Apex Model Simulation of Runoff and Sediment Losses From Agroforestry Buffers for Watersheds Under Pasture Management.

Hydrologic simulation models assist in predicting the effects of buffers on runoff and sediment losses from small watersheds.  The objective of this study was to calibrate, validate the APEX (Agricultural Policy/Environmental eXtender) model and simulate runoff and sediment losses from small watershed comparing values for agroforestry buffer watersheds versus control watersheds (no buffers).  The experimental design consisted of four watersheds under pasture management which were monitored from 2002 through 2008; two with agroforestry buffers (AgB) and two control watersheds (CW). The model was calibrated from 2002 to 2005 and validated from 2005 to 2008. The r² and Nash and Sutcliffe (NSE) values for the calibration and validation period of the runoff varied from 0.52 to 0.78 and 0.50 to 0.74, respectively. The model did not predict sediment loss well probably due to insufficient number of measured events and low measured sediment loss. Measured runoff was 57% higher for CW watersheds compared to AgB watersheds. Measured sediment loss was 95% higher for CW watersheds compared to AgB watersheds. After calibrating and validating the model, it was run for long-term scenario analyses for 10 years from 1999 to 2008. Buffer width had an influence on runoff. Simulated runoff decreased 24% when buffer width was doubled compared to losses associated with the measured buffer width. Simulated runoff from the CW watersheds was 11% higher with double stocking density (relative to measured density) compared to AgB watersheds with double stocking density. With half stocking density (relative to measured density), the AgB watershed had 18% lower runoff compared to CW. Results from this study imply that establishment of agroforestry buffers on grazed pasture watersheds reduce ru noff and sediment losses.