Multidecadal drought impacts on the Lower Colorado Basin with implications for future management

Overallocation of Colorado River water and groundwater alongside multidecadal drought underscore the need to understand water-resource dynamics. Here we assess water-storage variations using satellites, regional modeling, and monitoring to inform future management. Total water storage loss from Gravity Recovery and Climate Experiment (GRACE) satellites was dominated by Lower Basin declines (80% of total), exceeding Lake Mead capacity by 40%. These Lower Basin storage declines were dominated by groundwater depletion (60% of total), with cumulative depletion hotspots <= 11 m (2002-2023) and subsidence <= 1 m (2010-2024). Regional groundwater modeling shows intensive depletion (1940s-1970s) followed by partial recovery since the early 1980s from irrigation reduction, wet climate cycles (early 1980s-1990s), and Colorado River water transfers to Central Arizona. Managed aquifer recharge and incidental recharge from imported surface-water irrigation led to a 3-m average groundwater-level rise in Central Arizona Active Management Areas (2000-2023). Projected declines in Colorado River water transfers to Central Arizona could lead to further depletion and subsidence. Water transfers from agricultural to municipal/industrial sectors would improve future management. Understanding system dynamics related to climate and human drivers is essential for developing future conjunctive surface-water and groundwater management strategies.