Structural vulnerability regimes of water supply systems under climate change: A robustness-based analysis using multi-dimensional sustainability indicators

Climate change is expected to intensify hydrological variability and extreme events, increasing the vulnerability of basin-scale multi-sector water supply systems. Conventional assessments commonly summarize vulnerability using failure probability, providing limited insight into how failure-prone conditions are organized across system states. This study develops a multi-dimensional sustainability framework to characterize structural vulnerability regimes in the Seomjin River Basin, Korea. Natural runoff and water allocation are simulated using a coupled Soil and Water Assessment Tool (SWAT) and Korea-Water Evaluation and Planning system (K-WEAP) framework to evaluate domestic, industrial, agricultural, and instream flow sectors under future climate scenarios. A Classification and Regression Tree model combined with bootstrap resampling estimates conditional failure probability across a two-dimensional space defined by variability-based and risk-based sustainability metrics. The resulting fields reveal sector-specific vulnerability structures, including a compound-stress regime for domestic supply, a threshold-driven regime for agriculture, and a broadly distributed vulnerability basin for instream flow. Density weighting indicates that expected failure exposure (EFE) is highest for instream flow and agriculture (0.52 and 0.49, respectively) and substantially lower for domestic supply (0.07). Exceedance area curves further show that vulnerable regions can persist across broad probability ranges before contracting at higher thresholds. These results demonstrate that water-system vulnerability depends on the organization, exposure, and persistence of failure-prone states, rather than on failure probability alone.