Eutrophication Risk Assessment For Danjiangkou Reservoir Bays Based On Landuse Structure

Danjiangkou reservoir, as the waterhead area of the Middle Route of South-to-North Water Transfer Project, plays a decisive role in alleviating water shortage, improving the region ecological environment, and promoting regional economic and social sustainable development. Some of Danjiangkou Reservoir bays had showed some eutrophic phenomenon in different degrees, but the constant input of a large number of nutrients to the reservoir tributary and catchments, made the reservoir bay faced with larger trend of entrophic level increase. So it is of great significance for the protection of optimization layout on water resources to evaluate the level of eutrophic risk in reservoir bay.In this paper,62 reservoir bays and their catchment areas were chosen as the research object, and the remote sensing image data, DEM data, precipitation data and the water quality testing data of the water samples in the reservoir bays were all collected, and then, landscape ecology, hydrology, statistics, GIS theory and technology, and other research methods were applied to analyze the basic situation of water quality, and then made a eutrophication evaluation to them. After that, the landuse situation in each catchment area and the relationship between water quality and terrain factors were analyzed, obtaining the reservoir bay catchment area yield levels in accordance of the curve number method. At last, an eutrophication risk evaluation in each reservoir bay was made. The main conclusions are as followed:The average concentrations of TN, TP and Chla from reservoir bays were 1.29 mg/L, 0.08 mg/L, and 9.70μg/L, respectively, with some reaching even more than the V class water quality of upper limit in the surface water environment quality standard (GB3838-2002) regulation. Multiple water quality parameters were at a higher level, which accelerated the occurrence of eutrophication.The N/P ratio in each reservoir bay was almost above 7.2, and phosphorus became the restrictive factors in the process of eutrophication in each reservoir bay.Among the 62 reservoir bays on survey, there were five reservoir bays’ water quality in the middle nutrition state,33 reservoir bays in light eutrophication,18 reservoir bays in middle eutrophication,5 in severe eutrophication, otherwise one reservoir bay in extrem eutrophication, so the water quality has reached the state of eutrophication in 57 reservoir bays. By using reservoir bay nutrition state comprehensive rating parameter a to describe the superiority of water quality, the a values of each reservoir bay were between 1.42 and 5.85. And the average of a values in Danku and Hanku was 4.63 and 4.70, respectively, and the water quality situations in Danku and Hanku were similar.The catchment area in 62 reservoir bays had a total area of 819.41 km2, and the average area was 13.22 km2. The biggest area was NO.60 reservoir bay catchment area, 39.73 km2. The smallest one was NO.11 reservoir bay, which covered an area of 2.07 km2. There was a certain difference in the terrain factors between bays. NO.4 reservoir bay catchment area had the smallest average slope,1.04° NO.59 reservoir bays owned the largest average slope,21.36°. All reservoir bays’average elecation mostly distributed between 158.30 to 323.34 m. NO.4 reservoir bay owned the lowest average height, 147.19 m, and NO.56 reservoir bay had the highest average elevation,439.75 m.Landuse type in study area were mainly forest land and cropland, accounting for 39.6% and 36.5% of the total area, respectively, which were Followed by shrubs and grass; Urban land and bare land were lesser, accounting for a total of less than 7.5%. Landuse structure in each catchment was different from each other, of which crop land and forest land proportion got the biggest,2.1% to 77.8% and 0.8% to 75.9%, respectively.Cropland proportion was significantly and positively correlated with TN, TP and Chla, and Chla content, while forest land was opposite. Urban land proportion was significantly and positively correlated with Chla content, in addition, bare land and shrub land proportion was significantly and negatively correlated with TP. Of the terrain factors, average slope had a big influence to water quality, which was significantly and negatively correlated with TN, TP and Chla. Besides, average elevation was significantly and negatively correlated with TN. Cropland and urban land were able to promote water quality deterioration, while forest land and shrub land were just the opposite. Terrain factors affected runoff water quality mainly by influencing the distribution of landuse. Landuse and terrain factors explained the spatial variability of water qualities by 46.5% in total, of which landuse explained 43.9%, accounting for 94.4% of total explanation.The average TN concentration in runoff from each catchment varied from 3.88 to 10.47 mg/L, with a mean of 6.14 mg/L. The average TP concentration in runoff varied from 0.18 to 1.19mg/L, with a mean of 0.54 mg/L. N/P ratio of the weight in runoff was basically above 7.2, and phosphorus would be the restrictive factor in the process of eutrophication in the future.The results of eutrophication risk assessment showed that 7 bays were at the low risk level, and 27 bays were at medium risk, in addition, there were 24 and 4 bays at high risk and extremely high risk, respectively. Each bay of Danjiangkou reservoir stood a chance for eutrophication, and almost of them was at medium or high eutrophication risk. Among all the 295 bays, level from low to high risk of eutrophication in the number was 12,161, 117 and 5, respectively. Bays in the north of Danku and west of Hanku held higher eutrophication risks, while bays in the southeast of Danku and south of Hanku held lower eutrophication risks.