Bimodal Hydrographs in Semi-humid Forested Watershed: Controlling Factors and Generation Mechanism 

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. This study delves into the mechanisms behind delayed stormflow generation in a mountainous forested watershed within the semi-humid regions of North China. We assess the influence of soil water content and groundwater levels on the threshold behavior of delayed stormflow. Results indicate that the threshold behavior of the bimodal hydrograph is jointly controlled by soil water storage and groundwater levels, with soil water storage serving as the initiating factor for delayed stormflow. The groundwater replenishment and subsequent rise in groundwater level, crucial for the delayed stormflow, occur specifically when the soil water storage reaches 200 mm amid rainfall. At this point, shallow groundwater flow is promptly mobilized, swiftly moving into the channel and leading to the initiation of delayed stormflow. Notably, upon reaching a specific threshold groundwater level, each hillslope responds almost simultaneously, establishing a more extensive hydrological connectivity between the hillslopes and the stream channel. A substantial volume of shallow groundwater is released within a day, resulting in a hybrid bimodal hydrograph. These findings can enhance our understanding of the groundwater stormflow generation mechanism in semi-humid forest watersheds and contribute to the refinement of related runoff generation theories.