Absorption Characteristics Of Optically Active Constituents (OACs) And Their Remote Retrieval Based On Quasi-Analytical Algorithm (QAA)

Water problem is gradually obvious in Northeast China with the rapid development of economy and society as the old industrial bases. Remote sensing plays a vital role in the water quality assessment of lakes and reservoirs wiht many advantages. In recent years, the devolopment of semi-analytical/analytical models in retrieving water quality parameters is very urgent to get highly accurate estimations of water quality, and it is necessary to strengthen stufy of optical properties in regional water.Based on the in situ spectral measurements and laboratory analysis, the water optical properties were investigated linking to the water parameters in Chagan Lake, Songhua Lake, Shitoukoumen Reservoir and Xinlicheng Reservoir, Jilin Province. Furthermore, the absorption characteristic, resources and seasonal variation of optical active compounds（OACs） were all analyzed in detail. Based on the above analysis, the inversion accuracies for total absorption coefficients of Quasi-Analytical Algorithm were corrected and evaluated in the study.The results showed that Chagan Lake and Shitoukoumen Reservoir had become eutrophicated, and Xinlicheng Reservoir and Songhua Lake were both mesotrophic. Chl-a concentration in Shitoukoumen Reservoir was higher than other waters, and it also presented higher value in autumn than summer in all the studied waters. Remote sensing reflectance was rising with the increase of total suspended matter concentration. And the increase of Chl-a makes the reflectance valley of pigments was obvious.The non-water light absorption spectra was mainly nonalgal particles absorption in Songhua Lake in summer, Chagan Lake, and Shitoukoumen Reservoir, but it was phytoplankton absorption in Songhua Lake in autumn and Xinlicheng Reservoir. The absorption coefficient value of nonalgal particles at 440 nm showed the total tendency of Chagan Lake> Shitoukoumen Reservoir> Xinlicheng Reservoir> Songhua Lake, with the hihest value in summer Shitoukoumen Reservoir. It is obvious that the phytoplankton pigment was dominated by the Chl-a through the analyzing the ratio of pigments absorption coefficient（aph440/aph675）. The analysis of non-water light absorption contributions with different OACs showed that it was dominated by the nonalgal particles absorption in Chagan Lake with up to 50%, and dominated by CDOM absorption in Songhua Lakewith up to 70%. The difference was showed in Xinlicheng Reservoir that the phytoplankton absorption contribution is the highest in OACs. However, the absorption contribution in Shitoukoumen Reservoir was dominated by CDOM in autumn, by nonalgal particles in summer.Based on the whole analysis, ag（440） value was the highest in the Shitoukoumen Reservoir, especially in the summer, following by the values in Songhua Lake, Chagan Lake and Xinlicheng Reservoir. The changes of slope of spectrum（Sg） and the ratio of bands（M250/365） shared the same trend in different waters, but ag（440） was opposite. According to the Sg and M250/365, CDOM in Chagan Lake was mainly fulvic acid which has low molecular weight, but the composition of CDOM tends to humic acid in Songhua Lake and Shitoukoumen Reservoir.Improved QAA₇40 algorithm has a better performance when we shifted the reference wavelength from 670 nm to 740 nm, especially 700 nm is fit for water in Songhua Lake. Simultaneous, it also increases the uncertainty of the algorithm with high RMSE and APD. QAA₇40 can be applied in Chagan Lake and Shitoukoumen Lake, but they all show the results of underestimation especially at the shorter band. It has a better performance in Songhua Lake with QAA₇00, especially in summer.