Multivariate water environmental risk analysis in long-distance water supply project: A case study in China

The variation of water quality in long-distance water supply projects is often different from that in natural water bodies, especially the environmental risks of artificial open canal water transfer projects are seldom studied. The spatial heterogeneity of algae growth and the absence of universal reference standards for algae control often lead to water quality problems in these projects. The Middle Route (MR) of the South-to-North Water Transfer Project of China (SNWTPC), the world’s largest inter-basin water transfer project, has operated stably for six years. Its water resources have benefited more than 60 million people and an ecosystem cover more than 160,000 km². To understand the water environment risk of this mega hydro-project, this study focused on the relationships among three key parameters: water temperature (T), water discharge (Q), flow rate (V) and analyzed the spatial–temporal variation characteristics of the algal cell density (ACD) in the MR of the SNWTPC from January 2016 to December 2018, 36 months in total. The Copula functions were applied to identify and evaluate the multivariate risk variation of the water environment. Our result demonstrated that there was a significant positive correlation between T and ACD, and the ACD at the downstream has a 50% risk higher than 700 × 10⁴ cell/L in summer. Overall, the water quality status of the MR of the SNWTPC is quite well, and the ACD kept at an average level of 10⁶ cells/L during the monitoring period. Additionally, the ACD increased from upstream to downstream, showing the relatively higher ACD in summer and autumn than in spring and winter, with the ranges of 500 ~ 700 × 10⁴ cells/L and 200 ~ 300 × 10⁴ cells/L, respectively. The water temperature affected the ACD over the early-warning thresholds at different canal sections were as follows: 29, 26, and 21℃ from upstream to downstream, respectively. The influences of the hydrodynamic factors, water discharge and flow rate, impact on the ACD variation were analyzed to achieve the purpose of specific algae control for different canal reaches. Our study verified that the growth probabilities of the ACD under higher water temperature and water discharge in the MR of the SNWTPC than other natural water bodies.