An equation for evaluating the sediment transport capacity of overland ยฏow is a necessary part of a physically based soil erosion model describing sediment detachment and transport as distributed processes. At ยฎrst, for the hydraulic conditions of small-scale and large-scale roughness, the sediment transport capacity relationship used in the WEPP model is calibrated by Yalin and Govers' equation. The analysis shows that the transport coecient K t depends on the Shields parameter, Y, according to a semi-logarithmic (Yalin) or a linear (Govers) equation. The reliability of the semi-logarithmic equation is veriยฎed by Smart's, and Aziz and Scott's experimental data. Then the Low's formula, whose applicability is also proved by Smart's, and Aziz and Scott's data, is transformed as a stream power equation in which a stream power coecient, K SP , depending on Shields parameter, slope, sediment and water-speciยฎc weight, appears. A relationship between transport capacity and eective stream power is also proposed. Finally, the inยฏuence of rainfall on sediment transport capacity and the prediction of critical shear stress corresponding to overland ยฏow are examined.