Coupled surface runoff and subsurface flow model for catchment simulations

A distributed catchment scale numerical model for the simulation of coupled surface runoff and subsurface flow is presented. Starting from rainfall (evaporation) records, the model first determines the infiltration (exfiltration) rates in the soil, by evaluation of the soil field capacity at the specific conditions as calculated from the three-dimensional solution of the variably-saturated groundwater flow model (Richards' equation). The flow rate that remains or returns to the surface, the so called overland flow, is then routed via a diffusion wave surface runoff model based on a Muskingum-Cunge scheme with variable parameters. Both hillslope and channel flow are described, and a special algorithm is used for the simulation of pool/lake effects on stormflow response. The importance of including detailed subsurface flow descriptions in catchment simulations is shown in a simple test case characterized by the presence of a central depression.