Bimodal hydrographs in a semi-humid forested watershed: characteristics and occurrence conditions

Abstract. Bimodal runoff behavior, characterized by two distinct peaks in flow response, often leads to significant stormflow and associated flooding. Understanding and characterizing this phenomenon is crucial for effective flood forecasting. However, this runoff behavior has been understudied and poorly understood in semi-humid regions. In this study, we investigated the response characteristics and occurrence conditions of a bimodal hydrograph based on the hydrometric and isotope data spanning 10 years in a semi-humid forested watershed in north China. The main findings include that (1) the onset of the bimodal hydrograph exhibits a threshold behavior, with delayed streamflow peaks occurring when the sum of event rainfall (P) and antecedent soil moisture index (ASI) prior to the rainfall exceeds 200 mm; (2) isotopic hydrograph separation reveals that the delayed stormflow process is primarily driven by pre-event water, with increasing contributions of pre-event water during catchment wetting up; and (3) the dynamic variation in groundwater level precedes that of streamflow, establishing a hysteretic relationship wherein the groundwater level peaks before streamflow during delayed stormflow. These findings, supported by on-site observations, emphasize the dominance of shallow groundwater flow in the generation of delayed stormflow.