Crop yield response to water - Cra

The water budget methodWith this method, the tree root zone is considered a reservoir of soil water that is depletedas E and Tr take place. The soil reservoir of available water that the tree depletes through ETis allowed to lose water until a soil water threshold (allowable depletion) is reached, belowwhich water stress is detrimental to crop production, quality or both. At this point, irrigationmust be applied to refill the soil profile and the amount needed is equivalent to the ET c lossessince last irrigation. Box 14 describes the process of how to schedule irrigation with the waterbudget for tree crops.box 14Applying the water budget method of irrigation scheduling.Information needed• Available soil water holding capacity or total available water (TAW)Defined as the difference between field capacity and permanent wilting point, itvaries according to soil texture between 50 mm/m to 200 mm/m.• Rooting depthTree roots extend deeply into open soils and can reach several metres, but theirdepth may be much more limited by mechanical restrictions in the soil profile.• The allowable depletion(AD)This is the threshold level of the root zone storage capacity below which the level ofwater deficit in the tree is undesirable. At this point, an irrigation is applied. UsualAD levels vary between 50-70 percent of TAW.• The ET c rateThe ET c losses are accumulated until the allowable depletion level is reached.Practical considerationsAlthough tree roots may reach several metres, the effective depth of rooting for irrigationpurposes is considered much less for practical reasons. Even in deep, open soils, 1.5 to 2 m isthe maximum depth considered for water budget calculations. A 2 m soil profile can holdup to 400 mm of H 2 O, and if the AD is 50 percent of the TAW, the crop can extract 200 mmbefore the next irrigation is applied. At an ET c rate of 5 mm/day, the next irrigation wouldbe applied after 40 days! This is not practical for many reasons; for example, it wouldbe difficult to replenish that deficit in a reasonable time period, as it is very difficult toinfiltrate 200 mm of water into most soils within the standard irrigation time. Therefore,what is commonly done is to fix a certain depth of water to be applied, often muchless than the AD, (between 50 and 100 mm of water), and vary the irrigation intervalsaccording to the ET c loss. Thus, setting the AD is primarily a management decision, andmust consider all practical aspects of the farm irrigation processes.Yield Response to Water of Fruit Trees and Vines 279