A connection between topographically driven runoff generation and channel network structure

Runoff generation can be modelled using a topographic wetness index; the fraction of the catchment where the wetness index exceeds some threshold is assumed to be saturated. We show that distributions of a topographic index can share a common form across many catchments, particularly for sufficiently large catchments. This leads us to speculate that the topographic structure of drainage networks provides a unifying theme for runoff generation. We identify those catchments which share the common distribution of topographic index, and label them ''organized.'' Visual examination of maps of this index and of subcatchment area indicates that the organized catchments are those which have a ''proper'' network: The branching structure provides an orderly increase in subcatchment area as one proceeds downstream. We consider that the presence of a ''properly'' branched network is the essence of ''organization.'' It has previously been observed that catchments can have a power law distribution of subcatchment area: our data show that the distributions of wetness index and subcatchment area are closely connected for catchments with a well-developed channel network. Since the distribution of subcatchment area is a signature of the channel network, this permits the reparameterization of runoff generation models using channel network properties. As a result, the runoff generation model is a truly a catchment-scale model and can no longer be disaggregated to the point scale. This view of catchment response is closely related to the representative elementary area concept, where catchment response is ''simple'' for sufficiently large catchments and depends only on catchment-scale phenomena.