Water clarity changes in Lake Taihu over 36 years based on Landsat TM and OLI observations

Abstract

The water quality in Lake Taihu, which is located in the middle of the largest group of cities on the Yangtze River delta of China, has changed greatly with rapid economic and social development since the reform and opening up of China in the 1980s. Here, Landsat-5 TM and Landsat-8 OLI satellites launched in 1984 and 2013 were used to track 36-year changes in Secchi disk depth (Zsd), which is a key water quality and lake health indicator. First, the quasi-analytical algorithm (QAA)-based Zsd estimation model and commonly used empirical models were calibrated using in situ measured Zsd and TM/OLI band-simulated remote sensing reflectance (Rrs) obtained from 299 sampling points in 12 field cruises in Lake Taihu. Afterward, another 69 pairs of in situ measured Zsd data synchronized with TM images and 40 pairs of in situ measured Zsd data synchronized with OLI images were used to validate the calibrated Zsd estimation models. For the QAA-based method, which has been successfully applied to OLI data, we developed an approach for modeling the Rrs in the missing 443-nm band in TM using a virtual-band estimator algorithm; this enables the application of the method to TM data. By comparing with other calibrated empirical models, the accuracy of the QAA-based model was determined to be high for both OLI and TM, with an average unbiased relative error of < 24%. Second, we employed the QAA-based model to retrieve the spatial and temporal Zsd variation characteristics of Lake Taihu from 1984 to 2019, and analyzed the driving factors. The results suggest an increase in Zsd from 1984 to 1998, which was mainly caused by a decrease in wind speed. There was a turning point for Zsd changes around 1998, with a downtrend from 1999 to 2019, which was primarily attributed to increase in the frequency and area of cyanobacterial blooms associated with the low average wind speed and increasing air temperatures. This study provides the first analysis of the spatial and temporal characteristics of Zsd in Lake Taihu over the past 36 years, and the Zsd results and driving force analyses could facilitate local water resource conservation.