River corridor beads are important areas of floodplain-groundwater exchange within the Colorado River headwaters watershed

Authors: SCHULZ, EVAN - Colorado State University; MORRISON, RYAN - Colorado State University; BAILEY, RYAN - Colorado State University; Arnold, Jeffrey; White, Michael

Submitted to: Hydrological Processes
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 9/3/2024
Publication Date: 9/19/2024
Citation: Schulz, E.Y., Morrison, R.R., Bailey, R.T., Arnold, J.G., White, M.J. 2024. River corridor beads are important areas of floodplain-groundwater exchange within the Colorado River headwaters watershed. Hydrological Processes. https://doi.org/10.1002/hyp.15282.
DOI: https://doi.org/10.1002/hyp.15282

Interpretive Summary: Floodplains provide many important ecosystem services, but monitoring data to evaluate them is lacking. The assessment of floodplain function at large scales is best facilitated using models. In this study we paired the SWAT+ (Soil and Water Assessment Tool) model with an updated version of gwflow that includes floodplain interactions. This modeling system was tested in the Colorado River Headwaters basin to see if the addition of floodplain interactions improved model accuracy. We found that the inclusion of these interactions improved streamflow predictions and ground/surface water interactions. This combined model is useful to better inform land conservation policy by indicating priority locations where substantial hydrologic exchange occurs in floodplains.

Technical Abstract: Floodplains are essential ecosystems that provide a variety of economic, hydrologic, and ecologic services. Within floodplains, surface water-groundwater exchange plays an important role in facilitating biogeochemical processing and can have a strong influence on stream hydrology through infiltration or discharge of water. These functions can be difficult to assess due to the heterogeneity of floodplains and monitoring constraints, so numerical models are useful tools to estimate fluxes, especially at large scales. In this study, we use the SWAT+ (Soil and Water Assessment Tool) ecohydrological model to quantify magnitudes and spatiotemporal patterns of floodplain surface water-groundwater exchange in a mountainous watershed using an updated version of the gwflow module that directly calculates floodplain-aquifer exchange rates during periods of floodplain inundation. The gwflow module is a spatially distributed groundwater modeling subroutine within the SWAT+ code that uses a gridded network and physically based equations to predict groundwater storage, groundwater head, and groundwater fluxes. We used SWAT+ to model the 7,516 km2 Colorado River Headwaters watershed and streamflow data from USGS gages for calibration and testing. Models that included floodplain-groundwater interactions outperformed those without such interactions and provided valuable information about floodplain exchange rates and volumes. Our analyses on the location of floodplain fluxes in the watershed also show that wider areas of floodplains, “beads,” exchanged a higher net and per area volume of water, as well as higher rates of exchange, than narrower areas, “strings.” Study results show that floodplain channel-groundwater exchange is a valuable process to include in hydrologic models, and model outputs could inform land conservation practices by indicating priority locations where substantial hydrologic exchange occurs.