Integrated isotopic-hydrological method reveals karst plateau reservoir leakage dynamics in urban water systems

The karst plateau drinking water source is situated in southwestern China. The complex karst hydrogeology of the study area, characterized by heterogeneous fracturing, presents significant challenges for accurately quantifying lake and reservoir leakage. To address these challenges, we developed an integrated framework combining stable isotope (δD and δ¹⁸O), the water balance equation, and partial least squares-structural equation modeling (PLS-SEM) to account for evaporation effects, estimate karst reservoir leakage, and identify its driving mechanisms. Seasonal isotopic variations exhibit distinct patterns, with enriched δD and δ¹ ⁸O values during the rainy seasons and depleted values in the dry seasons, along with transitional mixtures during seasonal shifts. The evaporation line equation confirms strong evaporative fractionation effects. The integrated isotopic-hydrological method provides precise leakage quantification, estimating annual leakage at 10.4 × 10⁶ m³ , with 1.1 % of total reservoir capacity, which closely aligns with the reservoir's engineering design specifications, showing only a 0.1 % deviation. PLS-SEM identifies reservoir capacity as the primary control on leakage (β = 0.772), while humidity and precipitation exert secondary influences, highlighting the role of atmospheric interactions. These findings offer a transferable methodology for karst water management, providing quantitative benchmarks to address climate-induced hydrological stresses globally. • Spatiotemporal isotopic decoupling reveals rapid hydrological renewal. • Coupled isotopic- hydrological modeling achieves leakage quantification. • Capacity-driven thresholds govern karst leakage nonlinearity. • Anthropogenic regulation disrupts precipitation-leakage coupling.