Eutrophication risk assessment considering joint effects of water quality and water quantity for a receiving reservoir in the South-to-North Water Transfer Project, China

Evaluating the eutrophication risk of a receiving reservoir is crucial for scientific water transfer schemes. However mega water transfer projects would greatly affect both water quantity and water quality processes, making the eutrophication evaluation more difficult. This study assessed the eutrophication risk of a receiving reservoir (i.e., Miyun Reservoir) after the implementation of the world's largest water transfer project, namely, the South-to-North Water Transfer Project (SNWTP) in China. A new perspective of considering joint effects of water quantity indicator (i.e., water storage) and water quality indicator (i.e., Chlorophyll a concentration or Chla concentration for short) was proposed to assess the eutrophication risk. The GMM model was first introduced into the copula model to adaptively describe the marginal distribution of hydrological variables and to improve the accuracy of the marginal probability distribution for water storage. Besides, the Frank copula model was selected to establish the joint probability distribution function of water storage and Chla concentration from four candidate copula models. The eutrophication risk of the receiving reservoir was then assessed under ten water transfer scenarios concerning six amounts of water transfer in four certain periods. Results showed: 1) there was little eutrophication risk (< 0.0005) in Miyun Reservoir under the selected water transfer scenarios; 2) the probability of higher Chla concentration in Miyun Reservoir would increase with larger water storage after the implementation of the SNWTP, and 3) the probability (0.00027) of water quality deterioration in the reservoir under uniform water transfer was about half (0.00045) of the centralized water transfer. These findings can contribute to the eutrophication risk assessment and adaptive management of the world's largest water transfer project.