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 ter- rain. 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. (KEY TERMS: wetlands; runoff; topography; seepage face.) Hernandez, Tatiana, Mahmood Nachabe, Mark Ross, and Jayantha Obeysekera, 2003. Modeling Runoff From Variable Source Areas in Humid, Shallow Water Table Environments. J. of the American Water Resources Association (JAWRA) 39(1):75-85. INTRODUCTION Variable Source Areas (VSAs) of runoff are formed in forested wetlands fringing thousands of miles of creeks and rivers in the southeastern United States. In these environments, runoff is generated by satura- tion excess after the filling of a thin, unsaturated soil zone above a shallow water table. Runoff from VSAs, also recognized as saturation excess runoff, occurs when the water table rises and intersects the ground surface at a location above the streambed, creating a seepage face (Dunne and Black, 1970a; Govindaraju and Kavvas, 1991; Steenhuis et al., 1995; Boufadel et al., 1999). As the name implies, VSAs vary with space and time – they expand in the wet season and shrink in the dry season, creating the hydroperiod of forested wetlands, the period of time that soils remain saturat- ed during a year (Kautz, 1998). The hydroperiod of forested wetlands is a major environmental factor in determining the community of plant species in a wet- lands (Ewel, 1990). The extent of VSA depends on a number of factors such as rainfall variability, slope, slope curvature, stream incision, and initial water table depth. Earlier investigations by Dunne and Black (1970a) and Freeze (1972) suggested that these areas were the major contributor to runoff in humid environ- ments. For the Sleepers River Experimental Water- shed near Danville, Vermont, Dunne and Black (1970b) found that VSAs are controlled by topography, antecedent water content, soil moisture storage capacity, and rainfall intensity. Freeze (1972) noticed that subsurface stormflow was significant on convex hillsides with deeply incised channels and highly sat- urated hydraulic conductivity. Kirkby (1978) observed that topography with concave slopes had a larger con- tributing source area. 1 Paper No. 02010 of the Journal of the American Water Resources Association. Discussions are open until August 1, 2003. 2 Respectively, Water Resources Engineer, URS Corporation, 7650 West Courtney Campbell Causeway, Tampa, Florida 33607; Assistant Professor and Associate professor, University of South Florida, 4202 East Fowler Avenue, ENG 118, Tampa, Florida 33620; and Director, Hydrologic Systems Modeling, South Florida Water Management District, 3301 Gun Club Road, West Palm Beach, Florida 33406 (E-Mail/Nachabe: nachabe@eng.usf.edu). JOURNAL OF THE AMERICAN WATER RESOURCES ASSOCIATION 75 JAWRA JOURNAL OF THE AMERICAN WATER RESOURCES ASSOCIATION FEBRUARY AMERICAN WATER RESOURCES ASSOCIATION 2003 MODELING RUNOFF FROM VARIABLE SOURCE AREAS IN HUMID, SHALLOW WATER TABLE ENVIRONMENTS 1 Tatiana Hernandez, Mahmood Nachabe, Mark Ross, and Jayantha Obeysekera 2