In the arid western United States, most water systems are fully allocated. Water for new uses must usually be obtained through reallocation of water that has already been claimed and assigned within prior appropriation. In Idaho, new allocations of ground water were made until approximately 1990. Since then, new uses have nearly always been met by reallocation, via prior appropriation water-rights transfers or the operation of the Idaho Water Supply Bank. Using number of allocations or reallocations per year as a proxy for the effectiveness of allocation mechanisms, it appears that transfers and the Bank have not adequately served the needs of society. This indicates that Idaho's allocation of ground water resources is inefficient; that is, that total benefit to society could be improved by mechanisms that more readily facilitate reallocation of water. It appears that improvements to ground water banking could be made within the scope of existing Idaho water-banking statutes, to improve the allocation of resources, reduce conflict and facilitate economic growth.