Groundwater storage changes in the United States using baseflow recession method: Comparison with GRACE and well observations

Study region: This study focuses on over 1000 minimally disturbed watersheds across the contiguous United States (CONUS). These free-flowing basins were chosen to minimize human influence and allow natural groundwater–streamflow interactions to be observed and quantified at the national scale. Study focus: Long-term groundwater storage dynamics are quantified by analysing individual baseflow recession events using a novel algorithm applied to two decades of daily streamflow and precipitation records. Groundwater storage changes were estimated using an empirical discharge–storage relationship and then compared with GRACE-DA estimates from GLDAS 2.2 and with storage changes derived from monitoring-well water levels. New hydrological insights for the region: The baseflow recession parameter, drainage timescale (K), exhibits both spatial and temporal variability. Most watersheds show rising low flows due to increased precipitation, while warming and drought-prone regions show declining low flow trends. Baseflow-derived storage trends showed over 75 % agreement in sign with GRACE-DA estimates, while 65 % of watersheds exhibited similar directional trends when compared with well-based storage data. Consistent trends across all three methods were found in 57 % of watersheds. These results show that baseflow recession analysis is a reliable, high-resolution proxy for groundwater storage monitoring. It is especially useful in regions with sparse well-level observation networks or where satellite data are too coarse for small watersheds.