GRACE Reveals Contrasting Groundwater Responses to Surface-Water Shortages Across the Colorado River Basin

Abstract

Groundwater is a critical component of water supply across the Colorado River Basin (CRB), particularly during prolonged surface-water shortages. We use GRACE/FO satellite gravimetry together with complementary hydrologic datasets to assess groundwater-storage variability across the Upper and Lower CRB from 2004 to 2023 and to quantify relationships between groundwater drought and surface-water deficits.The satellite record shows persistent groundwater depletion across the basin, with losses accelerating after 2019 and during the 2020–2022 drought. During this period, groundwater losses reached 17.5 ± 3.5 km³ in the Upper CRB, exceeding the 9.4 ± 6.5 km³ lost in the Lower CRB despite the Upper Basin’s typically greater hydrologic resilience.Copula-based modeling combined with statistical inference further reveals contrasting groundwater–surface water relationships between the two subbasins. In the Upper CRB, groundwater drought probability increases strongly with surface-water deficits and exceeds 60% under moderate shortages. In contrast, parts of the Lower CRB show weaker statistical dependence, consistent with groundwater extraction operating partly independently of short-term surface-water availability under chronically limited supply.These results highlight distinct regional controls on groundwater vulnerability across the CRB and demonstrate the value of GRACE-based monitoring combined with probabilistic statistical approaches for quantifying the likelihood of groundwater drought under surface-water stress.