Extreme rainfall frequency distribution and its flood risk assessment in the Upper Hanjiang River Basin

The identification of flood hazards resulting from intense rainfall events is a critical task for flood management and disaster mitigation in river basins. This study utilizes extreme intensity and duration theory to analyze the frequency distribution of extreme rainfall and associated flood risk. By introducing equivalent daily rainfall intensity (Rf) and flood-waterlogging intensity index (Pf), a novel approach is proposed to evaluate flood-induced risk through frequency analysis. Results indicate spatio-temporal variability in extreme rainfall within the Upper Hanjiang River Basin, characterized by a higher frequency in the southern region and a lower frequency in the northern region. Rf is on average 33.9% higher than the maximum daily rainfall (RX1day), attributable to its capacity to capture multi-day accumulation effects. After comparing five distribution models, Rf follows the Pearson Type III distribution (P-III) and Generalized Extreme Value distribution (GEV), whereas Pf primarily follows the GEV distribution. Under a 20-year return period, UHRB registers Rf and Pf values of 157.3 mm and 2.14, respectively, with a pronounced south-to-north rainfall gradient featuring two distinct high-intensity cores: one localized in the southwestern mountainous terrain and the other in the northeastern low-mountain zone. Identified middle-high risk areas (25.4%) are concentrated in the region of Zhenba (ZB), Hanzhong (HZ), Shangnan (SN) and Shiyan (SY). The intensification of extreme rainfall events exacerbates flood risks. This study enhances flood risk evaluation methods and offers distribution maps for different-frequency extreme rainfall for the first time, supporting flood control, disaster reduction, water resource management, and basin engineering safety.