Assessing the cumulative impacts of geographically isolated wetlands on watershed hydrology using the SWAT model coupled with improved wetland modules

Authors: LEE, S. - University Of Maryland; YEO, I.-Y/ - University Of Newcastle; LANG, M.W. - Applied Ecological Services, Inc; Sadeghi, Ali; McCarty, Gregory; Moglen, Glenn; EVENSON, G.R. - Virginia Polytechnic Institution & State University

Submitted to: Journal of Environmental Management
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 6/2/2018
Publication Date: 6/7/2018
Citation: Lee, S., Yeo, I., Lang, M., Sadeghi, A.M., McCarty, G.W., Moglen, G.E., Evenson, G. 2018. Assessing the cumulative impacts of geographically isolated wetlands on watershed hydrology using the SWAT model coupled with improved wetland modules. Journal of Environmental Management. 223:37-48. https://doi.org/10.1016/j.jenvman.2018.06.006.
DOI: https://doi.org/10.1016/j.jenvman.2018.06.006

Interpretive Summary: Numerous studies have been reported on the impacts of riparian wetlands on downstream water flow and quality, but little is known about the impact of geographically isolated wetlands. To demonstrate the hydrological impacts of isolated wetlands on the downstream flow, we compared two model scenarios of Soil and Water Assessment Tool (or SWAT) model; one including all wetlands that connect to the stream network during the seasonal inundation and the other excluding all the connected wetlands. The study site was Tuckahoe Creek watershed, a sub-watershed located on the Coastal Plain of the Chesapeake Bay watershed. Our model simulation results indicated that geographically isolated wetlands served as important landscape features to control watershed hydrology. Further, based on our findings, we conclude that isolated wetlands exert significant impacts on maintenance of upstream hydrology and downstream flow for this region. Thus, the conservation and management of the geographically isolated wetland is critical for increasing the hydrological resilience of downstream flow, especially with respect to the expected climatic variability and extremes.

Technical Abstract: Despite recognizing the importance of wetlands in the Coastal Plain of the Chesapeake Bay Watershed (CBW) in terms of ecosystem services, our understanding of wetland functions has mostly been limited to individual wetlands and overall catchment-scale wetland functions have rarely been investigated. This study is aimed at assessing the cumulative impacts of wetlands on watershed hydrology for an agricultural watershed within the Coastal Plain of the CBW using the Soil and Water Assessment Tool (SWAT). We employed two improved wetland modules for enhanced representation of physical processes and spatial distribution of riparian wetlands (RWs) and geographically isolated wetlands (GIWs). This study focused on GIWs as their hydrological impacts on watershed hydrology are poorly understood and often underestimated. Multiple wetland scenarios were prepared by removing all or portions of the baseline GIW condition indicated by the U.S. Fish and Wildlife Service National Wetlands Inventory geospatial dataset. We further compared the impacts of GIWs and RWs on downstream flow (i.e., streamflow at the watershed outlet). Our simulation results showed that GIWs strongly influenced downstream flow by altering water transport mechanisms in upstream areas. Loss of all GIWs reduced both water routed to GIWs and water infiltrated into the soil through the bottom of GIWs, leading to an increase in surface runoff of 9% and a decrease in groundwater of 7% in upstream areas. These changes resulted in increased variability of downstream flow in response to extreme flow conditions. GIW loss also contributed to reducing inter-monthly variability of downstream flow and an increase in the baseflow contribution to streamflow. Loss of all GIWs was shown to cause a greater fluctuation of downstream flow than loss of all RWs for this study site, due to a greater total water storage capacity of GIWs. Our findings indicate that GIWs play a significant role in controlling hydrological processes in upstream areas and downstream flow and, therefore, protecting GIWs is important for enhanced hydrological resilience to extreme flow conditions in this region.