SWAT+MODFLOW: A New hydrologic model for simulating surface-subsurface flow in managed watersheds

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

Submitted to: Geoscientific Model Development
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 6/4/2025
Publication Date: 9/5/2025
Citation: Bailey, R.T., Abbas, S.A., Arnold, J.G., White, M.J. 2025. SWAT+MODFLOW: A New hydrologic model for simulating surface-subsurface flow in managed watersheds. Geoscientific Model Development. 18:5681-5697. https://doi.org/10.5194/gmd-18-5681-2025.
DOI: https://doi.org/10.5194/gmd-18-5681-2025

Interpretive Summary: A new hydrologic model was developed that combines SWAT+ and MODFLOW to simulate surface and subsurface water movement. SWAT+ focuses on processes in the landscape, like soil, channels, and reservoirs, while MODFLOW models groundwater and its interactions with surface features. The two models are connected through GIS tools that link SWAT+ objects to MODFLOW grid cells, allowing accurate mapping of water flows like recharge, irrigation, and groundwater exchange. The model is tested in two regions with irrigation management: the Arkansas River Basin in Colorado and the San Joaquin River Basin in California. This tool can help in identifying better strategies for water resources management in arid areas.

Technical Abstract: Coupled surface-subsurface hydrologic models are used worldwide to study historical patterns of water storage and hydrologic behaviour, investigate the impact of management strategies on water resources, and quantify the impact of changing climate, population, and policies. This study presents a new hydrologic model to simulate surface and subsurface in a physically based spatially distributed manner by linking the popular SWAT+ and MODFLOW modelling codes. Within this new code, SWAT+ simulates processes in the landscape, soils, channels, and reservoirs, whereas MODFLOW simulation groundwater processes and interaction with land surface features (soil, channels, canals, reservoirs, tile drains). Geographic connections between SWAT+ objects and MODFLOW grid cells are established a priori using a GIS and then read into the code to be used throughout the simulation to map hydrologic fluxes (recharge, soil water transfer, groundwater-channel exchange, canal seepage, tile drainage outflow, groundwater-reservoir exchange, pumping for irrigation) on a daily time step. The use and general accuracy of the model is demonstrated for two study regions that are subject to irrigation management: the Arkansas River Basin in Colorado and the San Joaquin River Basin in California. An accompanying tutorial and example model data allow for easy use of the model to other study regions. As both SWAT+ and MODFLOW are widely used worldwide for watershed and groundwater modelling, we expect that this new tool can be an important asset in many water resources projects.