From risk to resilience: Quantifying water security through reliability and robustness analysis with simulation–optimization framework

Study Region Taiwan’s six major municipalities face pronounced wet–dry seasonal contrasts and rely heavily on monsoon-driven inflows to sustain year-round supply. Despite high annual precipitation, the uneven temporal distribution, concentration of storage in a few key reservoirs, and the strong role of agricultural withdrawals create recurrent drought risks. These conditions make the region an effective setting for operationalizing water-security assessment. Study Focus Water security has been widely framed from a global or continental perspective, but applying this broad concept to regional-scale drought management requires more operational clarity. This study develops a context-sensitive analytical framework to address this need. Building on Hashimoto’s (1982) concepts, two indicators are formulated—annual reliability based on sector-specific tolerance thresholds and the maximum annual deficit in percent-days. An integrated simulation–optimization model is developed to quantify these indices, explicitly representing policy-driven allocation priorities. New Hydrological Insights for the Region Results reveal prototypical spatial patterns: frequent but manageable shortages in the north, rare but severe deficits in central Taiwan, and both frequent and severe events in the south. The framework also identifies conditions under which optimal management fails and quantifies the backup water required for drought resilience in reservoir-dependent regions.