Risk assessment of non-point source pollution export in Lasahe basin

Abstract: Identification of the high risk area of non-point source pollution has important practical significance to control non-point source pollution and improve the water environment quality. Lasahe River basin located in the Tibet Autonomous Region of China is important drinking-water source for Lhasa City, Linzhou County and Dangxiong County. Because the underdevelopment of industry and the less discharge of industrial pollutants in this area, non-point source pollution is the most important contributing factor for water pollution. Lhasa River basin is the region of agricultural production base with the densest population in the Tibet Autonomous Region. This study built the output risk model that includes rainfall, topography, and fertilization influence factor, identification the output risk region unit from basin non-point source pollution at all levels object. The risk probability of non-point source pollution was classed into five levels: lowest, lower, moderate, higher and highest. Data used in this study were mainly from remote sensing image, statistical yearbook, and the parameters in output risk model were collected from the literatures. The results showed that risk probability of non-point source pollution output in 1996 and 2010 was 50% and 46.3% respectively. 17.5% in 1996 and 12.6% in 2010 of the study area showed that risk probability of non-point source pollution was more than 70%. The risk probability of non-point source pollution was spatially heterogeneous, corresponding with the attributes of land use types. The areas with highest risk of non-point source pollution were concentrated in farmland, where agricultural activities strengthened, and concentrated in unused land with great ecological vulnerability and sensitivity to external interference. Areas with lowest and lower risk of non-point source pollution mainly distributed in grassland, which has relative stability and robustness. Our study also found that risk probability of non-point source pollution was closely related with slope, the risk enhanced as the slope increased, and moderate risk, higher risk and highest risk mainly distributed in the area with slope greater than 15 degree. Compared with 1996, the area that non-point source pollution degree of risk transformation from lowest and lower level to higher and highest level in 2010 was about 6674.3km2. Lasahe basin non-point source pollution risk probability was medium risk level, and degree of risk decreased in the local scope. And the area of highest risk reduced, and the area of low risk increased, but the area of medium and higher risk had a tendency to increase. Land use change, agricultural production and water-soil erosion are the material cause of the basin non-point source pollution. Areas characterized by conversion from grassland to farmland and from grassland to unused land showed markedly changes in risk probability of non-point source pollution. These results would offer useful information for limiting non-point pollution and helping promote better water quality in the Lhasa River basin. The risk assessment of non-point source pollution provide an easy, verifiable, viable way to identify areas prone to non-point source pollution, supporting to design and apply adaptive management strategies. Therefore, it needs to strengthen the ecological environment comprehensive treatment results, tackle non-point source pollution in advance, formulate plans for the development of ecological agriculture, and build controlling non-point source pollution migration vegetation buffer.