Crop yield response to water - Cra

FIGURE 14 Schematic representation of the crop response to water stress, as simulated by AquaCrop,with indication (dotted arrows) of the processes (a to e) affected by water stress. CC is thesimulated canopy cover, CC pot the potential canopy cover, Ks sto the water stress for stomatalclosure, K c,Tr the crop transpiration coefficient (determined by CC and K c,Trx ), ET o the referenceevapotranspiration, WP* the normalized water productivity and HI the harvest index (adjustedfrom Raes et al., 2009).CCCanopycoverCC potCCTranspirationtimea.Canopy expansionc.EarlycanopysenescenseEvapotranspirationWeather dataFAO-Penman MonteithEquationIrrigationRainfallTr = Ks stoK c,TrET oRunoffWP*Biomass (B)HIb. Stomatalclosured.Harvest indexadjustmentCapillaryriseZSoilwaterbalancee.Root zoneexpansionDeeppercolation timeYield (Y)RootingdepthTemperature stressBy using GDD as the thermal clock, much of the temperature effects on crops, such ason phenology and canopy expansion rate, are presumably accounted for. The effect oftemperature on transpiration is accounted for separately by ET o . Damaging effects of extremeor close to extreme temperatures, however, fall into the stress category and require differentconsiderations.In general AquaCrop simulates temperature stress effects with temperature stress coefficients,which vary from zero to 1.0 and are functions of air temperature or GDD. Value of GDD for agiven day may be considered as an integrated measure of the daily temperature. Lower andupper thresholds delineate the temperature window wherein the process is affected. Lackingmore definitive data currently, the shape of the Ks vs. temperature curve (Figure 15) is takento be logistic, and may be changed in the future when better data become available.38crop yield response to water