A new physically-based spatially-distributed groundwater flow module for SWAT+

Authors: BAILEY, RYAN - Colorado State University; BIEGER, KATRIN - Texas Agrilife Research; Arnold, Jeffrey; Bosch, David

Submitted to: Hydrology
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 10/6/2020
Publication Date: 10/9/2020
Citation: Bailey, R.T., Bieger, K., Arnold, J.G., Bosch, D.D. 2020. A new physically-based spatially-distributed groundwater flow module for SWAT+. Hydrology. 7(75). Article 7040075. https://doi.org/10.3390/hydrology7040075.
DOI: https://doi.org/10.3390/hydrology7040075

Interpretive Summary: The Soil and Water Assessment Tool (SWAT+) watershed model is used by USDA for national conservation policy assessment and across the US by EPA for environmental assessments. However, the groundwater component in SWAT is simplistic and not capable of simulating detailed groundwater pumping and surface-groundwater interaction scenarios. In this study, we developed a new physically based, spatially distributed groundwater flow module for the SWAT+ watershed model. The model accounts for recharge to the aquifer, groundwater pumping, evapotranspiration from the aquifer, and surface-groundwater interactions through the stream. The model was applied and validated on the ARS Little River Experimental Watershed (LREW) (327 km2) in southern Georgia, showing reasonable agreement with measured streamflow. The new groundwater flow module increased SWAT+ run time by only 20% which is considerably less than most spatially distributed groundwater models, making it feasible to run for national assessments. The linked model also allows detailed assessment of groundwater pumping and the impact on surface and ground water supplies needed to develop national water policy.

Technical Abstract: Watershed models are used worldwide to assist with water and nutrient management under conditions of changing climate, land use, and population. Of these models, the Soil and Water Assessment Tool (SWAT) and SWAT+ are the most widely used, although their performance in groundwater-driven watersheds can sometimes be poor due to a simplistic representation of groundwater processes. The purpose of this paper is to introduce a new physically-based spatially distributed groundwater flow module called gwflow for the SWAT+ watershed model. The module is embedded in the SWAT+ modeling code and is intended to replace the current SWAT+ aquifer module. The model accounts for recharge from SWAT+ Hydrologic Response Units (HRUs), lateral flow within the aquifer, Evapotranspiration (ET) from shallow groundwater, groundwater pumping, groundwater–surface water interactions through the streambed, and saturation excess flow. Groundwater head and groundwater storage are solved throughout the watershed domain using a water balance equation for each grid cell. The modified SWAT+ modeling code is applied to the Little River Experimental Watershed (LREW) (327 km2) in southern Georgia, USA for demonstration purposes. Using the gwflow module for the LREW increased run-time by 20% compared to the original SWAT+ modeling code. Results from an uncalibrated model are compared against streamflow discharge and groundwater head time series. Although further calibration is required if the LREW model is to be used for scenario analysis, results highlight the capabilities of the new SWAT+ code to simulate both land surface and subsurface hydrological processes and represent the watershed-wide water balance. Using the modified SWAT+ model can provide physically realistic groundwater flow gradients, fluxes, and interactions with streams for modeling studies that assess water supply and conservation practices. This paper also serves as a tutorial on modeling groundwater flow for general watershed modelers.