| The hydrological cycle is the connection of the earth circle-biocycle-atmosphere system. So, the hydrological cycle decides the formation and evolution pattern of water resources, and the water environment characteristic in a specific basin.Therefore, the research about hydrological cycle has an important implication to keep a basin with sustainable development. Because of the very complicated hydrology process, the hydrological model has inevitable becomes a power tool for studying hydrological cycle and simulating hydrology system. At the same time, the hydrological model is gradually becoming a power tool for overall grasping the laws of the hydrology process mechanism, scientific evaluating the water resources and its evolution pattern, and objective discovering the disharmony problem among human, water and ecosystem.The aim of the distributed hydrological model is to simulate each aspect during hydrological cycle with taking the basic knowledge of hydrology science as its foundation. Due to the spacial and non-linear characteristics of the hydrological system in a basin, the model starts with physical mechanism of the hydrological cycle, with adequately considering landform, geography, soil type, vegetation, land use and rainfall overall the basin, in order to comprehensively grasping the status and change of the main hydrological factor in a basin.The Weihe River is the biggest distributary of the Yellow River, flowing from west to east through three provinces (autonomous region) in Gansu, Ningxia and Shaanxi.The Weihe River basin not only significantly contributes to the management in the Yellow River, and occupies an important role in Shaanxi economy development, but also has an important strategic meaning in the west development. With the development of productive forces and the population growth, the conflict of the human and nature is increasingly severe, more and more problems appeared in the Weihe River basin such as the stream flow is decrease, the water pollution is serious, the hydrological disaster is severe, resulting in the deterioration of water environment, and the ecological enviroment in the watershed is damaged. All these issuses will prevent the sustainable development of the watershed and the community. Therefore, the research of the hydrological circle and the water environment in the Weihe river basin is of great significance. For promoting the harmony between human and nature, we need know well the theory of the hydrological cycle and the chang of water resources, and understand correctly the hydrology effect on the nature chang and the human activity in the Weihe river basin. On the basis of above scientific research, the protection and improvement for the water environment in Weihe river basin will be carried out accordingly.First, on the foundation of the water recycle mechanism and the research achievement on the distributed hydrological model, according to "hillslope" method, a geography based hydrological model was constructed in this paper. The model takes geography information system(GIS) and remote sensing(RS) as its technique supports, and withdraws the hydrology parameter informations such as geography and geography characteristic on the foundation of the digital elevation model (DEM). Accordingly, the whole basn is divided into some sub-basin, fter these Carrying on space through a convection area is long-lost, pursue class demarcation river valley is some sub-river valleys, sub-river valley along with remit to flow length again one by one in order divide the line to remit to flow zone for a series, each one remits to flow inside the zone again include some several what likeness of unit in"hillside" is finally the basic calculation unit that the river valley hydrology imitates.On each hydrology calculation unit, make use of model to reason rain to produce to flow and the ground path flow, again carry on course of river to remit to flow emulation, beg the exit cross section of an each statures river valley discharge, produce to flow to the whole ascent noodles of river valley by realization, the ground path flow, the course of river remit the emulation of flowing the etc. hydrology process.Secondly, for the natural state of the runoff and water quality characteristics in Weihe River basin, the constructed geography based hydrological model is applied to analyzing the main problem of the water environment in Weihe River basin. The hydrological components, including runoff generation from the hillslopes and the flow routing in the river network are modeled using physically-based approaches. The hydrological characteristics, the change of the flow and water quality, and the law of formation and transformation of water resources in Weihe River basin is also studied. The whole basin is divided into a number of sub-basins firstly, each sub-basin along the convergence of the length of interval is then divided into a series of convergence, and each convergence zone contains a number of geometric similarity of the hillslope unit as the basic calculation of hydrological modeling unit. Using the hillslope unit, the net rain of each hydrological unit is calculated firstly, then, the convergence of river network each sub-basin is calculated, and the whole export flow of each sub-basin is finally obtained. According to this simplified calculation method, the basin-wide hydrological processes is generalized and implemented. The results of this paper showed that the construction of the integrated hydrological and quality model can give a better simulation and forecast with less number of mesh and high-precision process, and has been satisfied with the analytical results of the water environment in the Weihe River basin. Through applying the model in the Weihe River basin to simulate natural water circle, the features and the applicability was well verified. The using of Basic Hydrology Unit could not only reflect the human activities impacting the rainfall-runoff process wit h physics-based parameters, but also greatly reduce the computation time.Roundly grasping the hydrology respond to the change of climate and characteristic in a basin, and effectively predicting the future trend of runoff, hydrological cycle and water resources in Weihe River basin are the science foundation to "maintaining healthy Weihe River, and achieveing human and water harmony". in order to explore climate change and human activities on water environment in response to the Weihe River basin, a quantitative description of the uncertainty change in water environment is discussed. The different rainfall and land-use scenarios are set to simulate the change and trend in the future of hydrology, water quality, water resources and water environment in the Weihe River basin, also using the integrated hydrological and quality model as the technical support. Finally, an objective assessment of the influence of natural changes and human activity on the uncertainty of the water environment changes in Weihe River basin is given. At the same time, the water quality under the different rate of flow in the main stream of the Weihe River in Shaanxi Province is evaluated and predicted.Finally, a distributed coupling model based on the geography based hydrological model and the river quality model was established in this paper, and the discharge and the quality of the main section on the Weihe River in Shaanxi piece was simulated simultaneously in order to reflect the trends of the amount of water and water quality changes. Accordingly, the spatial distribution of the main pollutants and the safety evaluation of water environment in Weihe River basin are also made. The results showed that water environment situation of Weihe River basin is still grave and serious, and it threats to the ecological health of river. On this basis, the proposal to restoring the water quality of Weihe River is proposed in order to protecting the water environment security, achieving sustainable development and building a harmonious society.
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