MODELING SUBSURFACE DRAINAGE AND SURFACE RUNOFF WITH WEPP

The U.S. Department of Agriculture's Water Erosion Prediction Project (WEPP) is a new technology based on fundamentals of hydrology, soil ph ysics, plant science, hydraulics, and erosion mechanics. WEPP hydrolog y includes simulation of excess rainfall using the Green-Ampt infiltra tion equation, surface runoff routing, evapotranspiration, percolation , and subsurface drainage to tiles (or ditch). Soil water flowing to a n artificial drain (tiles or ditch) reduces the antecedent water conte nt of soil and results in increasing soil-infiltration capacity and re duction of storm runoff. Hydrometeorological, vegetation, topographica l, and soil data from a poorly drained watershed subjected to subsurfa ce drainage system were used to evaluate the WEPP hydrology components . Model-simulated and measured storm-runoff hydrographs were compared for the pre- and postsubsurface drainage installation. The results ind icate that, in general, the model does an acceptable job of predicting storm and peak runoff rate for pre- and postsubsurface drainage insta llation.