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Soil loss from unpaved rural roads contributes largely to watershed sediment yield. A road erosion model is therefore urgently needed to evaluate road erosion production. Field flow simulation experiments were conducted on unpaved loess roads under five flow rates and three slope gradients in order to quantify road surface hydraulics and develop soil detachment prediction models. The results showed that flow velocity was highly related to flow rate but generally independent of slope gradient. The hydraulic roughness of the road surface was much less than that of cropland and was influenced significantly by relatively intensive erosion on steep slopes. Easily obtainable parameters such as flow rate and slope gradient could be used to estimate soil detachment accurately. Furthermore, power functions of chosen hydraulic parameters, (i.e., shear stress, stream power, and unit stream power) could be used to predict detachment rates. Among these parameters, flow shear stress was the best predictor. A detachment prediction model similar to that used in the WEPP model was proposed. Corresponding to the excess shear stress model in WEPP, loess road surface soil erodibility is 0.0066 s m-1 and critical shear stress is 3.25 Pa.