Abstract:In recent years, scholars all over the world have realized that subsurface storm flow mechanism is one of the important runoff generation mechanisms in small watersheds in humid mountainous area. It is crucial to further explore the subsurface storm flow mechanism for flash flood simulation. This paper divides the discharge process of subsurface storm flow into three stages: water storage stage, rapid discharge stage and surface discharge stage. In the water storage stage, the precipitation needs to supplement the depression on the bedrock or the relative impermeable layer first. The phenomenon of “large rainfall but small runoff” often occurs in this stage. In the rapid discharge stage, the depression on the bedrock or the relative impermeable layer has filled, the transient saturation area gradually expands, and the pre-event water discharges rapidly. The phenomenon of “small rainfall but large runoff” often occurs in this stage. In the surface discharge stage, the transient saturation area further extends to the ground surface and discharges in the form of surface runoff. Based on this, the water storage and discharge formula is proposed to quantitatively describe the three stages, and the subsurface storm flow-based mountain hydrological model (SSFM) is constructed. Taking Guanshan River basin in Danjiangkou City, Hubei Province as the study area, 12 measured flood processes are selected and simulated by SSFM. Comparing the simulation results with those of TOPMODEL, the distributed time-varying gain model and Xin’anjiang model, it can be seen that: 1) the average peak flow errors of the time-varying gain model (TVGM), Xin’anjiang model, TOPMODEL and SSFM are -29.61%, -51.74%, -29.08%, -24.82%, respectively, and the average runoff depth errors are -30.83%, -26.87%, -18.43%, -9.67%, respectively. 2) The “water storage stage” and “rapid discharge stage” can explain the variation characteristics of “large rainfall but small runoff” and “small rainfall but large runoff” in flash flood process of Guanshan River basin. The results show that due to the introduction of three-stage subsurface storm flow mechanism, the simulation effect of SSFM is obviously improved compared to other models. SSFM is more suitable for flash flood simulation in small watersheds in humid mountain area, and it has potential application to flash flood forecasting and early warning.