Abstract
AbstractFreshwater lakes are facing increasingly serious water quality problems. Remote sensing techniques are effective tools for monitoring spatiotemporal information of chromophoric dissolved organic matter (CDOM), a biochemical indicator for water quality. In this study, the Gradient Boosting Regression Tree (GBRT) model and Sentinel-2A/B imagery were combined to estimate low CDOM concentrations (0.003 m−1 < aCDOM(440) <1.787 m−1) in Xin'anjiang Reservoir, an important drinking water resource in Zhejiang Province, China, providing the CDOM distributions and dynamics with high spatial (10 m) and temporal (5 day) resolutions. The possible environmental factors that may affect CDOM spatiotemporal patterns and dynamics were analyzed using Sentinel-2 image-observed data in 2018. Results showed that CDOM in the reservoir exhibited a clear increased gradient from its transition and lacustrine zones to the riverine zones, indicating that the rivers carried a substantial load of organic matter to the lake. The precipitation may increase CDOM concentrations but it has a delayed effect, while it may also shortly decrease CDOM concentrations due to the rainwater dilution. We also found that the correlations between CDOM and water temperature, air pressure, and wind speed were very low, indicating that these factors may not have significant impacts on CDOM variations in the reservoir. This study demonstrated that the GBRT model and Sentinel-2 imagery have the potential to accurately monitor CDOM spatiotemporal variations in reservoirs with low CDOM concentrations, which advances our understanding on the relations between the dissolved organic matter and its coupling environmental factors in river-reservoir systems.
Highlights
Freshwater lakes and reservoirs in high population density regions are facing increasingly serious water-quality problems, which are caused by human activities and climate change (Li et al ; Mushtaq & Lala )
Our field measured data showed that there were seasonal variations: the measured aCDOM(440) in April was within 0.1–0.99 mÀ1, but in October the range changed to 0.7–1.8 mÀ1, demonstrating that chromophoric dissolved organic matter (CDOM) concentrations were higher in autumn than in the early spring
We found that the logarithmic model (R2 1⁄4 0.0009, root mean squared error (RMSE) 1⁄4 0.4578 mÀ1, see Figure 5(a)) and the polynomial model (R2 1⁄4 0.1125, RMSE 1⁄4 0.4389 mÀ1, see Figure 5(b)) are not able to retrieve low CDOM concentrations in Xin’anjiang Reservoir
Summary
Freshwater lakes and reservoirs in high population density regions are facing increasingly serious water-quality problems, which are caused by human activities and climate change (Li et al ; Mushtaq & Lala ). CDOM has a few negative effects on the processes of drinking water treatment (Zhang et al ), such as reducing the effectiveness of oxidants and disinfectants and producing undesirable disinfection by-products during oxidation processes (Baghoth et al ). Understanding the sources, concentration, and cycling of CDOM in freshwaters is important for managing aquatic resources and predicting the outcomes of environmental change (Olmanson et al ). As a result, monitoring CDOM in reservoirs and studying its spatiotemporal distribution are very important for effective water quality evaluation and drinking water conservation (Zhang et al )
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