A Multi-Model Time-Varying Framework for Groundwater Vulnerability Assessment Under Climate Change with the DRASTIC Index

Static groundwater vulnerability assessments assume time-invariant inputs—an assumption hard to defend under climate change, when depth to water and net recharge are precisely the parameters that evolve with the forcing. This study develops a time-varying, multi-model framework for groundwater vulnerability assessment and applies it to the Almyros coastal aquifer (Thessaly, Greece) through the DRASTIC index. Time-varying depth-to-water and net-recharge fields are simulated for a baseline (1991–2018) and two future periods (2031–2060, 2071–2100) using an Integrated Modelling System driven by 19 bias-corrected regional climate model realizations under two emission scenarios (RCP4.5, RCP8.5); the remaining DRASTIC parameters are held time-invariant. Validation at 73 monitoring-well locations against IMS-simulated baseline nitrate concentrations, with the IMS having been calibrated against observed nitrate measurements, yields significant correlations for all 19 realizations (r = 0.569–0.742, p < 0.001). The best-fit realization reveals a non-monotonic trajectory of the combined High and Very High vulnerability area—54.2% at baseline, 36.6% at mid-century, 47.3% by 2071–2100 under RCP8.5. Formal partitioning of the climate-driven variance attributes 97.7–99.6% to inter-model spread and finds the between-scenario share statistically unresolvable, so climate model choice dominates within-scenario uncertainty. Because DRASTIC responds directly to depth to water, the projected index falls where water tables deepen and rises where they are shallow; static assessments, fixed at the baseline, therefore systematically overestimate future vulnerability where water tables decline and underestimate it where they rise. The framework is portable to other index-based methods and delivers climate-aware, empirically constrained projections for adaptive groundwater management.