Study On The Distributed Precipitation-Runoff Model With Impacts Of Soil And Water Conservation Measures Of The Loess Plateau

The Loess Plateau is a place where water is deficient, but soil and water loss is severe. The human activities can change the hydrological conditions and ecological systems easily in this area. Specially, the soil and water conservation activities that take place almost on every hill slope of the Loess Plateau nowadays can lead to the responses of the water conversion and hydrological cycle strongly. For many years hydrologists have attempted to relate the hydrologic response of watersheds to human actives and soil and water conservation measures. In this case, studying the hydrological effects of the soil and water conservation activities will premise the improving of the ecological environment in this area. For example, studying on the changes of the precipitation-runoff process owing to the soil and water conservation activities in recently years can guidance the basin soil and water management, relaxed the crisis of the water resource, prevention to the heavy flood and soil erosion. Moreover, it shows the theory on which the water resource sustainable development can be realized in the Loess Plateau. The hydrological and geological elements changes owing to the soil and water conservation have the characters of high spatial variation in the basin. The distributed hydrological model can disperse the basin, and quantitative analysis the spatial variation of the elements. On this theory, this dissertation established a distributed hydrological model and studied the basin hydrological process impacted by the soil and water conservation using the model. The model dispersed the basin two times. The basin is dispersed into subbranch basins according to the natural water systems in the model, and the subbranch basins are dispersed into grid cells. In the grid cells scale, the hydrological processes such as the vegetation interception, the soil water infiltration and so on are simulated by the lumped hydrological model. Moreover, the flow concentration process is simulated in the subbranch basins scale. The soil and water conservation activities changed the elements of the grid cells such as the gradient, the length of slope, the Manning roughness coefficient, the types of vegetation, the ratio of vegetation coverage and so on. The effects of soil and water conservation on hydrological cycle and water conversion can be thought of through renewed the factors value in the grid cells scale. The paper analyses space plane interpolation method that is used by the former distributed hydrological models when simulated the distributing of the precipitation. And the paper present that this interpolation method cannot indicate the position of the precipitation center either the precipitation amount of the center. To make the matter worse, this method make man-made intercepted of the basin precipitation distribution. On the base of the analysis, a new mathematical model that able to indicate the precipitation center and the precipitation amount of the center is presented in the dissertation. Moreover, the model can simulate the continuous distributing character of natural precipitation. The model needn't the cockamamie distance calculation at all. Therefore, it is not only simpler but can be used more easily as well. On the simulating the vegetation interception and infiltration in the grid cells, the model parameters in the model have definite physical properties. This make the value of model parameters can be estimated. Additionally, the model has high resolution through discreteness. Because of all of these, the distributed hydrological model can simulate the hydrological process accurately. At the same time, this dissertation presented a way to dealing with the problems in detail when the geological factors such as the vegetation types, the vegetation coverage ratio, the gradient and so on have high spatial variation in one grid cell. The surface runoff formed in one cell will flow into the lower cells around it. The problem of water quantity distribution among the lower cells comes into being. The former distributed hydrological models used to deal with this problem by the single direction method or multi-direction method. Both methods determine the flow direction(s) by the slope factor alone. They neglected the effect of the Manning roughness coefficient on the water quantity distribution among the cells. However, there have different values of Manning roughness coefficient in the different directions of a grid cell or in different grid cells. The different Manning roughness values affect the surface runoff confluence. On this theory, the concept of vector roughness was first introduced in the paper. And a new multi-directions water quantity distribution model, which think of the Manning roughness effects, is deduced from one-dimension Saint-Venant equation and Manning equation. This new model improves the runoff confluence pattern of the distributed hydrological models in the grid cells. When the isochrone of the surface runoff is calculated, the paper presumes that the concentration time of a grid cell is the sum of the surface runoff flowing across the grid cell time and the lower cell concentration time. Accordingly, a physical based grid cellconcentration time equation developed in this paper. Because there are a few of lower grid cells around a grid cell, the lower cell concentration time of the grid cell can be regarded as the average concentration time of all of the lower grid cells. Therefore, after the precipitation begins, the surface runoff of a cell will reach the basin outlet in its concentration time later. And a cell's runoff hydrograph is formed. The runoff hydrograph of the whole basin will be obtained by linear adding the all grid cells'runoff hydrograph according to the concentration time order. After the distributed hydrological model constructed, the author program turbo C++ computer programs. The programs can carry out the distributed hydrological model simulation about the basin precipitation-runoff. The grid cells'data about the geography position, the gradient, the Manning roughness, the vegetation types, the vegetation coverage and so on are stored in data files. The main program read those data when there have precipitation process input. And a runoff process at the basin outlet, which responds the precipitation process input, will be outputted at high speed by computer. Meanwhile, the data can be refreshed on the conditions of the soil and water conservation. Therefore, the hydrological effects of the soil and water conservation in the basin can be real-time simulated. Finally, the distributed hydrological model is tested in the Nanyaogou small experimentation watershed of the Wudinghe middle reaches. Despite the limitations in the data availability, the results of river discharges predictions seem to be reasonable in the scale concerned. The result indicated that this distributed model can simulate the effects of the soil and water conservation activities on the hydrological cycle in the Loess Plateau, and has higher precision than the Shanbei lumped hydrological model. Moreover, the model not only has a sensitivity response to the increase the area of afforestation and terrace, but also to the precipitation process as well.