Evaluation of the Penman-Monteith model for estimating soybean evapotranspiration

The Penman-Monteith model with a variable surface canopy resistance (r(cv)) was evaluated to estimate hourly and daily crop evapotranspiration (ETc) over a soybean canopy for different soil water status and atmospheric conditions. The hourly values of r(cv) were computed as a function of environmental variables (air temperature, vapor pressure deficit, net radiation) and a normalized soil water factor (F), which varies between 0 (wilting point, theta(WP)) and 1 (field capacity, theta(FC)). The performance of the Penman-Monteith model (ETPM) was evaluated using hourly and daily values of ETc obtained from the combined aerodynamic method (ETR). On an hourly basis, the overall standard error of estimate (SEE) and the absolute relative error (ARE) were 0.06 mm h(-1) (41 W m(-2)) and 4.2%, respectively. On a daily basis, the SEE was 0.47 mm day(-1) and the ARE was 2.5%. The largest disagreements between ETPM and ETR were observed, on the hourly scale, under the combined influence of windy and dry atmospheric conditions. However, this did not affect daily estimates, since nighttime underestimations cancelled out daytime overestimations. Thus, daily performances of the Penman-Monteith model were good under soil water contents ranging from 0.31 to 0.2 (theta(FC) and theta(WP) being 0.33 and 0.17, respectively) and LAI ranging from 0.3 to 4.0. For this validation period, calculated values of r(cv) and F ranged between 44 s m(-1) and 551 s m(-1) and between 0.19 and 0.88, respectively.