Suspended solids concentration is an important parameter to evaluate the quality and environment of water. It has very important significance to get the time-spatial distribution information of suspended solids concentration in lakes for their environmental management based on the remote sensing technology. Poyang Lake is the largest freshwater lake in China, located south of the Yangtze River in northern Jiangxi Province. One of the most notable features of Poyang Lake is the large difference in water level between the wet and dry seasons; during the wet season, the reservoir stretches hundreds of miles, while shallower wetlands and reed-dominated areas are widespread throughout the dry season.
Data
The measured suspended solid concentrations and chlorophyll-a concentration data consisted of 16 data sets in January, April, July, and October from 2009 to 2012, obtained from the Poyang Lake Wetland Observation Station, Chinese Academy of Science. MOD09 data products were used for suspended solid concentration retrieval. As the second-level standard products of MODIS, they are obtained after atmospheric and aerosol corrections. MOD09 data products have higher temporal resolution and 250 and 500 m spectral resolution.
MODIS data products were obtained for the 5th–15th of January, April, July, and October from 2000 to 2013 and preprocessed by transferring the image projection to the Albers Conical Equal Area project. To extract the water boundary for Poyang Lake, a water boundary extraction method called the Floating Algae Index (FAI, Hu 2009) was used.
This method is based on an effective combination of the red, nearinfrared, and shortwave-infrared bands of MODIS imagery.
Method
Based on the semi-empirical method, sensitive bands with higher R2 values were selected through regression analysis and curve fitting by combining the values between in situ measured suspended solid concentrations and each band of the MODIS imagery. Selected sensitive bands were the modeling input factors used to obtain the nonlinear response relationship between the remote-sensing reflectance values and observed suspended solid concentrations.
The remaining measured suspended solid concentrations data (that were not used for modeling) and the spectral reflectance of the MODIS bands were then used to evaluate the accuracy of the seasonal retrieval models, as follows:
Because of the clear seasonal changes in the water boundary of Poyang Lake, the standard deviation was used to indicate the fluctuation of suspended solid concentrations in the same month across different years, calculated as follows:
where σ is the standard deviation of suspended solid concentrations, N is years, xi is the suspended solid concentration value of each year, and μ is the average value of suspended solid concentrations.
Results The four seasons’ inversion models were built according to the suspended solids concentration data of Poyang Lake continuously measured in spring, summer, autumn and winter season of 2009 and 2012 year and combining with the regression analysis of the lake’s multi-band MODIS images in the same period. Then they are used to obtain the suspended solids concentration of the lake in annual spring, summer, autumn and winter seasons from year 2000 to 2013.
The results show that the suspended solids concentration of Poyang Lake is higher in the center of the lake and the main channel while is relatively lower in coastal waters, what is more the concentration was increasing year by year from 2000 to 2013 with high value in spring and autumn but low in summer and winter. Human activities have a great influence on the suspended solids concentration changes, too.
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