A dynamic surface conductance to predict crop water use from partial to full canopy cover

The Penman–Monteith (P–M) equation has been widely used to predict crop water use or evapotranspiration (ET) due to its simplicity and biophysically robust framework. Surface conductance (Gs), a key variable reflecting crop physiological and soil physical responses to changing environment, often is a significant impediment to the practical application of the P–M equation. Here, we derived a dynamic biophysical model of Gs after incorporating the combined contributions of crop canopy and soil based on: (a) dynamic fraction of canopy cover; (b) response of stomata to radiation intercepted by crop canopy, vapor pressure deficit, and soil water availability in the root zone; and (c) soil evaporation coefficient affected by radiation reaching soil surface and soil moisture. The dynamic Gs model with the P–M equation can predict the variation of Gs and ET from partial to full canopy cover as crop growing. The model was parameterized by measurements using the eddy covariance technique over an irrigated maize field in 2009, and validated using independent data in 2010. We found good data-model agreements between ET predicted by the dynamic Gs model with P–M equation and measurements for both half-hourly and daily time-scales from partial to full canopy cover. The model also produced satisfactory estimation for soil evaporation. Therefore, the model is an alternative approach to predict ET using P–M equation for partial to full crop canopy cover.