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Title: CHAPTER 4 HILLSLOPE SURFACE HYDROLOGY - WEPP)

Author
 Stone, Jeffry Lane, Leonard Shirley, Edward Hernandez, M

 Submitted to: Water Erosion Prediction Project Documentation Publication Type: Government Publication Publication Acceptance Date: 7/15/1995 Publication Date: N/A Citation: Interpretive Summary: The WEPP hillslope surface hydrology component was designed to represent the important processes which govern the amount and rate of runoff from a hillslope. The following processes are considered: 1. infiltration - the movement of rainfall through the air-soil interface and into the root zone. 2. rainfall excess - the difference between the rainfall rate and the infiltration rate. 3. depression storage - the amount of rainfall excess which is trapped in small depressions along the hillslope and which eventually infiltrate into the soil. 4. runoff routing - the movement of the rainfall excess down the hillslope at a rate which is dependent on the hillslope characteristics. In addition, the hydrology component also considers the case when processes are not uniform in the downslope direction as would be the case when a strip crop management system is in place. Technical Abstract: Chapter 4 of the WEPP User Documentation describes the surface hydrology calculations of the hillslope component of the WEPP model. Infiltration is computed using a formulation of the Green- Ampt Mein-Larsen model for unsteady intermittent rainfall. Rainfall excess is computed as the difference between the rainfall rate and the infiltration rate when the rainfall rate is greater that the infiltration capacity. Depression storage is subtracted from the rainfall excess and is assumed to be satisfied before runoff begins. When the model is run in a single event mode, the kinematic wave model for a single plane is used to compute the runoff hydrograph. For most cases, however, the model is run in continuous simulation mode and an approximation based on a relationship using the average and peak rainfall excess rates, duration of rainfall excess, and time to kinematic equilibrium is used to compute the peak duration. For cases of partial equilibrium hydrographs, the runoff volume is adjusted to account for infiltration during the recession of the hydrograph. When the hillslope is non-homogeneous as is the case with strip cropping, runoff is computed using a simple volume balance of inflow and outflow and an aggregation method to compute an equivalent overland flow surface.