EVALUATION OF THE DETACHMENT-TRANSPORT COUPLING CONCEPT IN THE WEEP RILL EROSION EQUATION

The rill erosion equation in the current Water Erosion Prediction Proj ect (WEPP) model is based on the coupled detachment and transport proc esses concept proposed by Foster and Meyer in 1972. The first-order de tachment-transport coupling states that the rill detachment rate, D-r, is proportional to the difference between transport capacity, T-c, an d sediment load, q(s): D-r = alpha (T-c - q(s)), where alpha is a rate control constant. A held experiment was designed to examine the valid ity of this model. Flow channels, 0.2 m wide, with clear water introdu ced at the upslope end were used in the study. Sediment delivery from different channel lengths and inflow rates was measured. Data were col lected for three soils: Russell silt loam (fine-loamy, mixed, mesic Ty pic Hapludalf), Saybrook silt loam (fine-silty, mixed, mesic Typic Arg iudoll), and Sharpsburg silty clay (fine, montmorillonitic, mesic Typi c Argiudoll). Results showed that rill detachment and transport are no t coupled processes. In the upper reach of a channel, q(s) is limited by a soil-dependent detachment rate. For longer channels, transport ca pacity controls the sediment delivery. Experimental data supported the Meyer and Wischmeier model concept, in which detachment and transport processes are separated and sediment delivery is limited to the lesse r of the two. A slight modification to the Meyer and Wischmeier concep t is the inclusion of an ''overshoot'' situation (i.e., q(s) > T-c) wh en the sediment regime is shifted from a detachment-dominated to a tra nsport-dominated condition. Additional work is required to expand the database to develop a validated rill detachment and transport model.