Development and Application of a Time-Based Standardized Torrential Flood Index via Modified Inter-Amount Time (M-IAT)

As torrential flood-inducing heavy rainfall intensifies under climate change, new indicators for quantifying short-term precipitation concentration are essential. This study introduces the Modified Inter-Amount Time (M-IAT), which measures the duration required to reach critical precipitation thresholds, and develops the Standardized Torrential Flood Index (STFI) using Generalized Extreme Value (GEV) and Generalized Pareto Distribution (GPD) models. Analysis of 65 ASOS stations (1990–2024) shows that as critical rainfall values (CV) increase, the GPD model evaluates extreme temporal concentration more conservatively than the GEV model. Validation against 39 historical flood events revealed that the GPD-STFI median reached 3.72 (99.99th percentile) during actual damage occurrences, effectively identifying extreme risks. Conversely, the GEV-STFI established stable long-term and structural risk baselines for different regions. The STFI facilitates a paradigm shift from precipitation-centered forecasting to dynamic, hydrological response-time-centered warnings. This study presents an integrated risk management strategy by combining design-oriented GEV models with operation-oriented GPD models, providing a robust framework for flood mitigation.