Total Nitrogen and Total Phosphorus Pollution Reshaped the Relationship Between Water Supply and Demand in the Huaihe River Watershed, China.

As total nitrogen (TN) and total phosphorus (TP) pollution is the main source of water pollution in the Huaihe River watershed in China, it is important to understand how TN and TP pollution affect the relationship between water supply and demand. Quantifying their impacts and describing the spatiotemporal distribution of this relationships are necessary for furtherly deepening the theory of TN and TP pollution on water bodies, and this information is also particularly essential for managing water resources regionally. In this study, based on the potential water supply, the water demand and the Integrated Valuation of Ecosystem Services and Tradeoffs (In-VEST) water purification models, we estimated the TN and TP pollution from agricultural fertilizer, livestock and poultry breeding, and rural residents in the Huaihe River watershed and simulated TN and TP impacts on the relationship between water supply and demand. We found that if the impact of TN and TP pollution on water supply was not taken into account, on average, there was excess water supply in 79.20% of the watershed and excess demand in 20.80% of the rest during 1980–2018. Under the TN concentration limit, Grade-II (The water quality meets the secondary level of water body qualified in GB3838-2002, classified as Grade-II) water was the main water-supply type in 1980–2018, followed by Grade-I and Grade-III water. The total water shortage showed an inverted V-shaped trend: first increasing and then decreasing at the same period. The proportion of the water shortage of Grade-I water in the total water shortage was the largest, followed by Grade-II and Grade-III water. Areas with excess demand were located on the north bank of Wang-Beng, Yishuhe, and Huxi regions, although the water in these sub-watersheds met the water quality standards of Grade-I water. Under the TP concentration limit, Grade-II and Grade-I water were the main water-supply types. The overall water shortage trend first increased and then decreased, exhibiting an inverted V-shape from 1980 to 2018. The water shortages of Grade-I and Grade-II water showed similar inverted V-shape trend over time. Areas that met the water quality standard of Grade-I included the north banks of Wang-Beng and Huxi regions, where there was a surplus of demand. This paper suggests a way to analyze the interaction between water pollutants and the water supply-demand ratio as the example of TN and TP pollution at a watershed scale, which can broaden water pollution theory for relative water resources departments when water supply and demand will be evaluated. [ABSTRACT FROM AUTHOR]