WEPP (Water Erosion Prediction Project), a physical-process-based soil erosion prediction model partitions the hillslope soil erosion into rill erosion and inter-rill erosion, with independent prediction models. The sediment source equation for soil detachment in WEPP is conceptually rational, but yet to be verified experimentally or theoretically. Theoretical analysis determines that the detachment rate equation in WEPP is indeed a linear function of the sediment concentration in runoff. The parameters for the detachment function under given experimental conditions were estimated and were compared with those parameters, directly determined from the experimental data, to validate the WEPP detachment equation. Integration of the differential equation for detachment rate was also used to derive the sediment concentration function of rill length. The estimated parameters of the sediment yield function were compared with those in the sediment-rill length function as determined from experimental data, to further validate the detachment rate model in WEPP. The detachment rate equation in WEPP was validated with both the detachment rate and the sediment yield.