simplemeasure of soil moisture content. This was operationallyused successfully by Mungai et al. (2000),for example.Among radioactive methods, the neutron probemeasures the degree to which high-energy neutronsare thermalized in the soil by the hydrogen atomsin the water. It determines volumetric water contentindirectly in situ at specific soil depths using apredesigned network of access tubes (Ibrahim et al.,1999). The neutron scattering and slowing methodwas until recently the most widely used, and it isrelatively safe and simple to operate. The totalneutron count per unit time is proportional to themoisture content of a sphere of soil whose diameteris larger when the soil is drier. Soil moisture is measuredwith the gamma radiation probe by evaluatingdifferential attenuation of gamma rays as they passthrough dry and natural soils. This method generallyrequires two probes introduced simultaneouslyinto the soil a fixed distance apart, one carrying thegamma source and the other the receiver unit.Time domain reflectometry determines the soilwater content by measuring the dielectric constantof the soil, which is a function of the volumetricwater content. It is obtained by measuring thepropagation speed of alternating current pulses ofvery high frequency (>300 MHz). The pulses arereflected at inhomogeneities, either in the soil or atthe probe–soil interface, and the travel time betweenthe reflections is measured. The dielectric constantis determined on the basis of the travel time andthis allows for determination of the volumetricwater content of the soil. As with neutron scattering,this method can be used over a large range ofwater contents in the soil. It can be used directlywithin the soil or in access tubes. Compared to theneutron scattering method, the spatial resolution isbetter, calibration requirements are less severe andthe cost is lower (WMO, 2001b).Another important measurement needed in agricultureis the moisture content of grains, whichinfluences viability and general appearance of theseed before and after storage. It is important toknow the moisture content immediately afterharvest, prior to storage and shipment, after longperiods of storage, and so on. The methods formeasuring moisture content are generally classifiedas reference methods, routine methods andpractical methods. The phosphorous pentoxidemethod (in which moisture is absorbed by thechemical) and the Karl Fisher method (in whichwater is extracted from seed using a reagent) areconsidered reference methods. The “oven-drymethod” is categorized as a routine method inwhich the seed moisture is determined by removingthe moisture from the seeds in an oven. Amongthe practical methods, the determination of moisturecontent by using samples in infrared moisturemeters is easy compared to others. WMO TechnicalNote No. 101 deals with some of the above, butalso with practical methods using electrical resistancesensors. Abdalla et al. (2001) successfullyused the latter.2.4.1.6.3 Leaf wetness and dewReference should be made to WMO (1992, 2001b).The very large number of instruments that havebeen developed for the measurement of dew orduration of leaf wetness (WMO Technical NoteNo. 55) indicates that not even a moderately reliablemethod has yet been found. The two maincategories of leaf wetness duration (LWD) sensorsbeing used are mechanical sensors with recorders,and electric sensors that exploit the conductivityvariation as a function of wetness.In addition to electric conductivity measurementsof dew (variations on both natural and artificialsurfaces), the principles of mechanical dew measurementare: modification of the length of thesensor as a function of wetness; deformation of thesensor; water weighing (dew balance recorder); andadsorption on blotting paper, with or withoutchemical signalling. There is also visual judgementof drop size on prepared wooden surfaces (theDuvdevani dew gauge).Porcelain plates (Leick plates), pieces of cloth andother artificial objects can share in any dew fall or

