Spectral features analysis of water quality in Baiyangdian wetland, China

Baiyangdian wetland is the largest freshwater lake in the north of China with an area of 366 Km2. Recent years, the water quality of this area is degenerating. According to the field data in October 2006, the water quality belongs to organic pollution type. In September 2008, 67 water quality samples in Baiyangdian wetland were collected, with the parameters including pH, chlorophyll-a, dissolved oxygen (DO), blue-green algae and total dissolved solid. Evaluation result shows that Baiyangdian wetland was contaminated seriously with water quality of V grade referring to the GB3838-2002. Chlorophyll concentration in northern region is higher than that of southern region, which is up to 66.16 g/L. At the same time water upwelling radiance was measured at 18 sampling stations by using field spectroradiometer (ASD FieldSpec). To contrastively analyze the characters of spectral curves of water body at different sampling points, in-situ measured spectrums are normalized. By comparing spectral curves of water, with the chlorophyll concentrations of 6.64 g/L, 20.08 g/L and 63.03 g/L respectively, it can be found that the reflectance peaks get lower in the region of 560-575 nm along with the increase of chlorophyll concentrations. While at the band scope around 700 nm, reflectance peaks rise as chlorophyll concentrations increasing, accompanying a `red shift´ of peak. Through calculating correlation coefficient between Chlorophyll-a concentration and normalized reflection of water, the highest positive correlation is 0.9133 at band of 722 nm. The bands near 550 nm and 670 nm show a negative relationship, while bands near 700 nm represent positive correlation. These bands can be chosen for chlorophyll concentration retrieval by using remotely sensed data.