Vegetation and topographic influences on the connectivity of shallow groundwater between hillslopes and streams Ryan E. Emanuel, 1 * Anna G. Hazen, 2 Brian L. McGlynn 3 and Kelsey G. Jencso 4 1 Department of Forestry and Environmental Resources, North Carolina State University, Campus Box 8008, Raleigh, NC 27695, USA 2 Department of Geology, Appalachian State University, ASU Box 32067, Boone, NC 28607, USA 3 Division of Earth and Ocean Sciences, Nicholas School of the Environment, Duke University, Box 90227, Durham, NC 27708-0227, USA 4 Department of Forest Management, The University of Montana, 32 Campus Drive, Missoula, MT, 59812, USA ABSTRACT Little is known about the combined effects of vegetation and topography on hillslope water table dynamics. In forested headwater catchments, complex terrain and vegetation intersect to impose large spatial and temporal variability in the vertical and lateral redistribution of water from hillslopes to streams. Here, we demonstrate, using empirical data from the Northern Rocky Mountains, that vegetation interacts with landscape topography to influence hillslope–riparian–stream hydrologic connectivity. We compared a measured relationship between hillslope contributing area and hydrologic connectivity during the growing season to LiDAR-derived vegetation characteristics and found that two behavioural regimes emerged. Among some hillslopes, hydrologic connectivity decreased as vegetation density increased, demonstrating that growing season hydrologic connectivity is subject to the balance between evapotranspiration and lateral redistribution of soil water. Among other hillslopes, hydrologic connectivity increased as vegetation density increased. For the latter set of hillslopes, hydrologic connectivity cannot be explained by topography and vegetation alone. When we compared joint distributions of vegetation density and modelled solar irradiance between the two regimes as another indicator of evapotranspiration, we found that conditions were generally more favourable for higher transpiration on hillslopes where hydrologic connectivity decreased as vegetation density increased than on hillslopes where the opposite behaviour was observed. Our results demonstrate not only the importance of vegetation heterogeneity for hillslope–riparian–stream connectivity but also the importance of other spatially distributed variables such as energy availability when considering the influence of topography on hydrological processes. Copyright © 2013 John Wiley & Sons, Ltd. KEY WORDS hillslope; hydrology; vegetation; topography; connectivity; insolation Received 21 July 2012; Revised 2 June 2013; Accepted 10 June 2013 INTRODUCTION Topographic and vegetation influences on the soil water balance Understanding how topography and vegetation jointly affect hydrological processes in forested watersheds is a continuing challenge in hydrology. Topographic effects on hydrological processes have been studied intensively for several decades leading to conceptual frameworks such as the geomorphic instantaneous unit hydrograph (Shreve, 1969; Rodriguez-Iturbe and Valdes, 1979) and topographic similarity (Beven and Kirkby, 1979; Burt and Butcher, 1985). These conceptual frameworks and much empirical evidence suggest that topographic heterogeneity serves, at least in part, as a template for predicting the behaviour of a range of hydrological responses including soil water redistribution (Western et al ., 1999), streamflow response (Jencso et al., 2009; Jencso and McGlynn, 2011; Nippgen et al., 2011) and connectivity between hillslopes and streams (Genereux et al., 1993; Chen and Kumar, 2001; Jencso et al., 2009; Jencso and McGlynn, 2011). This study provides empirical evidence demonstrating that vegetation, coupled with topography, influences the behaviour of hydrological processes within forested watersheds. Vegetation is known to influence point, hillslope and watershed scale hydrological processes. Plants alter the soil water balance through processes that include transpiration (Rodriguez-Iturbe, 2000) and interception (Keim et al., 2006). These influences propagate through the subsurface and watershed network, especially during the growing season, to affect groundwater recharge (Gribovszki et al., 2010) and streamflow (Wondzell et al., 2007). Recent work using observations of stream discharge has suggested topographic and vegetative controls on watershed scale *Correspondence to: Ryan E. Emanuel, Department of Forestry and Environmental Resources, North Carolina State University, Campus Box 8008, Raleigh, NC 27695, USA. E-mail: ryan_emanuel@ncsu.edu ECOHYDROLOGY Ecohydrol. (2013) Published online in Wiley Online Library (wileyonlinelibrary.com) DOI: 10.1002/eco.1409 Copyright © 2013 John Wiley & Sons, Ltd.