In order to improve prediction of soil erosion it is necessary to describe the processes that contribute to soil erosion. One of these processes is rill erosion that occurs when overland flow of water becomes concentrated, and the flowing water results in the detachment and downslope transport of sediment. Field observations and principles of fluid mechanics, soil physics and soil mechanics were used to develope a model to describe the rill erosion process;Erosion data were collected on eighteen cropland soils in twelve states in the western part of the US during the 1987 USDA Water Erosion Prediction Project. Simulated rainfall and rill flow additions were applied to six preformed rills at each site, and erosion rates were calculated;Mathematical techniques were developed to include the effects of previously eroded suspended sediment on reducing erosion rates. A simplified relationship between sediment transport capacity and hydraulic shear in the form of T[subscript] c = B[superscript]'[tau][superscript]1.7 was derived, and a soil transport coefficient, B[superscript]', based on eroded aggregate size distributions was found for each soil;A general linear model technique was used to determine which rill flow hydraulic properties best accounted for variation in calculated detachment capacity. It was found that both hydraulic energy and Reynold's number better predict the calculated detachment capacities than did hydraulic shear. An equation was proposed in the form D[subscript] c = K[subscript] e (E[subscript] p - E[subscript] pc) where D[subscript] c = detachment capacity, K[subscript] e = flow energy erodability coefficient, E[subscript] p = potential energy of rill flow per unit area, and E[subscript] pc = the critical flow energy below which no erosion occurs. This equation was fitted to data from nine soils and the constants K[subscript] e and E[subscript] pc, were calculated for each soil;Regression techniques were used to relate measurable soil properties to the soil erodibility constants. The soil properties that best described erodibility were particle specific surface area, the water content of the soil at -33 kpa soil moisture potential, organic carbon, the clay content, Wischmeier's M, bulk density, and the cation exchange capacity.