Contents I. Introduction
Water quality is an important factor for human health and environment. This has been recognized many years ago. Therefore, large investments were made in Yangtze River to ensure the water security for the people who live in the Yangtze River basin. There is no question that with the end of the construction of the Three Gorges reservoir, the water quality in this area will be paid more and more attention in the world. With strengthening control of point source pollution, non-point source pollution has become the key factor to improve water quality in the reservoir area (Liu, et al., 2003). Obvious point sources of pollution could be found and were controlled easily. While non-point sources of pollution is difficult to be found because these sources are not discrete places but diffuse areas somewhere on the Earth's surface. To eliminate them, a practical method is to control the headstream of non-point source pollution now. So how to identify the regions in which non-point source pollution are easily occurred is an important premise to control them effectively (Lemunyon et al., 1993; He, et al., 1998). Geographical information system (GIS) is a tool for the management, query, visualization and analysis of spatially referenced information. Benefits of GIS are the ease of data update, data management, and data presentation in forms most suited to user requirements. Significant advancements in the application of GIS for solving non-point source pollution problems have taken place over the past several decades (Zhang et al., 2003). And also the universal soil loss equation (USLE) formulated by Wischmeire and Smith in 1978 is a possible model to handle this kind of problem. The USLE model was developed to estimate the annual amount of sediments eroded form the surface of the surface of an analysis area. Because of the similarities of erosion and the no-point source problems, the USLE also be used to pinpoint critical non-point pollution areas. In this work, a modified version of USLE is used to analyze non-point source pollution critical area in the Three Gorges Reservoir area of Chongqing with support of GIS technique. It is expected that the study will show where non-point source pollution are easily occurred and provide some support to land use planning and management, and at last arrive to the target of sustainable development. Location of the study area
