Reservoir operation with combined natural inflow and controlled inflow through interbasin transfer: Biliu reservoir in northeastern China

The essential problem for reservoir operation is how to deal with variability of inflow. The impacts of both climate change and human interferences on streamflow further complicate the issue of reservoir inflow variability. This paper presents a case study of a reservoir that has both natural inflow (NI) and controlled inflow (CI), and analyzes how CI, which can be a deterministic item, mitigates the effect of NI variability on reservoir operation. Through a case study of the Biliu River Reservoir in northeastern China, the following questions are addressed: (1) how will the combined NI and CI improve water supply reliability, and (2) can-and if so, how much-of the reservoir's storage can be reduced without lowering water supply reliability with a certain level of CI? It is found that CI complements NI in terms of water supply reliability through a nonlinear relationship, as shown by an inverse S curve; the two turning points in the curve specify a range of CI with respect to the stochastic nature of NI and the reservoir operation objectives. For the case study reservoir, if the amount of annual CI increases to about 54% of the NI, the primary reservoir storage capacity can be reduced by 40% while providing the same level of water supply reliability.