CHAPTER 2. AGRICULTURAL METEOROLOGICAL VARIABLES AND THEIR OBSERVATIONS 2–27distillation occurring on a given natural surface.Unless they are more or less flush with that surfaceand have similar physical properties (surfacestructure, heat capacity, shape, dimension, flexibility,colour and interception) they will notindicate reliably the amount of dew that thesurface receives. If exposed above the generallevel of their surroundings, as is normal withDuvdevani blocks and usually appears to be thecase with more refined “drosometer” devices,their behaviour will diverge from that of thesurface below, and the observed amounts of dewmay bear little relation to the dew on adjacentnatural surfaces.Weighing-type instruments, modified hygrographswith a hemp thread instead of a hair bundle, andsystems with surface electrodes that connect whenthe surface is wet, all have their problems (WMO,2001b). The surface electrode instruments are thesimplest to read, but again do not measure real leafwetness, because the sensor is a fake leaf, with, interalia, a different heat capacity.2.4.1.7 Precipitation (clouds andhydrometeors)Reference should be made to MeteorologicalOffice (1981) and WMO (1994b, 2008b). WMOTechnical Note Nos. 21, 83 and 97 also provideinformation and guidance concerning instrumentssuch as raingauges and totalizers, rainrecorders (float and tipping bucket types) andsnow gauges. Many of these require lower accuracyin agrometeorology than when they are usedfor standard climatological measurements. Forsome purposes no great precision in rainfall isneeded, for example in classifying days as either“wet” or “dry” for insurance claims or when onlyrough ideas are needed concerning accumulationof rainfall over agricultural fields throughout anongoing season for comparison with the sameperiod in earlier years, which is a topic of interestto most farmers. The same applies to (agricultural)environmental science teaching in schools.In Mali, the National Meteorological Directorateis of the opinion that farmers need to have ameans of measuring rainfall if they wish to derivethe full benefit of the agrometeorological informationdisseminated by rural radio, and farmerraingauges are now locally manufactured (Rijks,2003).A few additional remarks are appropriate here on anumber of instruments used for specific work andon their operation. With regard to hail measurement,observations cannot be automated, becausethe only useful observation method so far is the useof a network of hail pads. As for rainfall measurement,it should be noted that wind can have animpact, along with the height and shape of theraingauge, which are by far the most importantfactors determining errors.When cost is important, along with the need forhigh measuring densities, raingauges smaller in sizethan the normal standard are employed, but theyare unsuitable for snow. Sometimes these are madeof plastic and shaped like a wedge, other times theyare just plastic receptacles. Commercially the formerare often called “raingauges according to Diem” or“farmer raingauges”; the latter, if made of plastic,are known as “clear view raingauges”. Inexpensiveraingauges and small-size totalizer raingauges areused for studying the small-scale distribution ofprecipitation, as seen with limited mesoclimates,forest or crop interception, shelterbelt effects, andso on.In addition to the performance of routine rainfallmeasurements, agricultural practices call fordata on the amount, duration and intensity ofprecipitation at the time of floods and relateddisasters. As the severe weather systems affectingcoastal areas originate in seas and oceans,ocean-based data collection through ships andbuoys is necessary. Also, the installation of automaticweather stations that meet the necessarycriteria can help with monitoring and providingearly warning to coastal zones about hazardousweather. In vulnerable coastal zones a densenetwork of stations is needed to diagnose weather‐relatedhazards and plan measures aimed atmitigating their effects.Radar, sometimes in parallel with satellite remotesensing,is increasingly used to estimate both pointand area rainfall by analysing the characteristics ofcloud structure and water content. These datacomplement the surface raingauge networks inmonitoring and mapping rainfall distribution, butit is essential that representative actual observationsat the surface be used when taking decisions on thetrack of a storm for forecasting purposes. Suchderived rainfall data need ongoing intensitycalibration.Particular metadata for precipitation measurementinclude the diameter of the raingauge rim and itsheight above ground; the presence of a Nipherscreen or some other airflow modification feature;the presence of overflow storage; and a means, ifany, to deal with solid precipitation (such as heatingor a snow cross).

