How does the relationship between soil moisture and runoff evolve with increasing urbanization?

Urbanization changes the rainfall-runoff relationship mainly by increasing catchment imperviousness. This increase questions the importance of antecedent soil moisture in runoff generation in urbanized catchments, which are composed of a mix of rural and urban surfaces. Using a large sample of 70,227 events in 852 United States catchments, we quantified the relative importance of total impervious area (TIA) and antecedent soil moisture (ASM) in determining the runoff ratio (RR) at the event scale. To this end, we grouped the events into 30 classes of TIA and 30 classes of ASM, and we applied linear regression models to analyze the evolution of RR depending on TIA or ASM. The key findings are as follows: (1) Compared to event characteristics (precipitation amount and intensity), ASM showed a stronger ability in predicting the event-scale RR. (2) Even in catchments with high TIA, ASM still played an important role in runoff generation. (3) The impact of urbanization on the relationship between RR and ASM was relatively gradual, but became significant as TIA exceeded 5%. These empirical results are beneficial for advancing the process-based representation of the rainfall–runoff relationship at the scale of an urbanized catchment. They are potentially useful to revise the widely applied methods in practical urban hydrology by elucidating the interplay of ASM and TIA in influencing the catchment response.