There are a number of different definitions of runoff that have been used either explicitly or implicitly in hydrological analyses over the years. In what follows we will use a working definition that runoff is that part of the rainfall falling on a catchment area that eventually leaves the catchment as a surface streamflow, whatever the flow pathway that the water has followed on its way to the stream channel. Thus this definition includes both surface and subsurface runoff pathways. Dunne (1978) provides a review of field studies of surface and subsurface runoff generation processes that remains one of the best summaries available.
For a long time, following the work of Robert Horton in the 1930s, storm runoff was often taken to be equivalent to a purely surface runoff process. Horton suggested that the soil surface acted as a separating surface, between fast (surface) storm runoff processes and slow (subsurface) flow processes contributing to baseflow (Horton, 1933). This concept still underlies a great deal of hydrological analysis even though we now know that this is often not the case and that much of the runoff in a stream channel seen during a storm event may have followed subsurface flow pathways. C. R. Hursh, working at the same time as Horton, was demonstrating the importance of subsurface flow in storm hydrograph generation in the Coweeta catchments in North Carolina (e.g., Hursh, 1944).
The best basis for the analysis of runoff in a catchment is to allow that there may be a spectrum of surface and subsurface flow velocities and path lengths, which must be expected to change with the state of wetness of the catchment area and with the nature and spatial pattern of a rainfall event. In some conditions or locations, subsurface flow processes may dominate runoff generation; in other conditions or locations (even within the same catchment), surface flow process may dominate. Indeed, in
Handbook of Weather, Climate, and Water: Atmospheric Chemistry, Hydrology, and Societal Impacts, Edited by Thomas D. Potter and Bradley R. Colman. ISBN 0-471-21489-2 © 2003 John Wiley & Sons, Inc.
some conditions and locations, subsurface flow may saturate the soil and return to the surface as a return flow, the area of saturation also acting as a dynamic contributing area for surface runoff due to additional rainfall inputs, that will expand and contract as the catchment wets and dries (Fig. 1). Thus, it is only for convenience that, in what follows, we treat subsurface and surface runoff processes in turn.
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