The spatial and temporal distribution of soil surface characteristics is one of the key variables which influences overland flow hydraulics and soil erosion. Soil erosion by water involves the detachment of particles and their subsequent transportation by runoff. The two more important processes in sheet and rill erosion are rainfall and overland flow. During a rainfall event the detachment and transport of sediment is strongly dependent on the spatial and temporal distribution of overland flow hydraulics. However, knowledge of these space-time dynamics remains poor.
The main objective of this presentation is to show the possibilities of the two dimensional numerical models PRIM_2D (Plot Runoff and Infiltration Model 2D) and PSEM_2D (Plot Soil Erosion Model 2D) for the prediction of infiltration, hortonian overland flow hydraulics and soil erosion. PRIM_2D and PSEM_2D are two dimensional event model which are based on an explicit finite difference scheme coupling overland flow, infiltration, rainfall and hydraulic erosion and sediment transport/deposition processes for hillslopes. The hillslopes are represented by topographic elevations, soil hydraulics and soil erosion parameters. Infiltration is computed using a Green-Ampt model and overland flow using the depth-averaged two-dimensional unsteady flow equations (Saint Venant equations). Soil erosion is computed by combining the conservation equation of the sediment mass and a relation that relates sediment load to the flow transport capacity based on the shear stress approach. Sediment transport is defined as a purely advective process, dispersion is not considered. At this stage of development of the model we assume that the sediment concentration in runoff is sufficiently low so that the hydraulics of overland flow are not affected by the suspended sediment and may therefore be represented independently. The sediment are represented by a single particle size.
The model PRIM_2D has been used to simulate runoff production and transfer under contrasted soil surface characteristics (crusted soils) and microrelief conditions. The numerical tests presented illustrate the influence of the spatial variability of the microrelief, the roughness and the infiltration on the distribution of the flow depths, and also the magnitude and the direction of flow velocities. These results served to highlight the role played by the soil surface characteristics on the hydraulics of surface runoff. The performance of PSEM_2D is illustrated by comparison with analytical solution and result obtained from experiments. Some numerical experiments are presented to show the capabilities of the model.