ABSTRACT: This study evaluates a conceptual model developed for riparian zones in Ontario, Canada, that links landscape hydrogeological characteristics to riparian ground water hydrol- ogy and nitrate removal efficiency. Data from a range of riparian sites in the United States and Europe suggest that the riparian zone types identified in the model are consistent with patterns of riparian hydrology and nitrate flux and removal in many humid temperate landscapes. These data also support the view that a riparian width of less than 20 m is often sufficient for effective nitrate removal unless riparian sediments are coarse grained or nitrate transport occurs mainly in surface-fed ground water seeps. This study assesses the possibility of using topographic, soil, surficial geology, and vegetation maps to determine landscape attributes linked by the model to ripari- an zone hydrological functioning and nitrate removal efficiency. Although mappable data can help in determining broad classes of riparian zones, field visits are necessary to determine non- mappable riparian attributes such as seeps, organic horizons, and permeable sediment depth in the riparian zone. This research suggests that the conceptual model could be used for landscape management purposes in most temperate land- scapes with minor modifications and that the hydrological com- ponent of the model could be adapted for contaminants other than nitrate. (KEY TERMS: riparian zones; nitrate; landscape hydrogeology; streams; ground water.) Vidon, Philippe G. and Alan R. Hill, 2006. A Landscape-Based Approach to Estimate Riparian Hydrological and Nitrate Removal Functions. Journal of the American Water Resources Association (JAWRA) 42(4):1099-1112. INTRODUCTION Research on the hydrological and biogeochemical functioning of riparian zones has greatly improved our understanding of their potential in the regulation of stream water quality (Haycock et al., 1997; Hill, 2000). Although riparian zones generally represent a small area of the drainage basin, they are critical con- trol points for nonpoint source nitrogen fluxes within the watershed (Peterjohn and Correll, 1984; Gilliam, 1994; Hill, 1996; Rosenblatt et al., 2001). Most studies indicate nitrate removal rates in subsurface flow of at least 90 percent; however, nitrate reduction of only 44 to 48 percent has been observed in riparian sites in Maryland and Virginia (Correll et al., 1997; Snyder et al., 1998). Other studies have also shown that nitrate- rich ground water may flow under riparian zones and discharge to streams (Bohlke and Denver, 1995; Burt et al., 1999). Research therefore indicates that nitrate removal in riparian zones can be quite variable. Recent research has shown that climate and vege- tation cover are not critical for controlling nitrate removal variability in humid temperate riparian zones (Burt et al., 2002; Clement et al., 2002; Hefting et al., 2004). Rather, research indicates that nitrate removal in riparian zones is influenced mainly by landscape hydrogeological characteristics (Correll, 1997; Lowrance et al., 1997; Devito et al., 2000; Hill et al., 2000; Vidon and Hill, 2004a). Several water- shed and riparian zone cross section types have been identified based on depth to a confining layer and depth of channel incision (Correll, 1997). A confining 1 Paper No. 05085 of the Journal of the American Water Resources Association (JAWRA) (Copyright © 2006). Discussions are open until February 1, 2007. 2 Respectively, Assistant Professor, Department of Geology, Indiana University-Purdue University, Indianapolis, 723 West Michigan Street, SL130, Indianapolis, Indiana 46202-5132; and Professor, Department of Geography, York University, 4700 Keele Street, Toronto, Ontario, Canada, M3J 1P3 (E-Mail/Vidon: pvidon@iupui.edu). JOURNAL OF THE AMERICAN WATER RESOURCES ASSOCIATION 1099 JAWRA JOURNAL OF THE AMERICAN WATER RESOURCES ASSOCIATION AUGUST AMERICAN WATER RESOURCES ASSOCIATION 2006 A LANDSCAPE-BASED APPROACH TO ESTIMATE RIPARIAN HYDROLOGICAL AND NITRATE REMOVAL FUNCTIONS 1 Philippe G. Vidon and Alan R. Hill 2