Comparison of Evapotranspiration and Biomass Simulation in Winter Wheat under Conventional and Conservation Tillage Systems using APEX Model

Abstract

The Agricultural Policy/Environmental eXtender (APEX) is one of the hydrologic and water quality models being used in the USDA-ARS Long-Term Agroecosystem Research (LTAR) to evaluate current and develop sustainable agricultural production systems throughout the United States. Tillage practices used affect agricultural production, soil erosion, and evapotranspiration (ET), which is a major component of the hydrologic cycle. However, there is limited literature on the impact of tillage practices on parameterization and performance of APEX to simulate ET. The goals of this study were to 1) determine model parameters that affect ET simulation in APEX model for winter wheat (Triticum aestivum L.) fields managed under conservation and conventional tillage systems, and 2) evaluate the ability of APEX to simulate ET and daily biomass for each of these tillage systems in central Oklahoma. Tillage system had an impact on the number and type of APEX parameters sensitive for simulation of ET. Therefore, it is essential to perform a sensitivity analysis for study areas with different tillage management practices to determine appropriate parameters for inclusion in the calibration process. Overall, APEX simulated daily, biweekly, and monthly ET and daily biomass relatively well for both tillage systems. The Nash-Sutcliffe efficiency (NSE) for the simulated monthly ET of the calibrated model was 0.98 and 0.96 for conservation and conventional tillage fields, respectively. The percent bias (PBIAS) values for the ET simulation were within 3%, while PBIAS values for daily biomass simulation were within 25%, which indicates that the average simulated ET and daily biomass values were within 3% and 25% of the measured ET and daily biomass, respectively, in both tillage fields. This result indicates that a well-calibrated APEX model using appropriate sensitive parameters can simulate ET and biomass satisfactorily in both tillage systems.