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Crop water use occurs at potential rates until soil depletion equals the readily available water (RAW) (Figure 2). RAW is defined as in FAO56 (Equation 4) .The water stress coefficient (Ks) defines crop water use response relative to soil water depletion (Equation 5). Ks decreases linearly from 1 when soil depletion exceeds RAW to 0 at soil depletions levels greater than TAW (Figure 2). When Ks falls to 0.05, the crop is assumed to die. When the crop dies, the area is returned to fallow and no more irrigation requirements are generated for this crop. The crop can only be re-established if triggered by a new planting decision.
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The crop water stress (Ks) and available water asymptote to 0, meaning the actual crop water use progressively reduces as the crop gets more stressed. It therefore takes a long time for the crop water stress coefficient to reduce to 0.05 and cause crop death. |
Readily available water is defined by:
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