Chapter 13. WATERSHED MODEL CHANNEL HYDROLOGY AND EROSION PROCESSES

The Water Erosion Prediction Project (WEPP) watershed model is a process-based, continuous simulation model built as an extension of the WEPP hillslope model (Flanagan and Nearing, 1995). The model was developed to predict erosion effects from agricultural management practices and to accommodate spatial and temporal variability in topography, soil properties, and land use conditions within small agricultural watersheds. The model contains three primary components: hillslope, channel, and impoundment. The hillslope component calculates rainfall excess by a Green-Ampt Mein-Larson (GAML) infiltration equation; peak runoff rate by kinematic wave overland flow routing or simplified regression equations; interrill erosion as a process of soil detachment by raindrop impact and sediment delivery to rill flow areas; and rill erosion as a function of sediment detachment, sediment transport capacity, and the existing sediment load in the flow. The following hillslope hydrologic and erosion output is stored in a pass file and then read in and used by the channel and impoundment components: 1) storm duration (s); 2) overland flow time of concentration (h); 3) Rational equation dimensionless α ;4 ) runoff depth (m); 5) runoff volume (m 3 ); 6) peak runoff (m 3. s −1 ); 7) total sediment detachment at the end of the hillslope (kg); 8) total sediment deposition at the end of the hillslope ( kg); 9) sediment concentration by particle size class at the end of the hillslope ( kg . m −3 ); and 10) dimensionless fraction of each particle size in the eroded sediment. The channel component can be further divided into hydrology and erosion components. The channel hydrology component computes infiltration, evapotranspiration, soil water percolation, canopy rainfall interception, and surface depressional storage in the same manner as the hillslope hydrology component. Rainfall excess is calculated using the identical GAML infiltration routines as found in the hillslope hydrology component. The GAML equation is used regardless of whether the channel is within a developed soil (e.g., an ephemeral gully or grassed waterway) or has an alluvial bed with a high loss rate (e.g., a channel in an arid or semi-arid climate watershed). The WEPP watershed model offers two options for calculating the peak runoff rate at the channel (sub-watershed) or watershed outlet: a modified version of the Rational equation similar to that used in the EPIC model (Williams, 1995) or the method used in the Chemicals, Runoff, and Erosion from Agricultural Management Systems (CREAMS) model (Knisel, 1980). The reasoning behind the development of the WEPP watershed model erosion component is that watershed sediment yield is a result of detachment, transport, and deposition of sediment on overland (rill and interrill) flow areas and channel flow areas, that is, erosion from both hillslope areas and concentrated flow channels must be simulated by the watershed version. The movement of suspended sediment on rill, interrill, and channel flow areas is based on a steady-state erosion model developed by Foster and Meyer (1972) that solves the sediment continuity equation. Detachment, transport, and deposition within permanent channels (limited to grassed waterways, terrace channels or similar size) or ephemeral gullies are calculated by a steady-state solution to the sediment continuity equation. Flow depth and hydraulic shear stress along the channel are computed by regression equations based on a numerical solution of the steady-state spatially-varied flow equation.