Study Of Mountainous Forest Watershed Distributed Hydrological Model Based On DEM

The relationship between forest and water is always one of the key tasks in forest ecosystem studies,and is also the important content in hydrological cycle research. How the forest ecosystem influences moisture distributing in space and time, transporting and transform and hydrological cycle mechanism is the core issue in the research field. The relationship between forest and water is very complicated: it is affected not only by forest ecosystem development itself, but also by the space heterogeneities of mountainous atmosphere,plant, soil and underground rock and the time-space change of weather factors, such as precipitation and evaporation. All the above factors add the difficulties in describing quantitative runoff generation mechanism and the modulus of hydrological response in forest ecosystem. In recent decades, the methodology of the field has shifted from the traditional test of comparing watersheds to the later analysis on how the forest influences the runoff generation in a certain watershed scale, and finally to the present new stage of establishing distributed hydrological model based on physical process serving forest vegetation management. Under the support of Chinese National Nature Science fund "The Study of Watershed Scale Effect in Forest Watershed Hydrological Response and Distributed Hydrological Model" (NO. 30271042), this thesis builds a mountainous forest distributed hydrologic model with modern Geography Information System (GIS) technology and theory of deterministic hydrologic model, based on Xing'anjiang hydrologic conceptual model. By the simulation and verification of the model in the upper reach of Minjiang River Watershed, this model proves to be effective. Then, the response of forest cover change on Hydrologic process in the upper reach of Minjiang River Watershed is analyzed by using the model. In this study, the principal results are concluded as follows: Firstly, the data of hydrology, climate, DEM, digital vegetation map, digital soil map in the upper reach of Minjiang River was collected and organized, and the annual rainfall-runoff relations of eight gauging stations in the upper reach of Minjiang River were established and analyzed. Secondly, the digital drainage network in the upper reach of Mingjiang River was built based on grid DEM by using single flow direction. The whole upper reach of Mingjiang River was divided into 35 sub-basin, and was discretized into 143 Hydrological Response Units (HRUs), which is the basic simulation unit, through overlaying the land use and soil distribution digital maps. Thirdly, a traversal algorithm based on multil-subtrees is suggested for sub-basins topological relationship, which simplifies grids or sub-basins of the digital drainage network into a tree-structured nodded graph and uses recursive postorder traversal algorithm to establish topological relationship of grids or sub-basins. This method has solved the issue of flood routing sequence for complicated basin. Fourthly, a Mountainous Forest Distributed Conceptual Hydrological Model is suggested,which adds the modules, such as vegetation interception, vegetation transpiration, forest soil water, snow cover and snowmelt, and frozen earth,reflecting hydrologic characteristic of mountainous forest. The frozen earth is simulated according to 3 layered soil mode, corresponding with soil evaporation mode and soil water movement law in vertical direction. The simulation and verification of the model for 8 gauging stations proves the model to be effective. Fifthly, the hydrological effect to different forest coverage, such as annual and monthly runoff, runoff components, evaporation, frozen earth and flood process, was analyzed by using the above model and parameters. The result shows that,alongwith forest coverage increasing, annual total runoff decreases gradually,monthly total runoff decreases gradually in flood season and increases in dry season, which varies little in increasing range,due to the influence of frozen earth. Accordingly, flood discharge process decreases gradually in rising segment and the early recession segment,but quite opposite in the late recession segment; the values of evaporation and frozen earth are bigger in wood area than those in the woodless area. Finally,the changing law of hydrological variable with scale was discussed in uniform watershed spatial factor by using the above model. The result indicates that the values of annual maximum peak discharge modulus and annual runoff modulus vary greatly in micro-scale, and tend to decrease with watershed area's increasing, when reaching a critical value,the changing range of two values decreases, and tend to reach a steady value respectively.