Contrasting Groundwater Response to Surface Water Shortages in the Upper and Lower Colorado River Basins

Abstract

Groundwater plays a critical role in sustaining the water supply in the Colorado River Basin (CRB), a region experiencing persistent drought, population growth, and increasing water demand. Using GRACE/GRACE‐FO satellite observations combined with hydrological data sets, we analyze basin‐scale variations and long‐term trends in terrestrial and groundwater storage across the CRB from water year 2004–2023. We further apply a joint framework integrating copula‐based statistical modeling with statistical inference to quantify probabilistic relationships among storage‐based drought indicators and to characterize groundwater–surface water interactions. Results reveal sustained basin‐wide groundwater depletion over the past two decades, with accelerated losses since 2019 and during the water year 2020–2022 drought. Notably, during this recent drought period, groundwater losses were larger in the typically more resilient Upper CRB (17.5 ± 3.5 km 3 ) than in the Lower CRB (9.4 ± 6.5 km 3 ), indicating increasing vulnerability across both subbasins. Copula‐based analysis shows strong coupling between surface‐water deficits and groundwater drought probability in the Upper CRB, whereas parts of the Lower CRB exhibit weaker statistical dependence, consistent with groundwater extraction operating partly independently of surface water availability under chronically limited supply conditions. These findings provide a basin‐wide observational synthesis of groundwater–surface water dynamics and demonstrate the value of satellite‐based monitoring for diagnosing hydroclimatic stress in large managed river basins. The results highlight emerging shifts in groundwater vulnerability across the CRB and underscore the need for integrated water‐management strategies that jointly consider climatic pressures and sustained anthropogenic demand.