Geographically Isolated Wetlands and Hydrologic Connectivity: Quantifying Seasonal and Annual Downstream Effects using a Hybrid Modeling Approach

Geographically isolated wetlands (GIWs) are distinguishable as depressional landscape features that are entirely surrounded by uplands. This nomenclature does not necessarily imply functional isolation because these systems may exhibit a gradient of hydrologic, biological, or chemical connections to other surface waters. In this study, we examine the influence of GIWs on streamflow, a potential indicator of GIW hydrologic connectivity with surface waters. Our method involves the Spatial Stream Network (SSN) model, which considers spatial autocorrelation of model covariates explicitly, and the Soil and Water Assessment Tool (SWAT), which predicts streamflow across a network of 579 subbasins in the lower Neuse River Basin, North Carolina, USA. Our study results suggest that GIWs influence streamflow, though the effect is limited. The farther GIWs are from a stream, the greater their capacity to increase streamflow due to the physiographic setting and hypothesized connectivity, transit times and sequencing of watershed hydrologic connectivity in the study area across seasonal and annual scales. However, as the combined extent of GIWs and non-GIWs increases in subbasins, seasonal and annual streamflow decreases. Results also suggest that other landscape indicators of watershed-scale hydrology, when coupled with wetland features on the landscape, explain variations in seasonal and annual simulated streamflow. Our study findings begin to elucidate the aggregate influence of GIWs on streamflow, providing insights for future decision-making on GIW protection and management.