Evaluating evapotranspiration estimation methods in APEX model in irrigated cropping systems

Irrigated agriculture water management is crucial for sustainable food production in arid and semi-arid regions of the world. Improving water use efficiency requires accurate assessment of evapotranspiration (ET) rates. Hydrologic models, like the Agricultural Policy/Environmental eXtender (APEX) model uses different potential ET (ET_p) methods to simulate monthly ET. However, robust model parameter values need to be developed to maximize the model's ability to provide reasonable estimates of ET for a wide range of conditions. The objectives of this study were to determine sensitive ET parameters in irrigated cropping systems and compare monthly ET estimates using multiple performance criteria. Measured ET and crop yield data from lysimeter fields located in the USDA-ARS Bushland, Texas were used for model simulations. Multiple statistical measures and crop yield were used to determine sensitive parameters and evaluate model performance. The number of sensitive parameters were 6, 8, 6, and 6 for Hargreaves (Hrg), Penman-Montieth (Pn-M), Penman (Pn), and Priestley-Taylor (Pr-T) methods, respectively. Soil evaporation plant cover factor, root growth soil strength, maximum rain intercept, and rain intercept coefficient were found to be sensitive parameters in all ET_p methods. Based on model performance results, the estimated number of model runs needed to obtain one model run with robust ET parameter values is 130, 5, 14, and 13 for Hrg, Pn-M, Pn, and Pr-T, respectively. When all inputs are available, the Penman-Monteith method takes the shortest computation time to obtain one model run with robust parameter values. However, in areas with limited data, one can still obtain reasonable ET simulations using either the Priestley-Taylor or the Hargreaves method.