Improved APEX Model Simulation of Buffer Water Quality Benefits at Field Scale

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Abstract. Watershed models offer cost-effective means to quantify the environmental benefits of conservation practices. This study evaluated the ability of the Agricultural Policy/Environmental eXtender (APEX) model to simulate the effectiveness of upland buffers for reducing event runoff, sediment, and total phosphorus (TP) loadings. The study used 16 years of monitoring data (1993-2008) from three field-scale row crop watersheds. Two watersheds had buffers installed in the fall of 1997, one with upland agroforestry buffers (grass plus trees) and the second with upland grass buffers; the third watershed was maintained as a control. Objectives were to (1) determine the effect of upland buffers on APEX parameters, (2) evaluate the ability of APEX to estimate upland buffer effectiveness, and (3) evaluate the impact of alternative buffer placement options on water quality outcomes. After modification of the APEX code to improve simulation of infiltration in the buffers, we successfully calibrated APEX for no-buffer and upland buffer conditions for event runoff and TP. However, calibrated parameters from the no-buffer model simulated buffer conditions poorly and resulted in an overestimation of buffer effectiveness. Buffer effectiveness estimated with upland buffer parameters was similar to that obtained from measured data. Scenario analysis indicated that a combination of backslope and footslope agroforestry buffers was more effective than contour placement for reducing average annual edge-of-field runoff and total P losses. These results highlight the complexity of using APEX to assess upland buffers as well as potential problems when using APEX to quantify the benefits of conservation practices not included in the calibration dataset. Keywords: Agroforestry buffers, Buffer placement, Modeling, Phosphorus, Practice effectiveness, Watershed discretization.