Abstract: Poyang Lake is the largest freshwater lake in China. Affected by the monsoon climate, the seasonal and inter-annual changes of its water area are obvious, and the remarkable differences exist in the internal water area. Optical classification of lake water is of great significance for retrieval of lake water quality parameters and monitoring of lake water quality. Taking Poyang Lake as the study area, in this paper, the Poyang Lake waters were divided into four categories based on morphological characteristics of the measured spectral reflectance data, i.e., Special turbidi, Medium turbidi, Mild turbid, Generally Clear. We analyzeddd the results of each water body type. Considering the measured spectral data are limited to some discrete points of the lake area, which are not enough for the observation of the spatial distribution and dynamic change of all different water body types in Poyang Lake area, we use remote sensing image to observe the type of whole lake water body. Then, we selected some sampling points on the Landsat 8 OLI remote sensing images , and established a classification algorithm based on the slope according to their spectral shape. The Poyang Lake water bodies were finally divided into five types using a decision tree model: Extremely Turbid, Medium Turbid, Mild Turbid, Generally Clear, and Particularly Clear. Results from image classification were found to be consistent with the field observations. When the model is applied to other periods of remote sensing images for classification of Poyang Lake water body, comparison of image classification results showed that Poyang Lake presented three, four and four different types of water bodies in 2002, 2005 and 2009, respectively, and the water turbidity range showed dynamic changes. Turbid water areas were mainly distributed in the channel of Poyang Lake's water flow into the Yangtze River and near the Songmen Mountain at the end of September or at the beginning of October (in 2002, 2005, 20013), and turbid water areas were mainly distributed near the river estuary in Poyang Lake at the early May in 2009, the channel of Poyang Lake's water flow into the Yangtze River and near the Songmen Mountain failed to present a special turbidity. It was found that optical classification of lake water can help monitor the spatial and temporal variability of Poyang Lake water optical quality.

Key words: Poyang lake, reflectivity, water, classification