Crop yield response to water - Cra

occupied by air in the root zone. The function is assumed to be linear with a settable upperthreshold and the lower threshold fixed at zero (fully saturated soil). When the percentageair volume drops below the upper threshold, Ks aer starts to decrease below 1.0, causingproportional reduction in Tr.The sensitivity of the crop to waterlogging is specified by setting the upper threshold, andby indicating the number of days waterlogging must remain before the stress becomes fullyeffective and Tr is affected. It should be pointed out that so far aeration stress parametersgiven for the crops already calibrated are all default values, because definitive data for cropunder aeration stress are rare.Low soil fertility (or mineral nutrient stress)As already mentioned under Field management, AquaCrop does not simulate nutrient cyclesand balances, but provides the means to adjust for fertility effects with a set of soil fertilitystress coefficients, to simulate the impact on the growing capacity of the crop in terms offour pivotal components of productivity: canopy growth coefficient (CGC), maximum canopycover (CC x ), canopy decline, which includes a slow but substantial decline upon reaching CC xin addition to the senescence near maturity, and WP*. Accounting for the first three of thesecomponents, as affected by fertility, results in simulated pattern of CC vs. time very similar toplots based on measured data (Figure 16). The last component, WP*, is also adjusted downwardfor low fertility. The basis for making these adjustments are the following observations,well established in the literature: plants grown on soil deficient in nutrients (N, P, and/or K)produce leaves more slowly, with lower leaves senescing quite or very early but the upper andyoungest leaves remain green until maturity or very near maturity. Photosynthetic capacity offigure 16 Green canopy cover (CC) for unlimited (light shaded area) and limited (dark shaded area)soil fertility with indication of the processes resulting in (a) a reduced maximum canopycover, (b) a slower canopy development, as indicated by the reduced slope of CC vs. time inearly season, and (c) a continuous and slow decline of CC once the maximum canopy coveris reached.Green canopy cover(b)(a)(c)Growing cycle (days)40crop yield response to water