Runoff Production and Flood Frequency in Catchments of Order n: An Alternative Approach

A simple physically-based hydrological model is derived that takes account of the effect of spatial heterogeneities of topography and soil on runoff production. Both infiltration excess and saturation excess mechanisms of runoff production are simulated by the model. The model is computationally inexpensive and has been used to derive flood frequency characteristics for three small catchments by simulating hydrographs during a 100 year record of randomly generated rainstorms. Interstorm calculations are carried out analytically. For the range of parameter values studied it was found that all the maximum annual flood peak distributions are of extreme value 1 (Gumbel) type. The normalized distribution functions (growth curves) are remarkably similar over all the parameter sets considered. Runoff production in flood events for all the simulations was dominated by the saturation excess mechanism, even assuming very high hydraulic conductivities, and even where infiltration excess runoff is predicted as occurring over part of the catchment. It appears to be difficult to avoid surface saturation under the wet conditions associated with floods in a climatic regime typical of upland Britain.

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