The effects of dynamic soil moisture-dependent parameters on runoff and crop yield estimation for the APEX model

The APEX, or Agricultural Policy/Environmental Extender Model, can be used to evaluate the effects of management practices on important agricultural outcomes, including water and soil quality, erosion, and crop yields. APEX simulates many of the bio-physical processes that take place on a farm or small watershed. The parameters APEX uses to detail how these processes are simulated are typically determined through calibration and validation procedures based on a comparison of model output to observational data. However, traditional methods of parameterization assume a static set of optimal parameters for the full simulation. To investigate the validity of the assumption of stationarity in optimal parameter values, this study assessed whether the introduction of parameters allowed to vary based on soil moisture conditions could offer improved runoff and crop yield estimation for the APEX model. The null hypothesis examined was that optimal parameters under dry conditions were similar to those under wet conditions. Results indicated that dynamic parameters resulted in improvements in model performance for an objective function that combined mean absolute error of modeled output water yields and crop yields under calibration scenarios. Validation performance also improved, though at less magnitude. The calibration process for several dynamic parameters indicated changes in optimal values based on soil moisture levels. These results have potential implications for both the accuracy and confidence in APEX outcomes under current scenarios and for APEX outcomes under changing environmental conditions.