Crop yield response to water - Cra

threshold and 1.0 at the lower threshold (Figure 10). The shape of most of the Ks curves aretypically convex, and the degree of curvature is set during model calibration.water stressAquaCrop distinguishes stresses related to deficit and to excess water. In this publication, waterstress routinely refers to the stress caused by a lack of water, and stress caused by excessivewater is referred to as aeration stress. Water stress effects on productivity and water useprocesses are simulated by impacting: (1) canopy growth; (2) stomata conductance; (3) canopysenescence; (4) root deepening, and (5) harvest index. The normalized water productivity isassumed to be not impacted, based on extensive evaluation of the literature. The discoursethat follows discusses the first three impacted processes together, and includes root dependingat the end. Harvest index, a complex subject, is covered on its own in the last section on waterstress.water stress response functionsFor water stresses, the stress indicator is the root zone depletion (D r ), and the thresholds aresoil water depletions from the root zone expressed as fractions (p) of the total available soilwater (TAW). At the point when there is no depletion Ks = 1.0. As depletion progresses Ksdoes not drop below 1.0 until the upper threshold for stress effect is reached. This thresholdis referred to as p upper . Further increase in root zone depletion, brings about lower values ofKs, until the lower threshold (designated as p lower ) is reached, where Ks becomes zero and thestress effect is maximum (Figure 11). Further depletion below p lower has no additional effectand Ks remains zero. For water stresses the shape of the curve can vary between very convexto mildly convex to linear. Conceptually, the more convex the curve, the higher is the crop’scapacity to adjust and acclimate to the stress. A linear relationship indicates minimal or noFIGURE 10 The stress coefficient (Ks) for various degrees of stress and for 2 sample shapes of the Ks curve.No stress1.0convex0.8KsStress coefficient0.60.4linear0.2Full stress0.0UpperthresholdLowerthresholdStressindicator0.0 0.2 0.4 0.6 0.8 1.0Relative stress32crop yield response to water