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Optical Classifications of Poyang Lake Water and Long-Term Dynamics Based on MERIS Observations

Using observations from the Medium Resolution Imaging Spectrometer (MERIS) and an unsupervised (Iterative Self-Organizing Data Analysis Technique) clustering method, the class centers for four different optical classes (i.e., moderate turbid or Class I; turbid or Class II; extremely turbid or Class III; and clear waters or Class IV) of Poyang Lake water were determined. A squared-Mahalanobis distance-based classification scheme was then used to classify 320 MERIS data between 2003 and 2012 (∼2.7 images/month on average). Significant dynamics of the probability of occurrence (POO) for different classes were observed through long-term analysis. The extremely turbid and clear water classes dominated Poyang Lake, accounting for ∼70% of the total surface area within the entire time series. Class III and IV waters showed out-of-phase fluctuation patterns, while a large POO was observed in dry seasons for the extremely turbid water class (Class III), and the clear water class (Class IV) represented most of the lake in the wet seasons. The northern narrow water channel of Poyang Lake was generally characterized by Class II and III waters. In contrast, the high POO of Class IV was demonstrated in the southern main lake and some eastern and southern isolated sublakes. The hydrological analysis shows that the south-to-north water level gradients were positively correlated with the combined POOs of Classes II and III (R2 = 0.4) and that the gradients were negatively correlated with the POO of Class IV (R2 = 0.36). Significant relationships existed between the POOs of different classes and local precipitation. This study provides critical information for future environmental conservation planning.