Predicting Runoff and Sediment Yield from a Stiff-Stemmed Grass Hedge System for a Small Watershed

Grass hedges planted at regular intervals on the landscape offer many opportunities to reduce runoff and sediment from leaving fields. Objectives of this study were (1) to evaluate the ability of the WEPP watershed model to simulate grass hedge system effects of sediment trapping (TE), bench terracing (BT), and variable effective soil hydraulic conductivity (HC) on simulated hillslope runoff and sediment yield, and (2) to model the effects of measured effective hydraulic conductivity (Keff) values from a grass hedge management system by comparing predicted runoff and sediment yield values to those measured in a small watershed over an 11-year period. The study was conducted on a 6.6 ha watershed located in the deep loess hills region of southwestern Iowa. Narrow grass hedges of predominantly switchgrass (Panicum virgatum) were planted at 15.4 m intervals in 1991. The WEPP model simulated greater reductions in runoff (9%) and sediment yield (58%) from BT compared to TE and HC effects. Combination of all three effects gave the highest reductions in runoff (22%) and sediment yield (79%) compared to individual effects or any combination of two effects. The watershed model did not adequately simulate slope length reduction effects from the grass hedges. Runoff (r2 = 0.78) and sediment yield (r2 = 0.75) were comparable to observed data when measured Keff values for grass hedge, row crop, and channel areas were used as input data. Measured Keff data from grass hedge, row crop, and channel areas should be used for improved runoff and sediment yield predictions.