Integrating "Endurance" into Groundwater Resilience: Quantifying the True Buffer Capacity of Aquifer Systems During Droughts

Under escalating impacts of climate change, the frequency and intensity of hydroclimatic extremes, particularly prolonged droughts, pose a severe threat to global groundwater security. As aquifer systems serve as the primary buffer during droughts, accurately quantifying their resilience under unprecedented stress is essential for ensuring sustainable water availability and ecosystem stability. However, existing resilience methodologies are predominantly based on "Engineering Resilience", focusing strictly on the recovery rate of an aquifer after a disturbance. This approach leads to a misleading paradox: fractured, rocky aquifers are often characterised as "highly resilient" simply because they exhibit rapid hydraulic rebound compared to alluvial aquifers, despite their inability to sustain supply during the stress period itself. This recovery-centric view ignores the critical role of "Endurance" (Ecological Resilience), the qualitative capacity of a system to buffer shocks and resist state shifts during active drought events. To bridge this gap, this study proposes the Endurance-Recovery-Resilience (ERR) Framework. Our primary objective is to operationalize "Endurance" as a quantifiable metric alongside recovery, thereby capturing the "True Resilience" or buffer capacity of the aquifer system. The universal applicability of the ERR framework is evaluated through a comparative analysis of heterogeneous aquifer systems across two continental-scale domains: the Ganga River Basin (India) and major US Aquifer Systems. We contrast the drought response of extensive unconsolidated sedimentary basins (Gangetic Plain, High Plains, Central Valley) against fractured crystalline and basaltic aquifers (Bundelkhand/Vindhyan, Columbia Plateau, Piedmont) to test the framework's validity across diverse hydrogeological settings. The results reveal a fundamental divergence in system behavior. While rocky aquifers demonstrate high engineering resilience (rapid recovery), they exhibit critically low endurance, failing rapidly under drought stress. Conversely, alluvial systems demonstrate "True Buffering Capacity" (High Endurance), successfully maintaining hydraulic heads during extreme events, although they are prone to poor recovery trajectories during prolonged droughts. We conclude that resilience cannot be defined by recovery speed alone. By integrating Endurance, the ERR framework corrects the "rocky aquifer paradox," providing a robust tool for decision-makers to identify region-specific vulnerabilities. This highlights that water security strategies must differentiate between protecting the limited buffer of rocky systems and managing long-term depletion in high-endurance alluvial basins.