Inland Lake Water Quality Parameters Inversion And Eutrophication Assessment

It is recognized that, lakes, playing a very important role in preventing from flood and drought, regulating local climate, irrigation, water transport, aquatic products supply, tourism, et al, not only continuously supply the goods and materials which are essential to development of human society, but also the ecological protection. Unfortunately, due to the lack of people's environmental awareness, most of the lakes in China have suffered different degrees of eutrophication status, which has seriously affected the function of lakes in recent years. So, this paper presents the inversion the concentration of the water quality parameters of Lake Kuncheng, and the evaluation of eutrophication level of the lake by three methods, which provides a basis for prevention and cure of eutrophic lake.Accurate extraction of remote sensing information is one of the most important parts of inversing the concentration of water quality parameters. Firstly, remote sensing data preprocessing, consisting of the pretreatment of the measured hyperspectral data of the sampling points and the ETM+ remote sensing image of Lake Kuncheng, was done. For the measured hyperspectral data, the measured DN curves are converted to reflectance curves; and then reflectance curves were normalized and first order differential treated in order to remove the introduction of random measurement error, and to prepare to determinate the optimal band of the water quality parameters. For the ETM+, the author selected 16 ground control points around the lake uniformly, and used a second-order rational polynomial model and bilinear interpolation method to complete the precise geometrical correction, the result showed that the error was less than one pixel. Furthermore, 0.3058 was set as the threshold to delimitate the lake and the other ground-objects by MNDVI. Lastly, FLAASH and dark objects method were adopted to complete the precise atmospheric correction. It showed that the result from FLAASH was better (the error was between±15%).By analyzing the measured reflectance spectrum, it was found that the water quality of the sampling sites had typical spectral characteristics of the type-II water bodies. On this basis, With the help of the correlation analysis between the normalized reflectance spectrum,first derivative spectrum and the concentration of chlorophyll-a and suspended solids, it was found that the sensitive band of chlorophyll-a and suspended solids were respectively the 2nd, 3rd, 4th band of ETM+ and the 1st, 2nd band of ETM+. Based on the above analyses, the semi-empirical quantitative remote sensing inversion formula for chlorophyll-a and suspended solids, of which the root mean square error (RMSE) was respectively 1.409mg/m3 and 1.452mg/L, could meet the statistical significance test with the confidence coefficient of 99.9%, were built in this paper. What's more, the author applied the semi-analytical method to inverse the concentration of chlorophyll-a with the help of the inherent optical properties of water quality parameters. The quantitative inversion formula by the semi-analytical, with the RMSE of 2.172mg/m3, could meet the statistical significance test with the confidence coefficient of 99.5%, which is slightly poorer both in significance and precision compare to that by semi-empirical method.For the sake of assessing eutrophic level of water of the sampling points, the chlorophyll-a was used to estimate the concentration of the total phosphorus(TP), total nitrogen(TN) and CODMn. The results from the three assessment methods(TLI, the Grey Clustering method and the Fuzzy Comprehensive Evaluation method) showed that the water of all the sampling points were in moderately eutrophic status. According to the comparative analysis of the merits and the scope of application of the three methods, the comprehensively trophic level index method, which was able to work combined with remote sensing images, was chosen to assess the eutrophic level of the whole lake. In accordance with the result, the a distributional map and a statistic map of the water quality of Lake Kuncheng were produced, according to which, brief analysis were proposed combined with the survey around the Lake Kuncheng.