Effect of regulating upstream reservoir on water supply security facing saltwater intrusion

Freshwater availability in coastal cities is severely threatened by saltwater intrusion during the dry season. Some reservoirs are typically installed in the upper reaches to regulate the temporal distribution of water resources to ensure a steady supply. However, studies quantifying this effect are limited. This study improved a framework proposed by our previous work and conducted some scenario-based experiments to evaluate this effect. The study considered the following: First, a comprehensive matrix index is developed based on the D-vine copula function and Kendall distribution transformation method (KF) using historical monthly streamflow. Then, several monthly-based management scenarios, i.e., flat, descending, ascending, convex, and concave, are designed based on the index. The scenarios provide inputs to the water supply system facing saltwater intrusion. The Zhuhai-Macao water supply system was taken as a case study. Results demonstrated that the index performed satisfactorily in devising scenarios and holistically describing the temporal streamflow distribution characteristics and hydrological wetness-dryness conditions. The security situation of scenarios followed ascending>convex>descending>flat>concave with KF=0.10 and convex>ascending>flat>descending>concave with KF=0.05. Hence, the securest scenario for regulating reservoirs was the convex pattern, which avoided shortages by 100% and improved the increment of the remaining water in reservoirs by 4.92 times under KF=0.05 and 6.13 times under KF=0.10, separately, compared to the original streamflow. The worst scenario for regulating reservoirs was the concave pattern. Moreover, the ascending pattern can also be considered as a distribution for regulation, but the long period of extremely low streamflow in the early dry season should be avoided. The scenarios experiment can be applied as a management tool to alleviate water supply pressures in coastal cities facing saltwater intrusion.