Ke fs Kc

Kcmax represents an upper limit on the evaporation and transpiration from any cropped surface and is imposed to reflect the natural constraints placed on available energy represented by the difference Rn — G — H [see Eq. (5.1)]. Kcmax ranges from about

1.05 to 1.35 when using the grass reference ET0 and is presumed to change with climate similar to Kc and Kcb; thus

Kc max = max[{1.2 + [0.04(U2 - 2) - 0.004(RHmi„ - 45)](h/3)a3}, {Kcb + 0-05}],

where U2 is the mean value for daily wind speed at 2-m height over grass (ms-1), RHmin is the mean value for daily minimum relative humidity (%), h is the mean plant height (m), and max[-] is a function to select the maximum value of the parameters in braces ■ that are separated by the comma.

A single Kcmax can be computed. Then, variables U2, RHmin, and crop height refer to the midseason period. When more detailed computations are applied, the variables U2 and RHmin can be averaged for shorter periods, five days for example. The mean crop height h also can be estimated for the same time intervals but, for the initial period, h can be artificially considered the same as for the grass reference crop (h = 0.12 m).

The simple model used to estimate evaporation from soil is similar to the one used to compute Kcini and is illustrated in Fig. 5.6, where the evaporation rate is at the maximum rate until the depth of water evaporated, We, equals W1, the maximum cumulative depth of evaporation during stage 1. After We exceeds W1, the evaporation process is in stage 2, and the evaporation rate begins to decrease in proportion to (We - W1)/(WX - W1). Therefore, the coefficient for soil evaporation decay, Kd, is calculated as

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