Towards one-step estimation of crop water requirements

This article provides theoretical analyses that facilitate the use of the Penman-Monteith equation to make a one-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 are commonly available only at 2 m above the ground while, when using the Penman-Monteith equation, they are required at some level above the crop. To resolve this, a blending height is defined in the atmospheric boundary layer (ABL) where meteorological conditions are independent of the underlying crop. Expressions are derived to calculate the aerodynamic resistances to, and the vapor pressure deficit at, the blending height from climate variables at 2 m. Consequently, 2 m climate data can be used in the Penman-Monteith equation, either to estimate transpiration from surface resistance or to calculate surface resistance from measured transpiration. Second, no table of effective values currently exists for the surface resistance of 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 crop coefficient into equivalent surface resistance. To make this translation, it is necessary to specify the relationship between the radiative and aerodynamic energy inputs to evapotranspiration when the crop coefficients were calibrated. Finally, a Penman-Monteith-based, one-step estimation equation is derived that makes proper allowance for the different aerodynamic characteristics of crops in all conditions of atmospheric aridity, and that estimates crop evaporation for any crop of specified height from existing crop coefficients using standard 2 m climate data.