Water is an essential yet constrained commodity for human communities and ecological systems depend on it. With population growth and economic development, water resource is increasingly declining. Over the past two decades, because of changes in population and climate, and relative welfare increase, the per capita consumption of water has increased. Thus, optimal use of available water resources is inevitable. In this study, a mathematical programming model for optimal allocation of water among the sectors of agriculture, aquaculture and environment is used. The data for the study were collected from Golestan Regional Water Organization which related to the years of 2001 to 2013. The results showed that under the efficiencies of 37, 45 and 51 percent during the three-year planning horizon, the amount of water allocated to agriculture decreases, water allocation to the environment increases and water allocation to the aquaculture sector will remain unchanged. The acreage of some crops under these efficiencies will not change.