Due to the asymmetric information problem, inter-temporal water allocation efficiency requires a water allocation institution that allows water right holders to store their allocated water. Capacity sharing has been recommended, but the problem of internal spillage that may arise in a capacity sharing system causes concerns for the implementation of such institutions. In this paper, the optimality of water allocation within a capacity sharing system in presence of a spot water market is studied. We show that in the presence of a spot water market with zero transaction cost, an appropriately designed capacity sharing system will result in optimal water allocation.