Vegetation and topographic influences on the connectivity of shallow groundwater between hillslopes and streams

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.