Estimation of crop water requirements: extending the one-step approach to dual crop coefficients

Crop water requirements are commonly estimated with the FAO-56 methodology based upon a two-step approach: first a reference evapotranspiration (ET₀) is calculated from weather variables with the Penman–Monteith equation, then ET₀ is multiplied by a tabulated crop-specific coefficient (K_c) to determine the water requirement (ET_c) of a given crop under standard conditions. This method has been challenged to the benefit of a one-step approach, where crop evapotranspiration is directly calculated from a Penman–Monteith equation, its surface resistance replacing the crop coefficient. Whereas the transformation of the two-step approach into a one-step approach has been well documented when a single crop coefficient (K_c) is used, the case of dual crop coefficients (K_cb for the crop and K_e for the soil) has not been treated yet. The present paper examines this specific case. Using a full two-layer model as a reference, it is shown that the FAO-56 dual crop coefficient approach can be translated into a one-step approach based upon a modified combination equation. This equation has the basic form of the Penman–Monteith equation but its surface resistance is calculated as the parallel sum of a foliage resistance (replacing K_cb) and a soil surface resistance (replacing K_e). We also show that the foliage resistance, which depends on leaf stomatal resistance and leaf area, can be inferred from the basal crop coefficient (K_cb) in a way similar to the Matt–Shuttleworth method