Crop yield response to water - Cra

The WP parameter introduced in AquaCrop is normalized for atmospheric evaporative demand,defined by ET o , and for the CO 2 concentration of the atmosphere. The normalized biomasswater productivity (WP*) proved to be nearly constant for a given crop when mineral nutrientsare not limiting, regardless of water stress except for extremely severe cases. Calibration of WPand normalization for evaporative demands has been based on the equation:(5)WP* =ΣBTrET O[CO 2]The summation is taken over the time intervals spanning the period when B is produced.[CO 2 ] outside the bracket indicates that the normalized value is for a particular air CO 2concentration. For most crop species, WP* increases as air CO 2 concentration increases,allowing the simulation of impact on yield under various CO 2 and climate change scenarios.The equation is directly applicable when Tr and ET o data are for daily time intervals. WhenTr and ET o are available for time interval larger than daily, the normalization requirescaution. Background information and more details on normalization, including that for CO 2concentration, are given in Steduto et al. (2007).In the literature WP is commonly normalized for evaporative demand using air vapourpressure deficit (VPD) instead of ET o . The choice of using ET o was made because it has beendemonstrated to be superior and accounts for advective energy transfer, which is ignoredusing VPD (Steduto et al., 2007). WP* is conservative for a given level of mineral nutrition,but may be reduced by nutrient deficiencies, particularly nitrogen. The calibrated WP* inthe model for various crops are for situations where nutrients are ample. For nutrient limitedsituations, the model provides categories of soil fertility stress ranging from mild to severenutrient deficiencies, with corresponding lower default WP* values.The conservative nature of WP* is demonstrated in Figure 5, where cumulative B vs.cumulative Tr are plotted in (a), and cumulative B vs. cumulative normalized Tr (Tr/ET o )in (b), over the season for sweet sorghum (a C 4 crop), sunflower, wheat and chickpea (allthree are C 3 ). It is seen in Figure 5a that the regression lines for different crops are linearbut with different slopes. This means WP is constant for each crop but differs among thecrops. In Figure 5b it is seen that normalization by ET o has coalesced the lines for the threeC 3 crops into one, meaning their WP* are very similar. In this study sunflower was grownin May-August, wheat in February-May, and chickpea in April-June. So growth of thesecrops occurred in periods differing in atmospheric evaporative demand. Normalizing by ET oaccounted for the difference in evaporative demand and showed that the three crops havevery similar intrinsic water productivity (very similar WP*).The single value of WP*, as show in Figure 5b, is used for the entire crop cycle for most ofthe crops. However, for crops with yields high in fat and protein content, more photosyntheticassimilates or energy is required per unit of dry matter produced after flowering and duringthe grain/fruit filling stage. For such crops, AquaCrop uses a single value for the WP* up toflowering, then declining gradually towards a lower WP* value to account for yield composition.AquaCrop: concepts, rationale and operation 25