Development of a Sensitivity and Uncertainty Analysis Tool in R for Parametrization of the Apex Model.

Development of a sensitivity and uncertainty analysis tool in R for parametrization of the APEX model Mansour Talebizadeh; Daniel Moriasi; Amanda Nelson; Prasanna Gowda; Jean Steiner; and Patrick Starks Hydrologic models are used to simulate the responses of agricultural systems to different inputs and management strategies to identify alternative management practices to cope up with future climate and/or geophysical changes. The Agricultural Policy/Environmental eXtender (APEX) is a model developed to evaluate the impact of different land management strategies at the farm or small watersheds scales with respect to water availability, soil and water quality, plant growth, and economics. However, depending on the complexity of the study area, some of the APEX model parameters that are more sensitive than others (with respect to their effect on model performance) may require adjustment. Sensitivity analysis can be used as an effective means for detecting sensitive model parameters that affect the model performance which in turn could result in model simplification without compromising the performance of model, effective model calibration, as well as better allocation of resources regarding measurements of parameters in the study area. In this study, a flexible software APEXSENSUN was developed (in R language) to streamline the sensitivity analysis and calibration of the APEX model. The APEXSENSUN provides users with a wide range of sensitivity analysis methods that can be implemented for different study areas or goals. The software was applied to Rock Creek watershed, a small agricultural watershed located in northern Ohio, to detect most sensitive parameters that affect APEX’s performance regarding daily streamflow, total nitrogen, and total phosphorous loads simulation. Overall, the APEXSENSUN proved to be a very flexible and easy-to-use software for performing sensitivity analysis and model calibration and is expected to be useful to many practitioners and researchers to parameterize APEX. Keywords: APEXSENSUN; sensitivity analysis; uncertainty analysis; model calibration; APEX model; hydrology; water quality