2–28GUIDE TO AGRICULTURAL METEOROLOGICAL PRACTICES2.4.1.8 Evaporation and water balanceThe standard instruments that are used for measuringthe different components of the water balancefor climatological and hydrological purposes (suchas screened and open pan evaporimeters, or lysimeters)are also employed in agricultural meteorology.Reference is made to the same literature as for2.4.1.7 and to WMO (1984, 2001b).2.4.1.8.1 EvaporationWhile it is possible to estimate actual or potentialevapotranspiration from observed values of screenor open pan evaporimeters or from integrated sets ofmeteorological observations, more accurate, directobservations are often preferred. Actual evapotranspirationis measured by using soil evaporimeters orlysimeters, which are field tanks of varying types anddimensions, containing natural soil and a vegetationcover (grass, crops or small shrubs). Potentialevapotranspiration (PET) can be measured by lysimeterscontaining soil at field capacity and a growingplant cover. A surface at almost permanent fieldcapacity is obtained by regular irrigation or by maintaininga stable water table close to the soil surface.With lysimeters, strict control must be kept of infiltrationfrom excess rainfall. For the observation bylysimeters to be reliable, the conditions at the surfaceof the instrument and below it need to be very similarto the conditions of the surrounding soil.Among the different lysimeters, the most importantfor agricultural applications are the Thornthwaitelysimeters (of the drainage type), Popoff lysimeters(a combined drainage and weigh ing type), weighinglysimeters and hydraulic lysimeters (a more robustweighing type). Lysimeters are used to measureevaporation, transpiration, evapotranspiration (ET),effective rainfall, drainage, and chemical contentsof drainage water, and to study the climatic effectsof ET on the performance of crops. Lysimetry is oneof the most practical and accurate methods forshort-term ET measurements, but a number offactors cause a lysimeter to deviate from reality,such as changes in the hydrological boundaries,disturbance of soil during construction, conductionof heat by lateral walls, and so forth.Atmometers or “small-surface” evaporimeters arealso still in use. Of these, the inexpensive Picheevaporimeter can be utilized anywhere inmeteorology and agriculture if the physics are wellunderstood (Stigter and Uiso, 1981). Shaded Picheevaporimeters were used to replace humidity andwind speed data in the aerodynamic term of thePenman equation in Africa (WMO, 1989).Devices for measuring net radiation, soil heat fluxand sensible and advected heat are needed in energybudget methods, while continuous measurements ofwind speed, temperature and water vapour profilesare needed for the aerodynamic method (see alsoFAO, 1998; Hough et al., 1996). When adequateinstrumentation facilities and personnel are available,it is possible to compute actual evapotranspirationusing energy balance or mass transfer methods.Certain semi-empirical methods that require relativelysimple climatological measurements to provideestimates of PET are often of little value when evaporationis limited by water supply.Microlysimeters are very small lysimeters that canbe put into the ground and used to take soil evaporationmeasurements for short periods in such amanner that disturbance of the soil boundarycondition does not appreciably affect evaporationfrom the soil. Precautions to be taken and a measuringprotocol were given by Daamen et al. (1993)and operationally applied by Daamen et al. (1995)and Kinama et al. (2005).Particular metadata for pan evaporation are the pandimensions and rim height, and any employmentof pan defence against thirsty animals (such as wirenetting).2.4.1.8.2 IrrigationWater balance studies are incomplete withoutproper reference to different methods of irrigationbecause water of acceptable quality isbecoming an increasingly scarce resource for agriculture,while this sector accounts for the largestshare of water consumption. This was alreadydealt with in 2.3.1.8. Measurements and calculationsinclude soil moisture conditions, water useefficiencies and water flow conditions in canals ofdifferent dimensions, including the smallest fieldchannels (for example, Ibrahim et al., 1999, 2000,2002).2.4.1.9 Fluxes of weather variables (derivedfrom measured quantities)Reference should be made to Fritschen and Gay(1979), Kaimal and Finnigan (1994), Griffiths(1994), Murthy (1995, 2002), FAO (1998), andWMO (2001b).A reliable, but complex, method to measure atmosphericfluxes is that of “eddy covariances”. In thismethod very fast response devices such as hot-wire,hot-film, or sonic anemometers are used to measurewind, and similarly fast response sensors are used to

