Throughout human history, floods have resulted in significant environmental and economic damages globally, and these impacts are expected to increase with climate change. Despite the occurrence of significant floods in Iran in recent decades, the precise impact of intense rainfall and antecedent soil moisture (ASM) on flood occurrence and how this relationship differs at different locations remains an area that requires further investigation. In this study, we analyzed annual maximum floods at 963 catchments across Iran from 1972 to 2019 (47 years). Moreover, we evaluated the relationship between catchment area and the main factor responsible for flood generation. Our analysis reveals a shift in the dominant factor driving flood generation from rainfall to ASM as the catchment area expands. The correlation between the topographic wetness index and the ratio of the relative significance of ASM (S) and daily rainfall (R) (i.e., Soil moisture-to-Rainfall Ratio; SRR = SR.), is positive, while the correlation between the magnitude of annual floods and SRR ratio is negative. A quantitative estimation of potential flood probability in ungauged catchments in Iran is facilitated by establishing a framework based on the relationship between easily measurable catchment attributes and the primary phenomenon of flood generation.
