Flood process types and runoff coefficient variability in climatic regions of Iran

This study analysed the spatiotemporal variability of runoff coefficients (RCs) in four climatic regions based on 18 468 events recorded in 963 Iranian catchments. Five flood process types were identified using a classification scheme. The results show that winter and spring have higher mean RCs of 0.46 and 0.42, respectively, confirming the role of snowmelt and heavy precipitation in flood runoff generation in these seasons. Event saturation conditions (i.e. event rainfall depth) had a stronger impact on RC variability than pre-event saturation conditions (i.e. antecedent rainfall depth). Flood occurrence varies significantly by season and region, with short rains being the most common type of flooding. Rain-on-snow floods, snowmelt, and long-rain floods had higher RCs than other types, and significant differences in RCs were observed across the four climate regions using the non-parametric Kolmogorov-Smirnov test. The median flood time scale is between 1 and 20 days in all catchments.