Crop Coefficient Method

This chapter focuses on using the two-step approach to estimate evapotranspiration (ET) for farm crops, landscapes, and some natural vegetation. Two families of crop coefficient ($K_c$) curves for agricultural crops have been developed for the two commonly used reference crops (tall and short): Pruitt and others' grass-based curves and Wright and others’ alfalfa-based $K_c$ curves. Although several crop coefficient models use a curvilinear shape, the Food and Agricultural Organization’s (FAO) linear segment model is widely used and easy to formulate. FAO24 and FAO56 present the procedure for constructing crop (cover) coefficients. Many of the principles and procedures described for using the FAO grass-based crop coefficients also apply when using alfalfa-based crop coefficients. The dual $K_c$ method provides means to estimate effects of wetting frequency on total water consumption and to estimate ET year round.