Estimation of Hourly Evapotranspiration in Arid Regions by a Simple Parameterization of Canopy Resistance

Abstract

The Penman-Monteith (PM) equation for evapotranspiration (ET) estimation has been applied in the world. Its weak point is the estimation of the canopy resistance. This paper proposed a simple canopy resistance model for estimating ET on an hourly basis. A non-linear functional relationship between canopy resistance, aerodynamic resistance, and climatic resistance was proposed. For model validation, data was collected in an irrigated wheat field in the Hetao Irrigation District, which is the largest one along the Yellow River. Surface energy balance and near-surface weather variables were measured during the growth cycle of wheat. Measured hourly ET values were obtained by the Bowen ratio energy balance method. Regression of predicted versus measured ET data resulted in r2=0.97 with slope of 0.98 and intercept of 0.01 which are not different from 1 and 0 at 99% confidence. This study provided a simple way to estimate ET with a general available meteorological database which can be further applied in hydrologic models considering effects of variable canopy resistance.