Retrieval of total phosphorus mass concentration in Yanghe Reservoir based on Sentinel-2 satellite data
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Abstract:
The measured data of Yanghe Reservoir and Sentinel-2 satellite image data were used to construct an inversion model of total phosphorus mass concentration in the water body, and the temporal and spatial variation characteristics of total phosphorus concentration in Yanghe Reservoir are analyzed, which provided a new way for water quality monitoring of the reservoir, and provided a new method for the reservoir area. Scientific basis was provide for water environment management.Based on the linear regression method, an inversion model between the reflectivity of the remote sensing band and the measured mass concentration of total phosphorus in water was established, and the total phosphorus mass concentration in Yanghe Reservoir from 2016 to 2019 was inverted, and the spatiotemporal characteristics were analyzed.In terms of time distribution, the average concentration of total phosphorus mass concentration in the reservoir in spring and winter is lower than that in summer and autumn, and the water quality in summer and autumn is worse than other seasons. From 2016 to 2018, the water quality of Yanghe Reservoir was relatively good, reaching Class Ⅲ, while the water quality in some months was poor, exceeding Class Ⅲ, mainly in autumn. In 2019, the water quality of Yanghe Reservoir was poor, reaching Class Ⅳ as a whole, and Class Ⅴ in some months, and the excesses were concentrated in summer and autumn. In terms of spatial distribution, the mass concentration of total phosphorus in Yanghe Reservoir in non-flood seasons gradually increased from the east and west entrances and water intakes to the central area of the reservoir. During the flood season, the mass concentration of total phosphorus in the waters near the west entrance was significantly higher than that in other areas. The optimal Sentinel-2 satellite band combination (B3/B5) and the field measured data of the same period were used to establish a total phosphorus mass concentration inversion model. The coefficient of determination (R2) of the established model reaches 0.818, which indicates good applicability. The error analysis between the inversion value and the measured value shows that the average relative error is 19.5%, and the root mean square error is 0.008 67 mg/L, indicating a good model accuracy. However, due to the limitation of geographical conditions, the selected sampling points are few. In the follow-up research, it is necessary to further accumulate the measured data and optimize the regression algorithm. The reason for this phenomenon is that endogenous pollution may be the most important factor in the excess of total phosphorus concentration in Yanghe Reservoir. At the same time, according to the estimation results of pollution sources in the Yanghe Reservoir basin, the total phosphorus non-point source pollution load of the main tributaries on the west side and the Xiyang River accounts for nearly 2/3 of the whole basin. And it is mainly discharged from rural life and livestock and poultry breeding. It also reflects the reason for the higher total phosphorus mass concentration on the west side of the reservoir during the flood season.
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