HYDROLOGICAL PROCESSES SCIENTIFIC BRIEFING Hydrol. Process. 16, 2041 – 2046 (2002) Published online in Wiley InterScience (www.interscience.wiley.com). DOI: 10.1002/hyp.5030 Refined conceptualization of TOPMODEL for shallow subsurface flows M. Todd Walter, 1 * Tammo S. Steenhuis, 1 Vishal K. Mehta, 1 Dominique Thongs, 2 Mark Zion 2 and Elliot Schneiderman 2 1 Department of Biological and Environmental Engineering, Cornell University, Ithaca, NY 14853-5701 2 New York City Department of Environmental Protection Division of Water Supply Quality and Protection, Kingston, NY 12401, New York City *Correspondence to: M. Todd Walter, Senior Research Associate, Department of Biological and Environmental Engineering, Cornell University Ithaca, NY 14853-5701, USA. E-mail: mtw5@cornell.edu Abstract The TOPMODEL framework was used to derive expressions that account for saturated and unsaturated flow through shallow soil on a hillslope. The result- ing equations were the basis for a shallow-soil TOPMODEL (STOPMODEL). The common TOPMODEL theory implicitly assumes a water table below the entire watershed and this does not conceptually apply to systems hydrologi- cally controlled by shallow interflow of perched groundwater. STOPMODEL provides an approach for extending TOPMODEL’s conceptualization to apply to shallow, interflow-driven watersheds by using soil moisture deficit instead of water table depth as the state variable. Deriving STOPMODEL by using a hydraulic conductivity function that changes exponentially with soil moisture content results in equations that look very similar to those commonly associ- ated with TOPMODEL. This alternative way of conceptualizing TOPMODEL makes the modelling approach available to researchers, planners, and engi- neers who work in areas where TOPMODEL was previously believed to be unsuited, such as the New York City Watershed in the Catskills region of New York State. Copyright  2002 John Wiley & Sons, Ltd. Key Words TOPMODEL; STOPMODEL; distributed hydrological model; soil moisture; interflow; shallow subsurface flow; saturation excess; variable source areas Introduction The ‘TOPMODEL’ concept (Beven and Kirkby, 1979; Beven and Wood, 1983; O’Loughlin, 1986; Ambroise et al., 1996) is currently a popular watershed modelling tool (e.g. Anderson et al., 1997; Lamb et al., 1998; G¨ untner et al., 1999; Scanlon et al., 2000). It is popular because of its deceptive simplicity, clever use of geomorphology, and demonstrated applicability to a wide variety of situations. Even so, various hydrologists have noted the inappropriateness of TOPMODEL’s conceptual basis to describe, meaningfully, hydrologically shallow, hilly situations where transient, perched groundwater flow plays substantial roles in controlling watershed hydrology (Moore and Thompson, 1996; Woods et al., 1997; Frankenberger et al., 1999; Scanlon et al., 2000). Given the large, global distribution of such systems and the ubiquity of TOPMODEL’s use, it is interesting that the model has been so successful (Anderson et al., 1997). Even more to the point, many published studies clearly apply TOPMODEL to shallow, perched interflow-driven systems and the results are often good (Hornberger et al., 1985; Ambroise et al., 1996; Copyright  2002 John Wiley & Sons, Ltd. 2041 Accepted 27 April 2002