Abstract
Abstract. 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 (Kc) to determine the water requirement (ETc) 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 (Kc) is used, the case of dual crop coefficients (Kcb for the crop and Ke 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 Kcb) and a soil surface resistance (replacing Ke). We also show that the foliage resistance, which depends on leaf stomatal resistance and leaf area, can be inferred from the basal crop coefficient (Kcb) in a way similar to the Matt–Shuttleworth method.
Highlights
The well-known FAO-56 publication on crop evapotranspiration (Allen et al, 1998) is the outcome of a revision project concerning a previous publication (FAO-24) on the same subject (Doorenbos and Pruitt, 1977)
We have shown that the FAO-56 dual crop coefficient approach, where the crop coefficient Kc is split into two separate coefficients, can be translated into a onestep approach based upon a Penman–Monteith type equation (Eq 10), its surface resistance being the parallel sum of a soil and foliage resistance
This new form of the Penman– Monteith equation estimates fairly accurately crop evapotranspiration when compared to a full two-layer model
Summary
The well-known FAO-56 publication on crop evapotranspiration (Allen et al, 1998) is the outcome of a revision project concerning a previous publication (FAO-24) on the same subject (Doorenbos and Pruitt, 1977). The FAO-56 methodology (single or dual crop coefficients) is commonly called the two-step approach (Shuttleworth, 2007) because ET0 is first calculated from weather variables and empirically adjusted using crop-specific coefficients. Shuttleworth (2006) provided a theoretical background, called the Matt–Shuttleworth approach, to transform the currently available crop coefficients (Kc) into effective surface resistances (rs) to be used with the Penman–Monteith equation. This method, which in principle only applies to the single crop coefficient approach, has been thoroughly examined and discussed by Lhomme et al (2014) and Shuttleworth (2014). Numerical simulations will be performed to illustrate the advantages of this new form of the Penman–Monteith equation to estimate crop water requirements with a one-step approach
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