Non-sequential response in Mountainous Area of Southwest China

Non-sequential response, the phenomenon that the water storage change in the lower layer bigger than that of the adjacent upper layer within a set time interval, is often ignored because of lacking of distributed measured data at watershed scale, especially in mountainous area where extensive monitoring network is expensive and difficult to deploy. In this study, the subsurface non-sequential response in a mountainous watershed in Southwest China was investigated, combining field monitoring and numerical simulation. A physics-based numerical model (InHM) was employed to simulate the proportion and position of occurrence of the subsurface non-sequential response. The topographic wetness index (TWI = ln(a/tan b)) was adopted to distinguish the topographic zone corresponding to the non-sequential response occurrence at different depths. The results showed that the storage change in deep layer is not as fast as that in shallow and middle layers due to less disturbance, and the non-sequential response mainly came from the subsurface lateral flow which accumulated at the soil-bedrock interface. During a rainfall event, the shallow soil layer responded rapidly, the saturation increased, the non-sequential response moved from the hillslope zone in the middle layer to the channel zone in the shallow layer, and accumulated gradually at the soil-bedrock interface. In case of double-peak rainfall event, the occurrence proportion of non-sequential response increased, the depth expanded vertically, and the accumulated response shifted from the channel zone to the hillslope zone, which would affect the outlet runoff. Counter to the subconscious, non-sequential response could still happen even when precipitation stopped. The results improve our understanding of non-sequential response and provide a scientific basis for flash flood research in mountainous areas.