Reservoir is one of water management facilities. The role of the reservoir has become prominent in recent years due to increased demand for water, intensified pressure on limited water resources, more frequent extreme weather, and heightened degree of climate change. The management of multiple and multi-purpose reservoir systems is complex as it involves the issues of dimensionalities, nonlinearities and conflicts between different objectives. Optimization and simulation models have been used as a tool for providing quantitative information regarding optimal reservoir operation. This research demonstrates the use of optimization techniques to improve water allocation for multiple and multi-purpose reservoir systems. The optimal reservoir releases were determined by linear programing using IBM ILOG CPLEX Optimization Studio software. The objective function was developed to minimize water deficit in dry years under the constraints of reservoir storage, reservoir continuity, and water balance at each node of the river network. Two modeling approaches including time horizon and non-time horizon were applied to a case study of the Lower Chao Phraya River basin which is characterized by two reservoirs and a number of control structures along the channels. When compared to non- time horizon, the time horizon gave higher accuracy of the results. This suggested that using time horizon framework could provide beneficial support for better reservoir management and operational efficiency.