A CONNECTION BETWEEN TOPOGRAPHICALLY DRIVEN RUNOFF GENERATION AND CHANNEL NETWORK STRUCTURE

Runoff generation can be modelled using a topographic wetness index; t he fraction of the catchment where the wetness index exceeds some thre shold is assumed to be saturated. We show that distributions of a topo graphic index can share a common form across many catchments, particul arly for sufficiently large catchments. This leads us to speculate tha t the topographic structure of drainage networks provides a unifying t heme for runoff generation. We identify those catchments which share t he common distribution of topographic index, and label them ''organize d.'' Visual examination of maps of this index and of subcatchment area indicates that the organized catchments are those which have a ''prop er'' network: The branching structure provides an orderly increase in subcatchment area as one proceeds downstream. We consider that the pre sence of a ''properly'' branched network is the essence of ''organizat ion.'' It has previously been observed that catchments can have a powe r law distribution of subcatchment area: our data show that the distri butions of wetness index and subcatchment area are closely connected f or catchments with a well-developed channel network. Since the distrib ution of subcatchment area is a signature of the channel network, this permits the reparameterization of runoff generation models using chan nel network properties. As a result, the runoff generation model is a truly a catchment-scale model and can no longer be disaggregated to th e point scale. This view of catchment response is closely related to t he representative elementary area concept, where catchment response is ''simple'' for sufficiently large catchments and depends only on catc hment-scale phenomena.