Soil Erosion Control Mechanism And Modeling Of Forest For Water Resource Conservation In Miyun Reservoir Watershed Area In Beijing

Relationships between vegetation and soil erosion control are a challenging focus of forestry ecology and play important roles in drinking water source protection, ecological environment improvement and sustainable development in the Miyun reservoir watershed area in Beijing. This paper is to focus on the soil erosion control mechanisms of forests for water resource conservation through hierarchical and three scale spatial analysis and evaluation and a soil erosion control model based on clearly defined vegetation parameters was developed and soil erosion spatial patterns for typical rainfall events were simulated using GIS. The impacts of two representative water conservation forest stands (Pinus tabulaeformis Carr.and Robinia pseudoacacia L.) on surface water runoff, sedimentation, rainfall interception, raindrop intensity, and surface roughness were studied via long-term in-situ observations at the Beizhuang Forest Station of Miyun County over 1991-2000, associated with laboratory experiments and comprehensive observations. Data were measured at seven runoff plots (three of them being Platycladus orientalis plantation, the two of them being comparative small watersheds with mainly Robinia pseudoacacia plantation and clear-cutting and the other two being comparative small watersheds with Robinia pseudoacacia and non-forest land). This thesis is a contribution to the National Ninth Five-Year-Plan key project "Integrated Techniques and Demonstration of Forests for Water Resource Conservation in Mountainous Areas of North China"and to the Sino-German Forestry Technical Cooperation Project "Protection and Management of the Watershed of the Miyun Reservoir". Its main objective is to clarify the effects of forests on the soil erosion process in three plots different in size and thus to help to explain the soil erosion control mechanisms of forests in general. It further aims to provide more theoretical background for an improved management of the water conservation forests in the watershed area of the Miyun Reservoir. The main materials and methods are:. Canopy functions: The capacity of Pinus tabulaeformis Carr. and Robnia pseudoacacia L..stands to intercept rainfall and to reduce raindrops'intensity of influence on the soil are discussed, based on forest population investigations and hydrological experiments. The functions of two stands in their ability to intercept rainfall, slow down the surface water runoff and reduce soil erosion were measured, surface roughness coefficients and the methods to determine them in these two stands are presented. Runoff generation and sedimentation on the three plots are analyzed, soil erosion models based on the given vegetation parameters are developed. Spatial patterns of soil erosion in the experimental watersheds were simulated by accepted soil erosion models and integrated with GIS under typical rainfall events in 1996-1998. Research Results This study puts forward the conceptual model of rainfall interception in Pinus tabulaeformis Carr. and Robinia pseudoacacia L.stands with the canopy cover as an independent variable. It explains the soil erosion control mechanisms the canopy layer has by reducing the effect of rainfall amount and the "raindrop energy and by prolonging the rainfall duration based on the systemic analysis The capacity of absorbing and intercepting rainfall for three litters is in study by the various experimental methods, measurement methods of surface roughness coefficients were presented and further the soil erosion control mechanism of litters were illustrated. After ten years of field surveys and analysis, this thesis presents runoff effects according to two study scales (0.9hm2,2.7hm2) in the forest and non-forest areas within watershed area of the Miyun Reservoir. The results show that the runoff rates of two land types logarithmically increased with the rainfall depth increased. The runoff rates in the non-forest areas are higher than in the aforestated areas. Furthermore, the difference of runoff rates between them at the small watershed scales are higher than those at plot scales; with the increase of spatial scales from plot to small watershed, the runoff rates of forest increase far less than those of non-forest. And on the runoff plot scales (100m2), soil erosion amount negatively correlated with the forest cover; sediment reduction rate of forest at natural slope was 33.95~99.12%, and at small watershed was 51.86~100%, an increasing trend from natural slope to small watershed. Sedimentation and runoff yield analysis based on three spatial scales indicate that rainfall depth for forest to control soil erosion effectively remains within 75~90mm. Annual maximum and minimum soil erosion modules of water conservation forest on the natural slope and small watershed are 1.99t/hm2,5.15 t/hm2 and 0.08 t/hm2,0.03t/hm2, respectively, far less than the acceptable soil erosion module 10t/hm2?y, compared to 8.31t/hm2,15.07 t/hm2and1.01 t/hm2,0.11 t/hm2. in corresponding scales for non-forest areas. According to the results, cell-based soil erosion pattern under typical rainfall events on experimental watersheds are simulated through soil erosion models and hydrological models integrated with GIS (ARC/INFO). Results show that sediment yields of non-forest and forest areas account for 84.77～100% and 0-15.23% of total sediment yield respectively. The soil erosion control effect of the forest is very evident. The model is further validated on the experimental watershed by two typical rainfall events and the results show that this model can be used for soil erosion prediction in the study area with an acceptable accuracy.