Abstract
The quantitative inversion of the concentrations of water quality parameters could clarify the temporal and spatial distribution characteristic, migration, and conversion of water quality parameters. This study took the Danjiangkou Reservoir as the research object, and established an inversion model based on the reflectance of different band combinations of remote sensing analyses on Sentinel-2 images, combined with the water quality monitoring data of total nitrogen (TN) and ammonia nitrogen (NH3-N) of the sampling sites in February 2016. The inversion results of TN and NH3-N in 2020 were obtained, the variation of TN and NH3-N concentrations in the reservoir area were analyzed, and the factors accounting for the variation were discussed. The results indicated that the fitting accuracy using the established model was high for both TN and NH3-N, and R2 was 0.782 for TN and 0.851 for NH3-N, respectively, showing high predication accuracy, which could be suitable for remote sensing inversion of TN and NH3-N concentrations in the Danjiangkou Reservoir. The NH3-N concentration of the Danjiangkou Reservoir was in line with Class I from 2016 to 2020, while the TN concentration was between Class III and IV. The inter-annual changes indicated that the overall water quality had an upward trend. The main tributary in the northern of the Danjiangkou Reservoir had a heavy load of TN, and after entering the reservoir, the flow velocity decreased, which caused nitrogen to accumulate at the river entrance, leading to a high TN concentration. The large slope of the mountainous area cause soil erosion. The lost soil and water carried a large amount of pesticides and fertilizers, and the ground runoff carried a large amount of nitrogen into water body, which could account for the high NH3-N concentration on the east and west sides of the southern part of the Danjiangkou Reservoir.
Highlights
The South-to-North Water Diversion Project (SNWDP) is a major strategic infrastructure aimed at optimizing allocation of water resources across the river basin and benefiting the greatest number of people in China, and is the inter-basin water diversion project with major international influence [1].The Danjiangkou Reservoir, as the water source for the SNWDP, is one of the most important protectionWater 2020, 12, 2615; doi:10.3390/w12092615 www.mdpi.com/journal/waterWater 2020, 12, 2615 areas in the country
The water source for the middle route of the SNWDP is mainly located in the Danjiang Reservoir area, and the water source mainly comes from the Danjiang River basin that flows through the Shaanxi and Henan provinces (Figure 1)
The analysis indicated that the constructed inversion model had high prediction accuracy and was suitable for remote sensing inversion of total nitrogen (TN) and NH3 −N concentrations in the reservoir area of Sentinel−2 images
Summary
The South-to-North Water Diversion Project (SNWDP) is a major strategic infrastructure aimed at optimizing allocation of water resources across the river basin and benefiting the greatest number of people in China, and is the inter-basin water diversion project with major international influence [1]. Since some of water quality parameters are not optically active or lack hyperspectral data at fine spatial resolutions, not all parameters can be directly measured by current satellites, including total nitrogen and phosphorus and dissolved oxygen levels. The spatial resolution of some satellite data that was relatively low was only suitable for small-scale typical areas, and there were limitations in remote sensing inversion of TN and NH3 -N concentrations in large lakes. Sentinel-2 integrates the advantages of other satellite remote sensing data, which has the characteristics of short revisit time, high resolution, and strong spectral recognition ability, is suitable for monitoring water of large lakes and ensures more accurate results [29]. The current research mainly focuses on remote sensing inversion of water quality parameters such as chlorophyll-α, water turbidity, total suspended solids (TSS), chemical oxygen demand (COD). The temporal and spatial characteristics of TN and NH3 -N were analyzed, the pollution load of the river basin was revealed, and the sources of pollutants were analyzed, which could provide a scientific reference for water quality monitoring and diffuse pollution control of the Danjiangkou Reservoir
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