Estimation of Chlorophyll a Concentration Using NIR/Red Bands of MERIS and Classification Procedure in Inland Turbid Water

The classification criteria are established to classify the water of Taihu Lake into four classes based on above-water remote sensing reflectance (R_rs), i.e., types A to D. Among the four water types, type A spectra represented the case of waters where algal blooms or aquatic plants appeared, while type B is referred to the water with high suspended matter concentration and low chlorophyll a concentration (C_chla). Both types A and B were not suitable for retrieving C_chla from image data. Hence, three-band, four-band, and two-band ratio algorithms were constructed to retrieve C_chla from water types C and D. The obtained results showed that the relation trends between C_chla and R_rs were different between type C and type D waters. By using Medium Resolution Imaging Spectrometer images, acquired on November 11, 2007 and November 20, 2008, the C_chla of Taihu Lake was mapped by band 9/band 7 models; it could be concluded that the C_chla mainly ranged from 0 to 20 mg · m^-3, accounting for 83.70% of the whole lake area in 2007 image, while the area was 86.63% in 2008 image. The estimation accuracies varied from different C_chla ranges. The mean absolute percent errors obtained by band 9/band 7 models were 106.23%, 56.79%, 38.04%, 33.80%, and 58.74% for the ranges 0 mg · m^-3 <; C_chla <; = 5 mg · m^-3, 5 mg · m^-3 <; C_chla <; = 10 mg · m^-3, 10 C_chla <; = 20 mg · m^-3, 20 mg · m^-3 <; C_chla <; = and 30 mg · m^-3 <; C_chla, respectively. Correspondingly, the root-mean-square errors were 5.02, 4.45, 5.59, 8.72, and 32.55 mg · m^-3, respectively.