Preparing for the next drought: delineating safe groundwater reserves in post-subsidence cities

Cities recovering from land subsidence face a critical water-security dilemma: how to safely use groundwater in emergencies—such as prolonged droughts—without reactivating unrecoverable subsidence. Focusing on post-subsidence Tianjin, we develop a hydro-geomechanical framework that defines subsidence-safe pumping limits. By quantifying a regional-average intrinsic vertical hydraulic gradient (IVHG) in the confined aquifer system, we establish an IVHG-governed continuous aquifer model that replaces the traditional layered model. We then map the new pre-consolidation head (NPCH)—the geomechanical inelastic-elastic threshold—and derive the safe pumping buffer (SPB). By synthesizing these point-based thresholds, we delineate ten management zones, each with a representative set of NPCH and SPB values. For each zone, the SPB is converted to a safe pumping volume (SPV) by multiplying the zone’s areal extent and the aquifer’s elastic storage coefficient (S ke ). As of 2025, Tianjin’s city-scale SPV is about 0.16 km³ based on a regional-average S ke of 2.5 × 10⁻³ and is projected to about 0.3 km³ by 2028. The IVHG–NPCH–SPB–SPV framework thus provides post-subsidence cities with a transferable approach to convert recovering aquifers into strategically managed, subsidence-safe reserves for drought resilience.