CHAPTER 2. AGRICULTURAL METEOROLOGICAL VARIABLES AND THEIR OBSERVATIONS 2–29measure the remaining quantities. These include theinfrared gas analyser (for water vapour and CO 2 ) andfine-wire temperature sensors. The correlationbetween instantaneous departures from the mean ofthe wind and other variables provides an estimate ofthe flux. Eddy covariance systems use commerciallyavailable instruments such as a three-axis sonicanemometer and infrared gas analyser, controlled bysoftware that also calculates and displays the surfacefluxes of momentum, sensible and latent heat, andcarbon dioxide. The Bowen ratio (the ratio of thesensible to latent heat fluxes) energy balance methodis a reliable technique for obtaining evaporationrates and is one of the most frequently used methodsfor estimation of surface energy balance componentsand evaporation. The required observations aredifferences in temperature and humidity betweentwo levels or in a profile. The Bowen ratio energybalance system provides continuous estimation ofevaporative loss. This system is less complex thaneddy covariances and its needs as to maintenanceand power consumption are lower than for eddycovariances.In all the studies pertaining to flux measurements,the temperature profile observations are supportedwith direct measurement of the soil heat fluxdensity. Heat flux densities in the soil or in plantor animal tissues are measured close to the interfacebetween air and soil, plant and animal withtransducers or heat flux plates. Generally, theseinstruments are thermopiles whose output isproportional to the temperature differencebetween the sides of a plate crossed by the flux.Such thermopiles are usually constructed bywinding a constantan spiral on a glass or plasticplate, copper-plating half of each winding in sucha way that portions of the plated and non-platedconstantan remain exposed in the upper and thelower sides. The conductivity of the plate materialshould match the heat transmission of themedium measured. For soils, the small plates aretypically buried at a compromise depth of 10 cm.Burial beyond this depth makes them unrepresentativefor soil heat flux at the surface, but veryshallow placement leaves only a thin coveringsoil layer, which then may dry out or crack. Thepresence of plant roots also has to be considered(WMO, 2001b).2.4.1.10 Remote-sensing and GISReference should be made to Goel and Norman(1990), Milford (1994), and WMO (2008b). Theremotely sensed image is typically composed ofpicture elements (pixels), which vary in size from afew metres to a few kilometres across. For each pixelan associated digital number or brightness valuedepicts the average radiance from that pixel within aspectral band that is specified by the relevant sensor.For useful information, such as a vegetation index,to be derived from the raw data, it is usually necessaryto process the data from more than one band. Ageometrical correction is necessary to ensure thatthe location of each pixel in an image is accuratelyknown, a process known as rectification.Images may be transformed within a GIS, for exampleby principal component analysis, which createsnew images from the uncorrelated values of differentimages. This analysis is used for spectral patternrecognition and image enhancement. Two or moredifferent images may be combined to form a newone using a variety of different techniques. Thensupervised and unsupervised classifications aretaken up to find complexity of terrain. Finally,accuracy assessment is carried out to allow for theuse of all these techniques in operational agriculturalmeteorology. In this connection, the conceptsof GIS are useful for efficient planning and de cisionmakingat farmer level, for integrating informationfrom many sources, and for generating new information,such as the slope of a region, wind direction,possible flow of water as a result of disasters, andother risks. These aspects are discussed in furtherdetail in Chapter 4.2.4.1.11 Calibration of recorders, integratorsand automatic weather stationsReference should be made to Woodward and Sheehy(1983). Meteorological data can be obtained bydirect reading (instantaneous) of measuring instrumentsand also by instruments providing acontinuous record of the parameters over time,with mechanical, electrical or other analog or digitaldisplays. All the instruments have to be calibratedto meet comparability requirements and recalibrationsare essential after repairs or replacement ofkey parts of the instruments. The most commonway to calibrate is by comparison with standardinstruments that are kept at national centres andspecialized laboratories and are checked from timeto time against international standards.2.4.1.11.1 Mechanical and electrical devicesObservations with instruments that do not haveself-recording devices are made by individual readingsat the given observation times and written intoan appropriately designed observations book, inaccordance with the instructions. From this basicdocument, data can be transferred to monthlysummaries and extracted for special analysis.

