Event-Scale Limits of Reservoir Flood Moderation: Peak Attenuation, Storage Saturation, and Capacity Size-Dependent Constraints Across India

Reservoir regulation reshapes high-flow dynamics, yet the event-scale limits of flood moderation remain poorly quantified at national scales. We develop a framework to compare naturalized (NAT) and regulated (DAM) high-flow responses for 223 Indian reservoirs by coupling Noah-MP runoff with vector-based mizuRoute routing (2000–2022). High-flow events are identified using scenario-specific P99 thresholds and evaluated at the inflow peak (T) and the subsequent recession day (T+1) to quantify peak attenuation and the storage states that determine remaining flood buffer. Reservoirs reduce peak magnitudes relative to NAT conditions across all storage classes, from large (>1,000 MCM) to small (<1,000 MCM) systems. However, T+1 diagnostics reveal a consistent limitation: reservoirs exit most events nearly saturated, with a median post-event storage of 99.8 % of capacity and 74 % of events exceeding 90 % capacity. The T+1 storage fraction-attenuation relationship exhibits a near-zero slope across storage sizes, indicating that larger reservoirs attenuate more but still approach full capacity, while smaller reservoirs saturate even more rapidly. Instances where strong attenuation coincides with appreciable storage headroom are rare (<5 %). These findings highlight a national-scale constraint: Indian reservoirs effectively moderate individual peaks but rapidly expend available storage buffer, increasing vulnerability to multi-peak or persistent inflow sequences.