Evaluating the APEX model for simulating streamflow and water quality on ten agricultural watersheds in the U.S.

To better understand the Agricultural Policy Environmental Extender (APEX) model's strengths and limitations in simulating streamflow and water quality, we evaluated its performance under a diverse range of climatic, topographic, soil, cover, and land management conditions using three parameter settings: best professional judgment (BPJ), partially calibrated, and fully calibrated. A total of 18 calibration parameters governing streamflow, crop yield, and sediment, N, and P were adjusted. Hydrologic and water quality responses were simulated on ten USDA-ARS watersheds with heterogeneous forested, pasture/range, and/or corn-soybean cropping systems. Based on percent bias and Nash-Sutcliffe coefficient of efficiency test statistics, 0%, 0%, 15%, and 9% of the data sets were considered satisfactory or better in simulating monthly streamflow, sediment, N, and P, respectively, in the BPJ mode. However, when used with the fully calibrated settings, 73%, 43%, 38%, and 27% of the data sets were satisfactory or better. Findings indicate that when fully calibrated, APEX estimates streamflow very well, sediment and nitrogen moderately well, and phosphorus marginally well at a monthly time scale. However, when using the BPJ approach, APEX does not estimate watershed-level streamflow, sediment, or nutrients accurately at a monthly time step. Additionally, APEX lacks an element of robustness in simulating streamflow and water quality constituents when applied to the same watershed with a different period of record or to a nearby watershed with a similar period of record. Based on these findings, it is recommended that users exercise caution when employing APEX with the BPJ approach or in validation mode at watershed scales.