Evaluating the performance of the Flash Flood Guidance System: A Case study from Himachal Pradesh, India

Abstract

Flash floods are among the most challenging hydro-meteorological hazards to forecast due to their rapid onset, short hydrological response times, and limited warning, particularly in mountainous and densely populated regions. These challenges are being amplified by climate change through increases in short-duration intense rainfall. India is highly vulnerable to such extremes, especially across the Western Himalayan region, where complex terrain and strong monsoon interactions further exacerbate flood risk. To strengthen flash flood preparedness, the India Meteorological Department, in collaboration with the World Meteorological Organization and the Hydrologic Research Center, operationalized the South Asia Flash Flood Guidance System (SAsiaFFGS) in October 2020. The system integrates real-time hydrometeorological observations and numerical weather prediction outputs to generate Flash Flood Risk (FFR) at daily scales and Imminent and Persistent Flash Flood Threats (IFFT, PFFT) within short-term nowcasting windows at the watershed level. This study evaluates the performance of SAsiaFFGS during the extreme rainfall and flash flood event of July 2023 over Himachal Pradesh, a representative Western Himalayan state characterized by steep terrain and short hydrological response times. System generated risk and threat indices are assessed against observed rainfall, soil moisture evolution, and satellite-derived runoff. Results indicate that SAsiaFFGS successfully captured the spatial and temporal evolution of flash flood conditions, providing actionable lead time for operational early warning.