Modeling the effectiveness of conservation practices at Shoal Creek watershed, Texas, using APEX.

This study was conducted to evaluate the performance of the Agricultural Policy/Environmental eXtender (APEX) model using daily storm event runoff and sediment yields (1997-2005) collected at the outlet of the 22.5 km2 Shoal Creek watershed. This watershed only has intermittent streams. The watershed is within the U.S. Army's Fort Hood military reservation in central Texas. It received a combination of erosion control practices including implementation of gully plugs and contour soil ripping. APEX was calibrated and validated with a 183-subarea configuration delineated from a 5 m digital elevation model. Results from model calibration and validation confirmed that APEX was able to realistically estimate daily runoff and sediment yield for both the pre- and post-BMP conditions, as evidenced by R2 values ranging from 0.60 to 0.80 and Nash-Sutcliffe efficiency (EF) values ranging from 0.58 to 0.77 with an exception of 0.33. During the post-BMP period, the total sediment yield was significantly less than that from the pre-BMP period, even though the corresponding total precipitation amount from the post-BMP events (1025 mm) was more than that from the pre-BMP events (668 mm). The simulated sediment yield was summed up to 24.3 Mg ha-1 for the pre-BMP events and 7.6 Mg ha-1 for the post-BMP events, which were very close to the measured values of 24.9 and 8.1 Mg ha-1, respectively. The benefits of the existing BMPs were quantified as a reduction of 52% in runoff and 86% in sediment yield based on comparisons between predictions from the run using the pre-BMP model setup and measured values under BMP conditions. The results suggest that APEX is capable of simulating conservation practices on military landscapes, and that it a useful tool for scenario analysis to evaluate the effectiveness of conservation practices.

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