Assessing the impact of groundwater saturation excess runoff on hydrologic features and processes in a watershed modeling setting

Authors: ABBAS, SALAM - Colorado State University; BAILEY, RYAN - Colorado State University; Arnold, Jeffrey; White, Michael

Submitted to: Earth and Space Science Open Archive
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 11/16/2023
Publication Date: 11/16/2023
Citation: Abbas, S.A., Bailey, R.T., Arnold, J.G., White, M.J. 2023. Assessing the impact of groundwater saturation excess runoff on hydrologic features and processes in a watershed modeling setting. Earth and Space Science Open Archive. https://doi.org/10.22541/essoar.170016466.67156311.
DOI: https://doi.org/10.22541/essoar.170016466.67156311

Interpretive Summary: Groundwater saturation excess flow is an important yet often neglected aspect of hydrologic modeling. In this research, a computer model (SWAT+ and the new gwflow) was used to better understand how excess groundwater contributes to streamflow and wetland formation in Georgia's Little River Watershed. Including this excess groundwater as predicted by gwflow into SWAT+ predictions makes the combined model more accurate. Without it, the SWAT+ compensates incorrectly, such as by overestimating water from other sources. The models predicted that the amount of stream water coming from overflowing groundwater can vary significantly from year to year, influenced by the weather and the water already stored in the system. This has important implications for wetlands, as the model's predictions of where groundwater saturation excess flow often occurs match the actual locations of these ecosystems. This suggests that the model could be useful for forecasting how environmental changes might impact wetlands in the future or under differing land use, climate, or management.

Technical Abstract: Groundwater saturation excess flow can be a major surface runoff mechanism in humid regions, characterized by shallow aquifers and soil profiles that become saturated during wet periods or intense storm events. This process often plays an important role in the creation and maintenance of groundwater-dependent ecosystems and the overall water yield of a watershed. In this paper, we examine the process of groundwater saturation excess flow and assess its influence on hydrologic features (wetlands) and temporal patterns of watershed water yield. We do this by applying a surface-subsurface hydrologic model (SWAT+ with the physically based spatially distributed gwflow module for groundwater storage and flow) to the Little River Watershed, Georgia, USA, which has a high baseflow fraction and contains numerous wetlands, and for which groundwater saturation excess flow has been noted in past studies. The model is calibrated and tested against measured streamflow and groundwater head for the period 2000-2015, with and without groundwater saturation excess flow included in the gwflow module. Model results indicate that including groundwater saturation excess flow improves hydrologic estimation, and demonstrate connections between precipitation, recharge, saturation excess flow, and streamflow before and during storm events. Finally, we compare locations of consistent groundwater saturation excess flow (simulated) with mapped wetlands, demonstrating that the model can be used to explore impacts of system changes (land use, climate, management) on wetland development and maintenance.