The Impact Of Forest On Runoff Under Different SpaceScale On The Loess Plateau

The Loess Plateau of China is experiencing extreme water and soil loss and is one of the most severely impacted regions in the world. Serious water and soil loss has deteriorated the ecological environment of the Loess Plateau in China, and greatly restrictedg the social and economic development of the area. To improve the ecological environment and control and prevent water and soil loss in the Loess Plateau, the Chinese government successively launched several major ecological forestry engineering projects in the past decades. Although the revegetation reduced water and soil losses locally, however, it also caused soil desiccation, resulting in a severe imbalance between water supply and demand. The study found that the impact of forest on runoff was different in different spatial scales.In connection with the key scientific issues in the study forest on runoff and the reality of shortage of water resources and the deterioration of the ecological environment in Loess Plateau, the impact of forest on runoff was analyzed under the sloping lands scale, the small watershed scale and the large and medium watershed scale with fixed location observation and semi- fixed location observation for a long time, distributed hydro logical model and published literature to select watershed (≥100km2) located in the Loess area of the Loess Plateau. At the same time, the main influence factor of the runoff under different spatial scales was analyzed with gray correlation method.The main results showed that:(1) The main influence factors of runoff were not the same under the different space scale. The impact of rainfall intensity and canopy density on runoff was the greater in the sloping lands scale, followed by the soil water content before rainfall, rainfall and slope. The impact of rainfall on runoff was the greater in the small watershed scale, followed by watershed area, rainfall intensity, basin shape factor, forest coverage and channel gradient. The impact of forest coverage (45.34%) on runoff was the smaller than precipitation (54.66%) in the large and medium watershed scale.(2) The influence of forest on runoff was not the same under the different space scale.Compared with the pinus tabulaeformis forest, Robinia pseudoacacia forest had higher the canopy interception rate and the evapotranspiration, at the same time, Robinia pseudoacacia forest had lower sloping lands runoff under the condition of same rainfall.In the small watershed scale, for more forest watershed and less forest watershed, the change of forest coverage on the runoff forming process, the annual average runoff and its components, the runoff distribution during the year had a significant effect. The effects of more forest watershed in intercepting runoff and reducing flood peak was more obvious than less forest watershed, and its adjustment effects for runoff during the year was greater. The effects of protected natural forest in reducing surface runoff, increasing base flow, maintaining dry reason runoff were more significantly than the artificial forest watershed. So, the measure of closing hills for afforestation should be carried out to promote the natural recovery of secondary vegetation in small watershed management for the purpose of soil and water conservation in the loess area.Although revegetation could reduce the runoff of the loess area on the Loess Plateau to a certain extent, for the watershed with forest coverage< 25%, the revegetation would not have a large impact on local water resources, while for the watershed with forest coverage≥ 25%, the mean annual runoff coefficient decreases significantly as forest coverage increases.The results establish the theoretical foundation for in-depth study of hydrological scale conversion, and could provide a theoretical basis and data support for water resources security in Loess Plateau, china, and macro strategic decisions of sustainable development of the regional forestry.