Towards One-Step Estimation of Crop Water Requirements

This article provides theoretical analyses that facilitate the use of the Penman-Monteith equation to make aone-step estimate of crop water requirements. Reluctance to using a one-step estimate results from two outstanding issues,both of which are addressed. First, no method has been yet defined to handle the problem that meteorological variables arecommonly available only at 2 m above the ground while, when using the Penman-Monteith equation, they are required at somelevel above the crop. To resolve this, a blending height is defined in the atmospheric boundary layer (ABL) wheremeteorological conditions are independent of the underlying crop. Expressions are derived to calculate the aerodynamicresistances to, and the vapor pressure deficit at, the blending height from climate variables at 2 m. Consequently, 2 m climatedata can be used in the Penman-Monteith equation, either to estimate transpiration from surface resistance or to calculatesurface resistance from measured transpiration. Second, no table of effective values currently exists for the surface resistanceof different crops equivalent to that for the crop coefficient. This article calls for field studies to address this need. However,recognizing the need for an interim source of crop-specific surface resistances, a methodology is given for translating the cropcoefficient into equivalent surface resistance. To make this translation, it is necessary to specify the relationship between theradiative and aerodynamic energy inputs to evapotranspiration when the crop coefficients were calibrated. Finally, aPenman-Monteith-based, one-step estimation equation is derived that makes proper allowance for the different aerodynamiccharacteristics of crops in all conditions of atmospheric aridity, and that estimates crop evaporation for any crop of specifiedheight from existing crop coefficients using standard 2 m climate data.