A Realistic Simulation of the Evapotranspiration Processes.

Agricultural system simulation models that realistically simulate the ET processes are potential tools for the integration, synthesis, and extrapolation of field water management research results across soils and climates. However, accurate simulation of ET in system models is a challenge as the ET processes in natural landscapes are impacted by the dynamic soil, plant, and atmospheric processes; and as such, involve complex modeling, using the weather inputs and the soil-water-plant-atmosphere processes for biomass and grain yields. Limited availability of precise field measurements of ET, along with the crop growth data, have also precluded extensive testing and improvement of these models. Agricultural Model Inter-Comaprison and Improvement Group (AgMIP, unpublished data) reported a 2-fold variation in simulated ET among the 23 maize models developed around the world. These findings point towards the need for a more critical look at the ET simulations used in cropping system models and to evaluate them against precisely measured experimental data, such as in precision lysimeters. The objective of this study was to evaluate accuracy of Root Zone Water Quality Model (RZWQM) simulated silage corn daily ET against their water use measured with large precision lysimeters in the Texas High Plains. An extended approach, based on the Shuttleworth and Wallace method (extended S-W), was used to estimate potential crop ET (E and T separately) demand in RZWQM2. The Nimah and Hanks approach was used for crop water uptake and Richard’s Equation for soil water redistribution modeling. Simulations of biomass, leaf area index, soil water storage, and ET were found to be reasonably accurate for irrigation water management applications. The extended S-W approach showed the capability to estimate potential crop ET comparable to the lysimetric measurements, i.e., without the use of Kc.