2–30GUIDE TO AGRICULTURAL METEOROLOGICAL PRACTICESIn mechanical reading instruments, the changes inthe length of the sensing element or sensor forceare transmitted mechanically with or withoutamplification to a recording system that is usuallybased on a clock-driven paper strip of either thedrum or endless belt type. The variations in thegiven parameter over time are displayed in graphicalform or in a diagram chart. The main advantagesof mechanical recorders are their relatively low cost,easy maintenance and independence from an externalpower supply.In electrical recording instruments, sensors are usedthat produce electrical signals (voltage, differencesin potential, resistance, and so on), whichcorrespond to the parameters under consideration;or detectors are used in which initial mechanical“signals” (such as longitudinal changes androtation) are transformed into electrical impulsesby appropriate devices (such as a potentiometerand switches). Depending on the signal output ofthe sensor, different recorders are used, such as thenull-balance potentiometric recorder, thegalvanometric recorder and the Wheatstone bridgefor electrical resistance measurements.2.4.1.11.2 MicroprocessorsWith the advances in microelectronic technologiesin recent years, more and more instrumentsusing integrated circuits and microprocessors arebeing designed for the purpose of measuring meteorologicalparameters. Together with electricalsensors, the use of integrated circuit chips hasallowed the construction of highly sensitive andlow-weight digital readout instruments. They havethe advantage of built-in “conversion” from electricalsensor outputs to technical units, includingcomplex linearizations. The use of integrated electroniccircuits and microprocessor chips has led tothe construction of automatic environmentalcontrol systems and automatic weather stations(AWSs).Electronic integrators with memory capacity fordata storage that can be recalled are also available.For any particular logger memory, the duration ofthe record available depends on the number ofsensors and frequencies of observation.2.4.1.11.3 Automatic weather stationsReference is made to WMO (2001a, 2008b). AnAWS is defined as a meteorological station atwhich observations are made and transmittedautomatically. If required, they may beinterrogated either locally or from an editingstation. Most of the variables required foragricultural purposes can be measured byautomatic instrumentation. As the capabilities ofautomatic systems expand, the ratio of purelyautomatic stations to observer-staffed weatherstations (with or without automaticinstrumentation) is increasing steadily. Theguidance regarding siting and exposure, changesin instrumentation, and inspection andmaintenance apply equally to automatic weatherstations and to staffed weather stations. Automaticweather stations are used to satisfy several needs,ranging from a single aid to the observer atmanned stations to complete replacement ofobservers at fully automatic stations. A generalclassification of these stations includes stationsthat provide data in real time and those thatrecord data for offline analysis or analysis notperformed in real time. It is not unusual, however,for both these functions to be discharged by thesame AWS.When planning the installation and operation of anetwork of AWSs, it is of utmost importance toconsider the various problems associated withmaintenance and calibration facilities, with theirorganization, and with the training and educationof technical staff. In general, an AWS consists ofsensors installed around a meteorological towerhoused in appropriate environmental shields; acentral processing system for sensor data acquisitionand conversion into computer-readable format;and some peripheral equipment, such as a stabilizedand uninterruptible power supply.The agricultural meteorological demands made onsensors for use with AWSs are not very differentfrom those made on sensors for conventional use.The siting of an agricultural AWS is a very difficultmatter and much research remains to be done inthis area. The general principle is that a stationshould provide measurements that are, and remain,representative of the surrounding area, the size ofwhich depends on the agricultural meteorologicalapplication needed. The distance over which anystation-measured parameter can be extrapolatedalso varies, from small for precipitation to large forincoming radiation (Wieringa, 1998). An AWSusually forms part of a network of meteorologicalstations and transmits its processed data or messagesto a central network processing system by variousdata telecommunication means. The cost over afew years of servicing a network of automaticstations can greatly exceed the cost of their purchase.The sensors with electrical outputs show drifts intime and, consequently, need regular inspectionand calibration.

