Catchment‐scale runoff generation model by aggregation and similarity analyses

Runoff generation in natural catchments due to storm rainfall is highly complex and spatially and temporally heterogeneous. In recent work on seven small experimental catchments Larsen et al. (1994) showed that underlying the heterogeneity of runoff generation within the catchments, there is a degree of regularity between the catchments that could be quantified in terms of two dimensionless similarity parameters K₀* and f*. These two parameters, constants for a catchment, were able to characterize the relative dominance of the saturation excess (Dunne‐type) and infiltration excess (Horton‐type) mechanisms of runoff generation. Given that K*₀ and f* can characterize the type of runoff generation on any catchment, it may follow that they can be used to define a catchment‐scale runoff generation model. This idea is pursued in this paper. For the same catchments as studied by Larsen et al. (1994), a lumped, physically based model is developed that describes both the extent of saturated areas and the average infiltration capacity of the unsaturated areas during a storm. This is achieved by utilizing the distributed model used by Larsen et al. (1994) to aggregate the point‐scale runoff generation responses, up to the catchment scale, from which the functional form and the parameters of the catchment‐scale runoff generation model are inferred. The parameters of this lumped model are defined entirely in terms of the underlying distribution of topography, three similarity parameters K*₀, f* and B*, the normalized average water‐table depth, z*, and the normalized cumulative volume of infiltration, G*.