2.2.1. Overland Sediment Routing

The revised USLE [23] is applied to predict soil loss from upland area (sheet and rill), which is caused by rainfall and associated overland flow. In a single grid, the transport capacity is calculated as follows:

$$T\_{\rm ovrl} = 58390 \times \text{S}\_o^{1.664} \times q^{2.035} \times K \times \text{C} \times P \times dx \times dt \tag{8}$$

where *Tovrl* is the overland transport capacity (L3), here in the model m3, *q* is the unit flow discharge (L2 T−1), in this study m2/s, *K* is the soil erodibility factor, which is in t/acre, *C* is the dimensionless cropping-management factor, *P* is the conservation practice factor, which is dimensionless, *dx* is the grid size (L), which is m in this study, and *dt* is the time step (T), which is in second. In this study, *dx* and *dt* are the same as those used in overland routing. *So* is the water surface slope. The overland sediment transport capacity is used to transport suspended sediment first, and then the deposited sediment if any. After the suspended and deposited sediment in the source grid are transported, if there is still transport capacity left, the remaining capacity will be used to erode the original bed layer.
