Hydrologic Response In A Humid Steep Mountainous Watershed In The West Of China

China is a mountainous country with2/3of the land being mountainous area. The flash flood disasters, affected by the East Asian monsoon climate and steep topography and other factors, are frequent, and caused huge losses. In recent years, the study of flash flood has become one of the core contents of disaster reduction and prevention in China. Flash floods usually occur in the small watershed, which area is smaller than200km2. With the comprehensive effect of the weather, topography, vegetation and land use conditions, the hydrological response of mountainous watershed shows complex temporal and spatial distribution characteristics. As a result, reasonable representation of watershed hydrological response is of great significance not only to scientific research, but also in improving the study of disaster reduction and prevention technology.The flash floods in the humid western mountainous watershed of our country are frequent. The2008Wenchuan earthquake resulted in great surface damage leading to more flash floods and debris flow disaster.This paper selects the typical humid mountainous watershed, Longxihe watershed, locating in the upstream of the Yangtze River in the southwest of China, as the study area. Hydrological observation and measurement as well as simulation are conducted in this area. In addition, the hydrological response mechanism of watershed characteristicsbetween different watershed scales and the influence of earthquake are discussed. The main conclusions are as follows:(1) The abnormal phenomenon between hydrological response mechanism and runoff coefficient for small watershed. It is found that the Dunne overland flow and subsurface flow are dominant in the runoff, and both occur mainly in the channel and adjacent areas. The rainfall on the slope is mostly infiltrated and formed as shallow subsurface runoff, converging to the channel at last. The proportion of shallow subsurface runoff in the total runoff is relatively large, for example, it is up to69% during the late rainy season, while the proportion usually decreases with enhanced rainfall or the decreasing of initial soil moisture content. And this mechanism of hydrological response can well explain the anomalies that the runoff coefficient reaches to3.29in Jianpinggou catchment (sub-catchment of Longxi River watershed with area of3km2) during the observation time period.(2) The nonlinear jump behavior of peak runoff-rainfall for small watershed. Two nonlinear relationships are observed in this study. One is that when the runoff is dominated by shallow subsurface runoff, the power function can be used to represent the nonlinear surge law; the other is two linear function with different slope can be employed to fit the peak runoff-rainfall relationship, in other word, there exists a threshold. When that threshold is reached, then the runoff will increase sharply and the Dunne overland flow will be dominant in that case. Moreover, the rainfall threshold keeps on changing with different initial conditions. The greater the initial soil moisture content, the smaller the threshold.(3) The dependence of space scale for small watershed’s hydrological response characteristics. It reveals that hydrological response characteristics are tightly dependent on the space scale after comparing the situation in Longxi River watershed and other tributaries under the same rainfall and other initial conditions. The runoff coefficient decreases with the increasing of the catchment area, but increases as rainfall increases. And it is more sensitive to rainfall for smaller catchment area, that is,with the same rainfall increase, the increment of runoff coefficient in small catchment is greater than that in large catchment.(4) The influence to flash flood owing to the2008Wenchuan earthquake. The changes of underlying surface condition induced by Wenchuan earthquake result in significant flood characteristics change, especially at the local area. Take Jianpinggou catchment as an example, due to changes in the underlying surface, the peak flow after the earthquake is generally higher than the flow before earthquake about7%under the same rainfall, the flood under once-in-75-years rainfall condition after-earthquake is approximately equivalent to that of once-in-100-years rainfall condition before-earthquake; the flood of once-in-140-years rainfall after earthquake approximately equals to that of once-in-200-years rainfall condition before-earthquake. And the increased peak flow is enhanced with the increase of rainfall. When the rainfall increase from once-in-10-years to once-in-200-years, the increment for the latter situation is almost1.5times as that for the former one.This study, fundamentally showing the hydrological response mechanism and characteristics of small watershed in west humid mountainous areas and further deepening the understanding of hydrology for flash flood in small watershed, can efficiently provide scientific support for forecasting of flash flood.