INITIAL CONTRIBUTING AREA OF A SMALL WATERSHED

The initial contributing area (ICA) of a watershed at the onset of rain is defined as consisting of the stream surface and the area of surface saturation caused by the groundwater table intersecting the land surface above the elevation of the stream. Subsurface flow discharges upward within this area, so the ICA exhibits zero infiltration capacity and potential storage for rainfall. Thus, the ICA is the initial runoff-producing area in variable-source-area generation of storm runoff. The ICA of a 42-ha east-central Pennsylvania watershed is shown to be a function of prestorm baseflow; a prediction equation is developed using rainfall-runoff data from small rainstorms. This equation could be incorporated directly into a continuous streamflow model where a baseflow value is part of the simulation, or into an event-based hydrograph model where prestorm baseflow is specified as a design parameter. © 1990.