Delayed Stormflow Generation in a Semi-humid Forested Watershed Controlled by Soil Water Storage and Groundwater

Abstract. An analysis by Cui et al. (2024) of stormflow responses to rainfall in a mountainous forested watershed in the semi-humid regions of North China identified a distinct threshold for bimodal rainfall-runoff events, where delayed stormflow appeared to be influenced by shallow groundwater. This study further investigates the processes driving these bimodal events, focusing on the dynamics of soil water content (SWC) and groundwater level (GWL) during storm events. The results show that delayed stormflow is governed by the interplay between SWC and GWL. Delayed stormflow is initiated when SWC exceeds the soil’s water storage capacity, while its timing and volume are determined by GWL fluctuations. During rainfall, SWC increases rapidly; if it does not reach the soil's water-holding capacity, it stabilizes after the rainfall ends. Conversely, if SWC surpasses the soil's storage capacity, it decreases rapidly post-rainfall, with the excess rainwater infiltrating deeper to recharge groundwater, leading to a gradual rise in GWL. As GWL rises, increased hydraulic conductivity facilitates the movement of shallow groundwater into the stream channel, resulting in delayed stormflow. Simultaneously, the effective connection area between the stream channel and adjacent hillslopes expands vertically. At specific high GWL thresholds, GWL responses across the watershed converge, significantly increasing groundwater discharge and reducing lag times, often causing the delayed stormflow peak to merge with the direct stormflow peak. These findings enhance our understanding of delayed stormflow generation in similar regions and contribute to refining runoff generation theories.