Human induced turbidity changes in Poyang Lake between 2000 and 2010: Observations from MODIS

A robust retrieval algorithm to estimate concentrations of total suspended sediments (TSS) in Poyang Lake (the largest freshwater lake in China) was developed using Moderate Resolution Imaging Spectroradiometer (MODIS) medium-resolution (250 m) data from 2000 to 2010 and in situ data collected during two cruise surveys. The algorithm was based on atmospherically corrected surface reflectance at 645 nm, with 1240 nm data serving as a reference for aerosols and a nearest-neighbor method was used to avoid land adjacency effect. The algorithm showed an uncertainty of 30-40% for TSS ranging between 3 and 200 mg L-1. Long-term TSS distribution maps derived from MODIS data and the customized TSS algorithm showed significant variations in both space and time, with low TSS (<10 mg L-1) in wet seasons and much higher TSS (>15-20 mg L-1) in dry seasons for the south lake, and generally higher TSS in the north lake. The TSS difference between the north and the south increased significantly after 2002, with mean TSS often reaching >40 mg L-1 in the north. While the TSS seasonality was attributed to the seasonal changes of the lake's circulation, the inter-annual variations were primarily driven by sand dredging activities, regulated by management policies. The case study here provides baseline water quality information for future restoration efforts in Poyang Lake, and more generally, an approach to assess water quality changes in similar water bodies, which have resulted from either climate variability or human activities.