This situation is true only in the Middle and Lower Rio Grande Basin, where purpose of use determines priority to the water stored in Falcon and Amistad Reservoirs. In those reservoirs, municipal and industrial rights have priority over irrigation rights if and when water shortages require that supplies be allocated. As a result, no priority dates exist for rights to water stored in Falcon and Amistad Reservoirs. Elsewhere in Texas, the doctrine of prior appropriation governs. The state of Texas has issued about 6,500 water right documents.
Some water rights authorize users to impound water in a lake or reservoir for later use.
If downstream users have water rights with older priority dates, they can require that streamflows into the reservoir be passed through the dam to satisfy their needs. Once the water is stored, however, the downstream water right holders cannot call on that water for use. Lake and reservoir owners store this water both for their own use and for sale to others. Water released from storage may be intended for users several miles downstream who have purchased the additional water for irrigation, industrial, or other purposes. In most areas of the state, the honor system governs compliance with water rights.