37-8 As above, so below? A framework for integrating long-term streamflow and groundwater level trends across the United States

Session: Innovations in Research of Groundwater-Surface Water Interactions over Multiple Spatio-Temporal Scales

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Climate, land use, and other elements of change are driving long-term trends in annual depth to groundwater table and low streamflow. Typically, these trends, when analyzed across large scales, are considered discretely despite the well-established concept that groundwater and surface water comprise a single resource through the lens of water availability (i.e., quantity and quality). Here, we present alternate approaches for pairing and aggregating groundwater level and streamflow data at the national scale for joint trend analysis with the goal of better understanding how these major water budget components respond simultaneously and interactively to various drivers. First, individual Mann-Kendall trends were computed for 21-year (2000-2020), 31-year (1990-2020), and 41-year (1980-2020) periods for annual mean groundwater level and annual 7-day low streamflow records across the United States. These results were initially compared broadly across the U.S., with no regional aggregation, for increasing, decreasing, or stable trends. Then, these individual trends were gathered by region using several different spatially contiguous and non-contiguous classifications based on subsurface (i.e., aquifer, geology) and surface (i.e., topography/watershed boundary, landscape factors) characteristics. Correlation between individual trends and factors such as watershed fraction within a given region, median depth to groundwater, and baseflow fraction were calculated to identify important variables. Finally, Regional Kendall analyses were performed for groups of sites aggregated by the regional classifications. Results identify areas where groundwater level and low streamflow trends are responding similarly, potentially indicating well-connected surface and subsurface resources. Results also highlight regions where trends are opposing in direction or significance, suggesting potential differences in resilience or vulnerability to change in the groundwater system vs. the stream network. The approach and findings developed within this study provide a basis for additional driver attribution work and for more holistic water quantity and quality management. 

Geological Society of America Abstracts with Program. Vol. 57, No. 6, 2025

doi: 10.1130/abs/2025AM-8869

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