Spatial variability and temporal changes in inter-catchment groundwater flow across the contiguous United States

The assumption of a closed water balance in catchments is central to many hydrological applications. However, emerging evidence suggests that catchments can lose water to surrounding areas, extending their influence beyond topographic boundaries. Understanding whether a topographic catchment acts as a groundwater importer or exporter—and how these roles evolve temporally—is crucial for effective water resource management. Previous studies using the water balance approach often relied on long-term averages and single catchments, limiting insight into temporal dynamics. This study examines inter-catchment groundwater flow (IGF) using precipitation, evapotranspiration, and discharge data from 685 gauging stations across the contiguous United States for the period 1981–2020. By employing a moving window averaging approach, we quantified IGF variability across 330 subcatchments formed by neighboring gauging stations. To enhance robustness, three independent evapotranspiration datasets and three window intervals (10, 15, and 20 years) were examined. IGF was derived as a residual term in the water balance equation, where positive IGF indicates a losing catchment and vice versa. We found that more than 60% of the subcatchments exhibit clearly increasing water losses over the study period. The median value of the annual increase of IGF has been quantified up to 0.36% of annual discharge. In this contribution, we will further explore drivers of the observed spatial variability in IGF using five time-invariant catchment descriptors (e.g., topographic characteristics, distance to the coast) and three time-variant descriptors (e.g., land use changes, precipitation seasonality) to understand possible controls of groundwater interactions. Comparison against the Budyko framework and case studies informed from the existing literature will support the reliability of our findings. This study offers new insights into the dynamic behavior of IGF and its drivers in the gauged subcatchments, advancing the understanding of groundwater interactions and informing sustainable water management practices.