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Evaluation of the Detachment-Transport Coupling Concept in the WEPP Rill Erosion Equation

Agronomy Dep. and USDA-ARS National Soil Erosion Research Lab., 1196 SOIL Bldg., Purdue Univ., West Lafayette, IN 47907-1196

Joe M. Bradford

USDA-ARS Subtropical Agricultural Research Lab., 2413 East Highway 83, Weslaco, TX 78596

*Corresponding author (chihua{at}ecn.purdue.edu).

ABSTRACT

The rill erosion equation in the current Water Erosion Prediction Project (WEPP) model is based on the coupled detachment and transport processes concept proposed by Foster and Meyer in 1972. The first-order detachment-transport coupling states that the rill detachment rate, D_r, is proportional to the difference between transport capacity, T_c, and sediment load, q_s: D_r = a (T_c – q_s), where a is a rate control constant. A field experiment was designed to examine the validity of this model. Flow channels, 0.2 m wide, with clear water introduced at the upslope end were used in the study. Sediment delivery from different channel lengths and inflow rates was measured. Data were collected for three soils: Russell silt loam (fine-loamy, mixed, mesic Typic Hapludalf), Saybrook silt loam (fine-silty, mixed, mesic Typic Argiudoll), and Sharpsburg silty clay (fine, montmorillonitic, mesic Typic Argiudoll). Results showed that rill detachment and transport are not coupled processes. In the upper reach of a channel, q_s is limited by a soil-dependent detachment rate. For longer channels, transport capacity 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 lesser of the two. A slight modification to the Meyer and Wischmeier concept is the inclusion of an "overshoot" situation (i.e., q_s > T_c) when the sediment regime is shifted from a detachment-dominated to a transport-dominated condition. Additional work is required to expand the database to develop a validated rill detachment and transport model.

Received for publication June 19, 1995.

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