*2.2. Soil Erosion and Sediment Transport Model*

The sediment algorithm is adapted from the CASC2D-SED model, which is a fully distributed, physically based hydrological and upland erosion model developed at Colorado State University. CASC2D-SED is capable of reproducing hydrograph and sediment graph at different grid scales [22]. The code is converted from C to Fortran, and then incorporated intoWRF-Hydro as an independent module.

On watershed scale, the annual gross erosion includes upland erosion, gully erosion as well as local bank erosion [13]. In the model, three sediment size groups—sand, silt, and clay—are included. Each sediment group can be presented in three states: suspended, deposited, or in bed layer. The bed layer represents the original soil layer and thus serves as the source of the sediment to the model domain. Once eroded from the bed layer, the sediment becomes suspended. After settling down, it goes into deposited.

As shown in Figure 2, sediment is first eroded from and transported through overland area by surface runoff. At the beginning of each time step, the total transport capacity is calculated with the revised USLE [23] over upland area. With the transport capacity, the suspended sediment is transported first, followed by the deposited part. At last, if there is still transport capacity left (excessive transport capacity), it will be used to erode the bed layer. During these processes, both the wash load and bed load transportation are considered. The different treatment of transport of the fine materials as wash load and coarse materials as bed load are achieved mainly by assigning different settling velocities to different sediment size groups.

In the channel, the sediment is carried by streamflow through the river network delineated in the model, meanwhile settling and resuspension processes are calculated. According to the Engelund and Hansen (1967) equation [24], the transport capacity is calculated for each sediment group separately in the channel. With the transport capacity, the suspended part of each sediment group is transported first by advective process. Then, bed material will be transported with excessive capacity. Channel erosion is not considered by the model yet. The different treatment of fine materials and coarse materials is accomplished by the differences in transport capacity as well as settling velocity between sediment groups.

Once sediment is transported from the "source grid (where the sediment comes from)" to the "sink grid (where the sediment goes to)", all the transported materials will stay in suspension first and then settle down at assigned settling velocities. By the end of each computational step, the deposited sediment is added to the original layer and the net accretion/erosion is updated.
