Tracking historical chlorophyll-a change in the guanting reservoir, Northern China, based on landsat series inter-sensor normalization
2021
Zhang, Fangfang | Li, Junsheng | Yan, Bokun | Yu, Junchuan | Wang, Chao | Wang, Shenglei | Shen, Qian | Wu, Yanhong | Zhang, Bing
The Guanting Reservoir supplied drinking water to Beijing until 1997, following which the water quality of the reservoir deteriorated. The chlorophyll-a concentration (C cₕₗ₋ₐ) of water is an important indicator of eutrophication. Therefore, changes in the C cₕₗ₋ₐ of the Guanting Reservoir should be monitored and analysed. For more than 30 years, the monitoring of C cₕₗ₋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 C cₕₗ₋ₐ using these sensors. To address this issue, this study utilized inter-sensor normalization models of the different sensors and a unified C cₕₗ₋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 (R ᵣₛ) between OLI and TM/ETM+ was corrected to 1–5%. The unified model of C cₕₗ₋ₐ estimation employed the normalized ratio index of blue R B and near-infrared R NIR remote sensing reflectance: (R NIR – R B): (R NIR + R B). The MRE for estimating C cₕₗ₋ₐ was 25.7% and the root-mean-square error (RMSE) was 5.65 mg m–³. The C cₕₗ₋ₐ of the Guanting Reservoir was then estimated for years between 1985 and 2019. During this time, C cₕₗ₋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.
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