Assessment of Seasonal Streamflow Controls Using SWAT +: Roles of Wetland Connectivity and Storage

Abstract

Wetlands regulate seasonal streamflow in landscapes by storing, redistributing, and gradually releasing water through interconnected hydrologic pathways. However, the relative influence of wetland connectivity and storage capacity on seasonal flow regimes remains poorly understood. In this study, we used SWAT+ to conduct controlled numerical experiments in a wetland‐rich, forested coastal‐plain watershed in northern Florida. Progressive reductions in modelled Hydrologic Response Unit (HRU)‐wetland routing interactions, representing changes in wetland connectivity within the SWAT+ framework (RC‐25, RC‐50, and NO‐WET), and independent storage‐enhancement scenarios (HS‐25 and HS‐50) were implemented to evaluate their relative influence on seasonal streamflow. Model performance was evaluated against monthly streamflow observations for 1999–2023, and hydrologic responses were assessed using seasonal water yield, seasonal water‐balance diagnostics, and flow‐duration curves. Reductions in HRU‐wetland routing interactions produced substantially larger hydrologic responses than storage enhancement. Annual water yield declined by approximately 20%–35%, with the largest impacts occurring during summer and autumn. Partial routing reduction decreased warm‐season water yields by approximately 15%–30%, while complete removal of HRU‐wetland routing interactions reduced summer water yield by more than 50% and low‐flow metrics (Q95) by up to 80%. Seasonal water‐balance diagnostics indicated that these responses were primarily associated with reductions in wetland outflow and surface runoff rather than changes in evapotranspiration. In contrast, storage‐enhancement scenarios altered seasonal water yield by less than 5% and produced only minor changes in outlet‐scale flow behaviour. These findings suggest that modelled wetland connectivity exerted a stronger influence on seasonal streamflow than comparable increases in wetland storage capacity. The results further indicate that the watershed‐scale hydrologic influence of wetlands depended not only on their capacity to store water but also on the degree to which stored water remained connected to the stream network through hydrologic routing pathways.