poster - International Conference of Agricultural Engineering

Daily Penman-Monteith sensitivity analysis in many subclasses climates
based on extended-De Martonne classification
Bahram Bakhtiari 1* , Amin Baghizadeh
1 Department of Water Engineering, College of Agriculture, Shahid Bahonar University of
Kerman, Kerman, Iran
2
International Center for Science, High Technology & Environmental Sciences, Kerman, Iran
Corresponding Author. Email: Drbakhtiari@uk.ac.ir
Abstract
ASCE daily Penman-Monteith model used in reference evapotranspiration (ET o ) estimation,
has many climatic parameters. To get useful results from the model, every parameter is
required to have a sensible value. In this study, a local sensitivity analysis of the
standardized daily ASCE Penman-Monteith evapotranspiration equation to time variation of
four climatic variables, net radiation (R n ), vapor pressure deficit (VPD), wind speed (U 2 ) and
air temperature (T) have been realized. Four different types of subclasses of arid and
semiarid climates in Kerman province (South east of Iran) have been studied using daily
data over a 17-year period (1998-2005) database. The studied regions of Kerman province
based on extended-De Martonne classification as follows: Baft (semiarid cool), Bam (arid
moderate), Kerman (arid cool) and Jiroft (arid warm). The results showed that, sensitivity
coefficients of studied climatic variables were positive in all stations. Net radiation was the
most sensitive variable in general, followed by, vapor pressure deficit, wind speed and air
temperature. The sensitivity of evapotranspimtion rates to changes in vapor pressure deficit,
wind speed, and air temperature is less in arid warm area than in arid and semiarid cool
regions.
Keywords: Climate data, Daily Penman–Monteith, evapotranspiration, Sensitivity coefficients
1. Introduction
Precise quantifications of crop evapotranspiration (ET c ) in irrigated agriculture are
consequential for scheduling irrigation. With increasing pressure on water resources from
competing sectors, great emphasis has been placed on water use efficiency in irrigated
fields (Hatfield et al., 1996), particularly in semiarid environment irrigation projects. Accurate
estimation of ET c is also essential for optimizing crop production and management practices
to minimize surface and groundwater degradation. The quantification of evapotranspiration is
normally based on the determination of reference evapotranspiration (ET o ). Reference
evapotranspiration is defined as ‘‘the rate of evapotranspiration from an extensive area of
0.08–0.15 m high, uniform, actively growing, green grass that completely shades the soil and
is provided with unlimited water and nutrients’’ (Allen et al., 1994). More recently, Allen et al.
(1998) elaborated on the concept of ET o , referring to an ideal 0.12 m high crop with a fixed
surface resistance of 70 s m-1 and an albedo of 0.23. ET o is widely used to estimate crop
water use and water requirements by using appropriate crop coefficients (K c ). The crop
coefficient is a dimensionless number that is multiplied by the ET o value to arrive at a crop
ET (ET c ) estimate; ET c = K c ×ET o . The K c represents the integrated effect of changes in leaf
area, plant height, irrigation method, rate of crop development, crop planting date, leaf area,
canopy resistance, albedo, soil and climate conditions, and management practices
(Doorenbos and Pruitt 1977). Different equations have been developed in attempts to model
ET o , including water budget (e.g., Guitjens, 1982), mass-transfer (e.g., Harbeck, 1962),
combination (e.g., Penman, 1948), radiation (e.g., Priestley and Taylor, 1972), and
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