Crop yield response to water - Cra

acclimation. The stress thresholds, as well as the curve shape, are set by calibration and shouldbe based on knowledge of the crop’s drought resistance or tolerance.Being the middle link in the soil-plant-atmosphere continuum, the plant water status dependsnot only on soil-water status, but also on the rate of transpiration determined by atmosphericevaporative demand. The crop is more sensitive to soil-water depletion on days of high ET o ,and less on days of low ET o . For simplicity, instead of modelling the soil-plant-atmospherecontinuum, AquaCrop adjusts the thresholds of the Ks curve according to ET o , a measureof evaporative demand. As the threshold is set for environments with ET o = 5 mm/day, themodel automatically adjusts the thresholds each day according to daily ET o when running asimulation. The extent of the adjustment is depicted in Figure 11.FIGURE 11 Sample Ks curve for canopy expansion. The thick blue line represents Ks for days whenET o = 5 mm/day. The line on the left indicates that the value of Ks decreases (stronger stresseffect) when ET o increases, and the line on the right that Ks increases when ET o decreases. Thehatched area spans the range of adjustment as dictated by ET o .HighETOKs curve forcanopy expansion growthwith ETO = 5 mm/dayWater stress coefficient (Ks, leaf)1.00.80.60.40.2LowETO0.0Fieldcapacityp upperp lowerRoot zone depletionOf the first three processes affected by water stress, extensive studies have shown thatexpansion of the leaf (hence the canopy) is the most sensitive, and stomatal conductance issubstantially less sensitive. Depending on the species, leaf (hence canopy) senescence may beequally or slightly less sensitive than stomatal conductance (Bradford and Hsiao, 1982). Settingof the three upper thresholds for water stress for a crop should be consistent with theseobservations. Differences in the Ks curves for the three processes can be seen in the examplefor maize in Figure 12.AquACrop: ConCepts, rAtionAle And operAtion 33