ABSTRACT
The TOPMODEL concept (Beven and Kirkby 1979, Beven and Wood 1983, O’Loughlin 1986, Ambroise et al. 1996) has been a popular watershed modeling tool (e.g., Anderson et al. 1997, Lamb et al. 1998, Guntner et al. 1999, Scanlon et al. 2000). It is widely used because of its conceptual simplicity of runoff generation, innovative use of topographical data, and demonstrated applicability to a wide variety of situations. In recent years, however, various hydrologists have noted the inappropriateness of TOPMODEL’s conceptual basis to meaningfully describe hydrologically shallow, hilly situations where transient, perched groundwater flow plays a substantial role in runoff generation processes (Moore and Thompson 1996, Woods et al. 1997, Frankenberger et al. 1999, Scanlon et al. 2000). Their observation in forested catchments has suggested the presence of such a storm flow zone perched above low-conductivity layers in the soil or a slowly moving wetting front (Hammermeister et al. 1982a). A transient occurrence of storm flow through the macroporous region of the shallow subsurface may result in the rapid rise of the hydrograph. Data collected from subsurface weirs (Scanlon et al. 2000) showed that this flow occurs quickly enough to contribute to peak stream discharge, and that a greater percentage of precipitation is converted to subsurface flow in the lower hill slopes.
