Estimating Sediment Delivery Ratios for Grassed Waterways using WEPP

Grassed waterways (GWWs) transport sediment and nutrients from upland source areas to receiving waters. Watershed planners have a critical need to understand GWW sediment delivery to optimally target source area management practices. Better physically based tools are needed to estimate sediment delivery by GWWs. This study developed several distributed sediment delivery ratio (SDR) regressions for GWWs using the process‐based Water Erosion Prediction Project (WEPP) model to provide simple equations to estimate sediment delivery for planning applications. Water Erosion Prediction Project was calibrated and validated for runoff and sediment yield for large 30.2‐ha and smaller 5·7‐ha nested watersheds with terraces and a common GWW outlet. A crop rotation of corn, oat and alfalfa and fall tillage using chisel plow were used in the nested watersheds. A hypothetical management case without terraces using corn, oat and alfalfa rotation with chisel plow as fall tillage was also evaluated for the 5·7‐ha watershed and the GWW. The length, slope, Manning's roughness coefficient and infiltration rate for the GWW were varied and SDRs calculated for 30 representative (in terms of daily rainfall) days over a 20‐year period of simulated climate. Regressions were developed for the existing (terraced) and hypothetical (non‐terraced) management scenarios for early (April–July), late (August–October) and full (April–October) growing seasons. Equations developed for the non‐terrace watershed had higher R² values compared to the terraced watershed suggesting that channel and rainfall parameters were better able to explain the variation in SDR for the non‐terraced watershed. Manning's roughness coefficient was the most significant parameter for predicting SDR for both the terraced and non‐terraced watersheds. The equations developed here can be used to estimate SDRs for watersheds that are drained via GWWs having similar physical characteristics: slope (1–5%), Mannings's roughness coefficient (0·1–0·3), length (0·15–1 km) and infiltration rate (0·025–25 mm h⁻¹). The SDRs can be used to estimate sediment yield, which is an essential element for making land management decisions but is rarely measured. Copyright © 2017 John Wiley & Sons, Ltd.