Crop yield response to water - Cra

Determining irrigation requirementsThere are a number of losses from irrigated agriculture, often unavoidable, associated withthe application of irrigation water that the grower must consider when determining theactual water requirements of an orchard or vineyard. It should be noted that in this context,the term ‘losses’ does not apply to E and Tr, which are consumptive uses of water rather thanlosses.Until now, when determining the ET c , a situation has been considered in which the calculationapplies to an ideal, uniform orchard with all the trees having the same ET c . However, theuniformity of irrigation water application over a field is not perfect and some areas getmore water than others. To adequately irrigate areas that get less, the system must applymore water than required to meet the overall needs of the field. Thus, some areas of thefield will receive water in excess of ET and this can result in the deep percolation (loss) ofwater below the root zone. Additionally, some irrigation water may inadvertently run offthe field and this is also considered a loss, at least for that particular field.Whether deep percolation and runoff are true losses depend on the scale under consideration(field, farm, district, basin), and whether any or all of these losses can be recovered. Forexample, if runoff from one field is collected and then applied to the same or an adjacentfield, it is not a true loss. The same applies to deep percolation that enters a groundwatertable and is eventually pumped and reused, although its quality may be degraded. If waterenters a saline sink, such as a perched saline water table or the ultimate saline sink, theocean, it is a true loss.To maintain a favourable salt balance, some deep percolation of water is needed totransport the salt introduced by the irrigation water out of the root zone. The amount ofdeep percolation required is referred to as the ‘leaching fraction’ and depends on irrigationwater quality as well as the crop sensitivity to salinity. Methods have been established toestimate appropriate leaching fractions and can be obtained in FAO I&D No. 29. Leaching ofexcess salts is a requisite for the sustainabiliy of irrigated agriculture.Application efficiency is used to express irrigation efficacy and is described as the percentageof applied water that is available for tree use. Variations depend on the irrigation systemand skill of the irrigator and on whether an individual field, series of fields, entire farm orregion is considered. The chart in Box 7 indicates the disposition of irrigation water intoconsumptive and non-consumptive uses, and into beneficial and non-beneficial uses. It isimportant for the grower to understand that some irrigation losses are unavoidable, butthat they should be minimized.For the grower, it is important to have high application efficiency and as good distributionuniformity of applied water as possible. Distribution uniformity (DU) for surface and sprinklerirrigation methods applied to systems operating with trees and vines can be determinedusing farm-system evaluation techniques. With microirrigation systems, it is relatively easyto measure DU by checking either emitter flow rates or operating pressures throughout thesystem. In orchards or vineyards under microirrigation, it is possible to attain DU values of 80to 90 percent and thus, equally high application efficiencies if the systems are well designed,maintained and managed.268crop yield response to water