Ecosystem processes at the watershed scale: hydrologic vegetation gradient as an indicator for lateral hydrologic connectivity of headwater catchments
Lateral water flow in catchments can produce important patterns in water and nutrient fluxes and stores and also influences the long-term spatial development of forest ecosystems. Specifically, patterns of vegetation type and density along hydrologic flow paths can represent a signal of the redistribution of water and nitrogen mediated by lateral hydrologic flow. This study explores the use of emergent vegetation patterns to infer ecohydrologic processes and feedbacks in forested headwater catchments. We suggest a hydrologic gradient of vegetation density as an indicator of lateral connectivity within headwater catchments. We define the hydrologic vegetation gradient (HVG) as the increase of normalized difference vegetation index per unit increase of the topographic wetness index. HVG are estimated in different headwater catchments in the Coweeta Hydrologic Laboratory using summer IKONOS imagery. We use recession slope analysis with gauge data and a distributed ecohydrological model to characterize the patterns of seasonal flow regimes within the catchments. Correlations between HVG, catchment runoff, early recession parameters, and model parameters show the interactive role of vegetation and lateral hydrologic connectivity of systems in addition to climatic and geomorphic controls. This suggests that HVG effectively represents the level of partitioning between localized water use and lateral water flow along hydrologic flow paths, especially during the growing season. It also presents the potential to use simple remotely sensed hydrologic vegetation gradients as an indicator of lateral hydrologic connectivity to extrapolate recession behavior and key model parameters of distributed hydrological models for ungauged headwater catchments.