MODELING RUNOFF FROM VARIABLE SOURCE AREAS IN HUMID,SHALLOW WATER TABLE ENVIRONMENTS1

ABSTRACT: Variable Source Areas (VSAs) are zones with water saturated soils in forested wetlands fringing streams and creeks. Runoff from these areas is generated by saturation excess after a shallow water table rises and inundates the ground surface. In humid regions, like Florida and the Southeast, VSAs are believed to produce most of the runoff in shallow water table environments. Modeling the spatial extent and temporal fluctuation of a VSA is difficult because the formation of a VSA depends on a number of hydrological and morphological factors like rainfall intensity, soil texture, water table depth, and topographic attributes of the terrain. In this paper, we couple a digital elevation model with a two‐dimensional variable saturation model to illustrate the formation of a VSA at the hillside scale. The topography derived from the digital elevation model forms the upper domain geometry for the two‐dimensional finite element simulations of variable saturated flow. The objectives are: (1) to model the spatial and dynamic fluctuation of a VSA, and (2) to understand the roles of rainfall variability and terrain attributes on the formation of a VSA. Results show that hillsides with shallow water table depths, low saturated hydraulic conductivity, mild slopes, and concave slope curvature were more susceptible to runoff from a variable source. Runoff from a variable source showed little sensitivity to rainfall intensity. In general, landscapes with steep slopes generated a small VSA and a seepage face that vanished rapidly with time. In contrast, flat terrains are more amenable to VSA and retain ground surface inundation for longer periods of time.