Adjustments for Management Practices

The values given for Kcend for vegetable and cereal crops reflect crop and water management practices particular to those crops. Vegetable crops often are irrigated frequently until they are harvested fresh, before the plants have reached senescence and died. Therefore, Kcend values for these crops are relatively high (e.g., 0.90 for crucifers, green beans, and peppers). Cereal crops generally are allowed to reach senescence and to dry out in the field before harvest. Therefore, both the soil surface and the vegetation are drier with cereals and, consequently, the value for Kcend is small (e.g., 0.35 for maize and grain if dried in the field, but about 0.60 if harvested at high grain moisture).

When the local water management and harvest timing practices are known to deviate from those typical of the values presented in Table 5.1, the user should make adjustments to the values for Kcmid and Kcend, and to the length of the late-season period.

When stand density, height, or leaf area of the crop are less than that attained under perfect or normal crop and irrigation management conditions, the values for Kc are reduced. The reduction in Kcmid for poor stands can vary from 0.1 up to 0.5. Adjustment of Kc is made according to the amount of effective (green) leaf area relative to that for healthy vegetation having normal plant density (pristine conditions). Then,

where (Kc)cor is the crop coefficient to be used for poor stands, Kc is the Kcmid for pristine conditions (from Table 5.1), and Acm is the adjustment factor relative to the actual crop and irrigation management conditions (0-0.5). Acm can be approximated through a green cover ratio of type:

or similar ratio, referring to the midseason period.

A more complete approach to estimate Kc for nonpristine and unknown conditions is presented below in "Kc for Nonpristine and Unknown Conditions".

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