CHAPTER 2. AGRICULTURAL METEOROLOGICAL VARIABLES AND THEIR OBSERVATIONS 2–312.4.2 Measurement of biological andrelated phenomenaReference should be made to the literaturementioned in 2.3.2. While that section and Chapters4, 6, 7 and 10 through 14 deal with biological measurementsand related phenomena, there are a fewadditional issues that have recently received muchattention and are not widely dealt with elsewherein this Guide. These concern measurements at ornear agricultural meteorological stations.2.4.2.1 Measurement of soil erosionA universal soil loss equation has been developedto measure/estimate water and wind erosionfactors. It is discussed in Hudson (1993) andChapter 10 of this Guide. Water erosion field measurementsare dealt with in Hudson (1993) andWMO (1994b) and particle analyses are discussedin Vining and Sharma (1994), while general fieldmeasurements for wind erosion are covered byZobeck et al. (2003). Spaan and Stigter (1991) andMohammed et al. (1995, 1996) discuss the operationaluse of simple field measurements in winderosion studies. Soil erosion (deflation) and deposition(accumulation) occur as a consequence oftransport and these are scientifically quantified asheight differences (for example, Mohammed et al.,1995; Sivakumar et al., 1998). When properlydesigned and carefully executed, erosion pinsprovide sound data on these changes. They aremeaningful and visibly impressive to farmers andextension workers. They allow large numbers ofmeasurements to be taken at low cost and areextremely useful to measure the changes in surfaceelevations of soils exposed to wind and/or watererosion (for example, Hudson, 1993).2.4.2.2 Measurement of runoffThe equipment for measurement of runoff includesweirs and Parshall flumes, which are suitable formeasuring the runoff from small watersheds, andwater-recording equipment, such as water storagerecorders that continuously record the water levelin a stream. In studies of agriculture on slopinglands, runoff plots are successfully managed andsoil loss and water runoff can be quantified (forexample, Kinama et al., 2007).2.4.2.3 Measurement of leaf area, canopystructure and photosynthesisIt is desirable to express plant growth on the basisof leaf area. The leaves are the primary photosyntheticorgans of the plant. After destructivesampling (removing the leaves from the plant),the leaf area can be measured by using a leaf areameter. This instrument is portable, but expensive.It has a transparent belt conveyer to spread theleaves and has a digital display to indicate the leafarea. More simply, the leaf area can also be estimatedby using the following methods: the length× width × constant method, the dry weightmethod, and the paper weight method. The leafarea is normally expressed in relation to groundarea as the leaf area index (LAI), which is the ratioof the total leaf area of a plant to the ground areaoccupied by the plant. To achieve higher production,a plant should be able to utilize a maximumamount of light, for which optimum spacingshould be followed. The LAI helps to derive optimumspacing to utilize the maximum sunlight forphotosynthesis.There are different optical methods for measuringLAI that are well established. Canopy structures canalso be quantified in this way. Details may be foundin Pearcy et al. (1989), Russell et al. (1989), Goeland Norman (1990), and Baker and Bland (1994).These references also include details on leaf, plantand stand photosynthesis measurements, and theirconsequences for development and growth can befound there as well. Many methods are used successfully,but they are not as accurate or rapid as IRGAsystems. A sensitive technique for rapid measurementsof CO 2 concentrations with attached leavessealed in Plexiglas chambers is also used. Otherrelated instruments include those measuringstomatal conductance, sap flow, leaf water potential,dendrometers, and the like. The literaturereferred to above contains details.Measurements of crop production that includethe weight of dry matter above ground, total drymatter, economic yield, and so forth are frequentlytaken at agricultural meteorological stations or inadjacent fields. These are useful in correlatingproduction to climatic variables over periods thatrange from weeks to the entire season.

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