ABSTRACT
The Guanting Reservoir supplied drinking water to Beijing until 1997, following which the water quality of the reservoir deteriorated. The chlorophyll-a concentration (Cchl-a) of water is an important indicator of eutrophication. Therefore, changes in the Cchl-a of the Guanting Reservoir should be monitored and analysed. For more than 30 years, the monitoring of Cchl-a in inland waterbodies has only been possible using the Landsat 5 Thematic Mapper (TM), Landsat 7 Enhanced Thematic Mapper (ETM), and Landsat 8 Operational Land Imager (OLI). However, there are data consistency problems in monitoring Cchl-a using these sensors. To address this issue, this study utilized inter-sensor normalization models of the different sensors and a unified Cchl-a estimation model for Landsat series data with a long time span. After inter-sensor normalization, the mean relative error (MRE) of remote sensing reflectance (Rrs) between OLI and TM/ETM+ was corrected to 1–5%. The unified model of Cchl-a estimation employed the normalized ratio index of blue RB and near-infrared RNIR remote sensing reflectance: (RNIR – RB): (RNIR + RB). The MRE for estimating Cchl-a was 25.7% and the root-mean-square error (RMSE) was 5.65 mg m–3. The Cchl-a of the Guanting Reservoir was then estimated for years between 1985 and 2019. During this time, Cchl-a had a distinct seasonal distribution trend; the annual changes go through four stages, and each period shows different change characteristics. This study is the first to systematically understand the history of chlorophyll-a changes in the Guanting Reservoir over the past 30 years and provides references data for its eutrophication process and management.
Acknowledgements
We would like to thank the National Aeronautics and Space Administration (NASA) for providing Landsat TM/ETM+/OLI data, and the Big Earth Data Platform for Three Poles for providing China meteorological forcing dataset.
Disclosure statement
No potential conflict of interest was reported by the authors.