Estimating crop coefficients from fraction of ground cover and height

The FAO-56 procedure for estimating the crop coefficient Kc as a function of fraction of ground cover and crop height has been formalized in this study using a density coefficient Kd. The density coefficient is multiplied by a basal Kc representing full cover conditions, Kcb full, to produce a basal crop coefficient that represents actual conditions of ET and vegetation coverage when the soil surface is dry. Kcb full is estimated primarily as a function of crop height. Kcb full can be adjusted for tree crops by multiplying by a reduction factor (Fr) estimated using a mean leaf stomatal resistance term. The estimate for basal crop coefficient, Kcb, is further modified for tree crops if some type of ground-cover exists understory or between trees. The single (mean) crop coefficient is similarly estimated and is adjusted using a Ksoil coefficient that represents background evaporation from wet soil. The Kc estimation procedure was applied to the development periods for seven vegetable crops grown in California. The average root mean square error between estimated and measured Kc was 0.13. The Kc estimation procedure was also used to estimate Kc during midseason periods of horticultural crops (trees and vines) reported in the literature. Values for mean leaf stomatal resistance and the Fr reduction factor were derived that explain the literature Kc values and that provide a consistent means to estimate Kc over a broad range of fraction of ground cover