Allocation of water resources from river basins draining into reservoirs is a challenging task, particularly, if the ecological water requirement is taken into account. In this study, copula functions were used to analyze hydrologically the possible synchronous occurrence of dry and wet regimes in the Liaohe River. The ecological streamflow requirements were considered during the water resource allocation. The following results were obtained. (1) The probability distribution with the highest goodness-of-fit for the Yinna River and Zhangjiabao reservoir inflow was the log-normal distribution and that for the reservoir inflow of other rivers was the generalized extreme-value distribution. Further, the Gumbel and Frank copulas had the best fitting efficacy for the inflow series of the reservoirs. (2) The synchronous wet/dry events occurred at a higher probability of more than 27% at 4 out of 7 pairs of water reservoirs and at the rate of 13%, 12%, and 13% at the other three pairs of water reservoirs, i.e., Baishi vs. Jinling, Baishi vs. Qingshan, and Zhangjiabao vs. Biliuhe, respectively. Further, the probability of synchronous dry instream events was high in these reservoirs. (3) The method of minimal monthly runoff at 90% guarantee rate suggest the best ecological runoff at the nine water reservoirs. (4) Dry-dry events occurred at a higher frequency during low-flow periods. The water resource allocation or operation cannot be executed when the reservoir instream flow is lower than the minimum ecological streamflow. Joint water operation of the reservoirs based on the occurrence probability of dry-dry events and minimum ecological streamflow can satisfy the regional water requirement.