Experimental, verification of a mechanistic model to partition evapotranspiration into soil water and plant evaporation

A model that separates evapotranspiration (ET) into soil water evaporation (E) and plant transpiration (T), and calculates parameters associated with the water and energy balance of the soil and crop surfaces is required to understand the fate of water in different cropping systems. ENWATBAL, a mechanistic ET model, is such a model. Our objective was to test the model by comparing calculated values of ET, E, T, net irradiance (R-n), soil heat flux (G), and;oil surface temperature (T-s) with measured ones. Three field experiments were conducted throughout 1993-1995 in a structureless sandy soil either bare or planted with sorghum in Tottori, Japan. E was measured by lysimeter using both micro- and an in situ large weighing lysimeter, ET was measured with the weighing lysimeter, and T was calculated by difference (T = ET-E). In these experiments, R-n, G, and T-s were also measured using standard methods. Three statistical tests, i.e., bias, RMSE and linear regression analysis were used to evaluate model performance. In all three experiments, measured and calculated values of E and T from both a bare soil and a cropped surface were in agreement with a bias close to 0.0 mm day(-1) and RMSE <1.0 mm day(-1). In these comparisons, slopes and intercepts were not significantly (P = 0.05) different from 1.0 and 0.0, respectively. Furthermore, experimental results indicated that ENWATBAL correctly calculated daily ET with a bias = -0.60 mm day(-1), RMSE = 0.89 mm day(-1) and a slope and intercept not significantly different from 1.0 and zero. Measured and calculated values of R-n and G over a bare soil were also in agreement. However, there are differences between the midnight and diurnal values of measured and calculated T-s. Therefore, we concluded that the ENWATBAL model calculated ET, E, T, R-n, and G within an acceptable accuracy and range for the soil and environmental conditions of our test. It is suggested that for our soil, model accuracy on the calculation of E and T-s could be improved by including the formation of a dry topsoil layer. (C) 1999 Elsevier Science B.V. All rights reserved.