Abstract Understanding baseflow dynamics is essential for sustainable ecosystem and water resource management. While most studies have documented climate‐baseflow relationships at regional scales, observational evidence of how groundwater influences baseflow dynamics across different spatiotemporal scales remains limited, substantially constraining mechanistic understanding. To address this gap, we conducted a systematic continental‐scale analysis of groundwater‐baseflow linkages across 18 major US watersheds from 1980 to 2020. We analyzed daily discharge from 7,016 stations and estimated baseflow using a recursive digital filter. We also identified 470 paired shallow groundwater well‐gauge sites within 5‐km proximity to evaluate hydraulic connectivity across multiple time scales. Key findings were as follows: (a) Annual baseflow trends revealed pronounced east‐west spatial clustering, with predominantly increasing trends in eastern watersheds and decreasing trends in western watersheds. (b) Seasonal analysis indicated strong synchronicity between groundwater level fluctuations and baseflow dynamics, with matched trend directions from 68.75% (fall) to 92.86% (summer) of paired sites. Spearman correlations further demonstrated robust positive seasonal relationships ( ρ = 0.686–0.791, all p < 0.005). (c) At the monthly scale, linear regression and Spearman correlation analyses showed weak associations between baseflow and climate variables ( R 2 = 0.02–0.31), but substantially stronger associations with upstream groundwater levels ( R 2 = 0.61–0.95). (d) At the daily scale, strong associations between baseflow dynamics and groundwater levels ( R 2 = 0.63–0.95) further support direct hydraulic connectivity between groundwater and river systems. Collectively, these findings indicate that groundwater is the dominant control on baseflow temporal variability across multiple timescales, whereas climate influences may emerge at annual or longer time scales due to groundwater travel times and long system memory. Our results underscore the importance of groundwater monitoring for improving baseflow prediction and water resources management.
Al‐Aizari et al. (Wed,) studied this question.