Monitoring water clarity dynamics in Hongze Lake under the influence of the south-to-north water diversion project...

The South-to-North Water Diversion Project (SNWDP), one of world's largest inter-basin water transfer projects, relies on Hongze Lake, China's fourth largest freshwater lake, as a key regulating and storage reservoir in the Jiangsu section. The water quality of Hongze Lake is crucial for ensuring the safety and reliability of water supply. Secchi disk depth (Z_SD, m) is a critical metric for evaluating water quality and ecosystem status. For inland highly turbid waters of Hongze Lake, the QAA_HZ algorithm based on Sentinel-3 OLCI imagery was developed to estimate water clarity. The algorithm outperformed other QAA algorithms and traditional empirical models, with MAPE and RMSE values of 14.60% and 0.06 m, respectively. When applied to other inland turbid lakes, such as Dongting Lake, all seven evaluation metrics indicated that the algorithm exhibits good stability and transferability. Analysis of OLCI images revealed spatiotemporal variations in Hongze Lake from 2017 to 2023, showing a long-term decline in Z_SD, with lower values in summer and higher in spring, consistent across the three lake bays. Among the meteorological variables, wind speed with a three-day lag was identified as the most significant driver, exerting a negative influence on water clarity, comparable to the effect of anthropogenic pressures represented by Gross Domestic Product (GDP). During the operation of the SNWDP, Z_SD in Hongze Lake is generally elevated during diversion periods compared to non-diversion periods, primarily driven by the combined effects of upstream inflows and algal dynamics. This study reduces uncertainties in semi-analytical models for inland turbid water bodies and offers robust data support for water quality assessment and ecological management related to SNWDP.