In its recent decision in Mississippi v. Tennessee, ___ U.S. ___, 142 S. Ct. 31 (Nov. 22, 2021), the U.S. Supreme Court, adjudicating a water rights dispute between two states, has for the first time determined that the doctrine of “equitable apportionment” should be applied to subsurface waters. More common in arid parts of the country, the doctrine has been developed to distribute equitably shared water resources among States with competing claims to ensure that each has an equal right to make a reasonable use of the limited resource. This doctrine has previously been applied to interstate rivers and streams, and even to groundwater withdrawals, but only in cases where the flow of interstate surface waters was affected.
Mississippi had filed suit, seeking more than $615 million in damages on the basis that water withdrawals in Tennessee from the Middle Claiborne Aquifer—which underlies both states—amounts to an illegal taking of water originating in and belonging to the State of Mississippi. In a logical extension of its prior decisions, the Supreme Court accepted that interstate groundwater aquifers are as much water bodies that flow naturally between states as are surface waters such as streams and rivers. As Mississippi claimed that groundwater withdrawals in Tennessee have reduced groundwater storage and water pressure within its boundaries, the Supreme Court determined that this resource should be distributed equitably between the two states under the doctrine of equitable apportionment.
Water wars have a long history among the western states. As climate change impacts increase, it is likely that no state will be immune from disputes over water with its neighbors. The extension of the doctrine of equitable apportionment may foretell an increase in interstate litigation over increasingly scarce water resources.