California's Drought Has Now Caused Irreversible Damage to a Crucial Aquifer

SignificanceGroundwater extraction can causeboth reversible and irreversibleground deformation, with thelatter leading to permanent landsubsidence and loss of ground-water storage capacity. Satelliteobservations of land surface de-formation reveal that large areasof the Sacramento Valley abruptlytransitioned from reversible toirreversible compaction over the2020–2022 California drought—atransition not anticipated from thegroundwater monitoring networkalone.

Subsidence rates reachingseveral decimeters per yearand not following groundwaterlevels indicate lastingdamage to the aquifer systemand underscore the need formore conservative groundwatermanagement. By imaging thissharp transition at the regionalscale, our analysis demonstratesthe potential of space-basedmonitoring for early detectionof groundwater overdraftand the resulting loss of aquiferstorage capacity worldwide.Groundwater extraction decreases water pressure in aquifer systems, causing reversibleor irreversible deformation of the water-bearing layers that manifests as recoverable orpermanent displacements of the land surface, respectively.

Detecting and forecastingwhen and where an aquifer system transitions from a reversible, poroelastic regime, toan irreversible, inelastic regime remains a crucial challenge given the complex, hetero-geneous nature of aquifer systems. Here we leverage high-resolution measurements ofground deformation and groundwater levels from 2016 to 2022 to characterize bothregimes at the regional scale and show that a critical transition occurred in large areas ofthe Sacramento Valley during California’s 2020–2022 extreme drought.

Our analysisreveals that, while deformation remained primarily poroelastic during the 2016–2020 interdrought period, land subsidence in areas of intense groundwater extractionaccelerated abruptly in 2021, with subsidence rates exceeding the inferred poroelasticrates by several decimeters per year. Such rapid and extensive land subsidence indicatessevere inelastic compaction and loss of storage capacity of the underlying aquifersystem, which pose a serious threat to California’s water resources and infrastructure.

A comparison of present-day deformation with historical groundwater levels revealsthat this abrupt transition was not predictable based on the available groundwaterrecords alone.groundwater | subsidence | drought | geodesyAs global demand for groundwater intensifies in response to climate change, its abilityto act as a buffer against surface water variability becomes increasingly critical (1). Agrowing concern, however, is that intensive exploitation of groundwater resources candamage aquifer systems and hence threaten future groundwater availability (2). Removingwater stored in the pores of an aquifer system stresses its solid matrix since pressurizedwater helps support the weight of overlying materials (3). The system can respond tothis stress perturbation either elastically or inelastically. In the poroelastic regime, water-bearing pores deform elastically: They contract as fluid pressure decreases during netgroundwater discharge and expand when fluid pressure increases during recharge (4),causing no permanent damage to the porous rock matrix.

Current theories imply thatwhen fluid pressure reaches a historic low during intensive groundwater extraction, the system might enter the inelastic regime in which pores permanently contract, leading to irreversible compaction, reduction of storage capacity and permeability that may impede aquifer recharge (5, 6). While poroelastic deformation results in reversible displacementsof the Earth’s surface, inelastic deformation leads to permanent land subsidence (Fig. 1A).Measurements of surface deformation over exploited aquifer systems provide a way tomonitor groundwater management practices and mitigate risks including infrastructure damage, increased flood vulnerability and other socioeconomic consequences of land subsidence (7).

Over the last two decades, space-based geodetic techniques like Global NavigationSatellite Systems (GNSS) and Interferometric Synthetic Aperture Radar (InSAR) have revolutionized aquifer monitoring by enabling the measurement of surface deformation with unprecedented spatial coverage, decametric resolution, and millimeter/y accuracy.

Measurements of surface deformation provide depth-integrated insight into the complex mechanical response of heterogeneous aquifer systems to internal pressure changes (Fig.1A). These observations complement basin-scale estimates of groundwater mass changes(8, 9) inferred from the Gravity Recovery and Climate Experiment and Follow-On(GRACE/-FO) satellite missions (10, 11), as well as local-scale in-situ hydrogeophysical(12, 13) and hydrological monitoring (14). Geodetic techniques have been used toPNAS 2026 Vol. 123 No. 31 e2526041123 https://doi.org/10.1073/pnas.2526041123 1 of 11