where Ya,k and Yp,k are the actual and potential yields for the period k, Ta,k and Tp,k are the actual and potential crop transpiration, respectively, and Ky,k is a coefficient which depends on the crop and on the crop growing period k. Final relative yields, i.e. the ratio between Ya and Yp, are calculated as the product of the relative yields of each growing stage k.

In SWAP, irrigations may be prescribed at fixed times or scheduled in accordance with certain criteria. According to Kroes and Van Dam (2003), the irrigation scheduling criteria applied in SWAP are similar to those found in CROPWAT (Smith, 1992), IRSIS (Raes et al., 1988) and others. SWAP can therefore be used to select the irrigation management that maximizes crop yield, minimizes irrigation costs, optimally distributes a limited water supply or optimizes production from a limited irrigation system capacity (Kroes and Van Dam, 2003).

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