Review on the simulation of overland flow in hydrological models

Overland flow is that part of rainfall which exceeds the infiltration capacity of the soil surface and any depression storage, which usually is assumed to flow downslope as a uniform thin sheet of water. In reality however, overland flow takes place along gullies and rivulets as non-steady non-uniform flow. It is one of the basic processes in the catchment hydrological cycle that has significant roles in processes such as soil erosion, sediment and pollutant transport. In this paper, different approaches of modeling and simulating overland flow including their pros and cons, assumptions and limitations are summarized and analyzed. From the literature survey, it can be concluded that the distributed conceptual approach based on the time-area relationship and linear reservoir has further development potential. In this study, a new approach based on the watershed variable isochrone method that uses the DEM and has the potential to represent the non-linear effects of the runoff generating mechanism is proposed. To implement this approach it is necessary to monitor runoff resulting from urban low impact developments (LID).