Lisa Rosenof, Associate Member, University of Cincinnati Law Review
“Whiskey is for drinking; water is for fighting over!
Both surface water and ground water flow naturally without regard to political boundaries, making water rights issues contentious and difficult to resolve. Contemporaneous with significant climate change and heightened environmental concerns, the Supreme Court has seen an increasing number of water-related lawsuits between states.
Generally, “sharing is caring” is a best practice often taught to children, but it is also one that has been applied by the Supreme Court to states as it relates to the sharing of interstate water resources. In prior disputes over surface water between states, the Court has consistently applied the doctrine of equitable apportionment, which grants the Court the authority to determine the percentage of the disputed water each state is allowed to use. However, the Supreme Court recently ruled, in its decision of Mississippi v. Tennessee, that the doctrine applies to groundwater as well.
Consequently, this article will explore the background of interstate water conflict and provide insight on the implications of the Court’s decision of Mississippi v. Tennessee.
II. Background of Interstate Water Conflict
The U.S. Supreme Court has jurisdiction to hear claims or controversies between two states—including interstate water issues. Importantly, there are three methods in which conflicts between states over interstate waters may be resolved: (1) legislative apportionment, (2) interstate water compacts, or (3) judicial apportionment. States typically do not resort to legislative apportionment because it requires more technical expertise than can be met with a Congressional hearing. Interstate water compacts are used to allocate water for future use between states. Interstate water compacts are often the preferred method of conflict resolution because they allow for flexibility in that they are tailored to each situation, they incorporate water resource management experts in the negotiation process, and they have a much better enforcement mechanism to monitor and enforce the compact. Finally, judicial apportionment requires the Supreme Court to create a solution to an interstate water conflict.
In judicial apportionment proceedings, the federal common law doctrine of “equitable apportionment” guides Supreme Court decisions. The doctrine favors a fair distribution of water between the disputing states by asking the Supreme Court to issue a decree stating how much water each state can use. Equitable apportionment is a flexible doctrine applied on a case-by-case basis, depending on the facts of each dispute, and the Court is free to disregard the existing regimes and fashion a more equitable resolution.
For the Court to equitably apportion an interstate water source, the states must first meet their individual burdens of proof. State A, typically the state seeking apportionment because it is being affected by the other state’s use or proposed use, must demonstrate by clear and convincing evidence that it is being harmed by the actions of another state. Once State A has met this burden of proof, the burden shifts to the defending state, State B, to prove by clear and convincing evidence that the diversions complained of should be allowed to continue. Only after both states have met this burden will the Court conduct an equitable apportionment analysis.
Federal common law has long applied equitable apportionment in disputes between states over the division of surface waters of rivers and streams passing between states. However, equitable apportionment had not been applied to underground waters until the Supreme Court’s November 2021 decision of Mississippi v. Tennessee.
The case involved a dispute over the Middle Claiborne Aquifer, a single hydrological unit that is interconnected beneath both Mississippi and Tennessee. Mississippi sued Tennessee alleging that Tennessee was taking Mississippi’s groundwater by allowing a Tennessee utility company to pump groundwater from the aquifer. Mississippi argued that the groundwater stored in the aquifer lied entirely within its state and would never flow into Tennessee if it were not for the pumping. The Court ultimately held that Mississippi did not have absolute ownership of groundwater in the aquifer within its boundaries, but instead that the water was subject to equitable apportionment.
The Court identified three criteria for determining when equitable apportionment is the appropriate doctrine to govern an interstate dispute: (1) when transboundary resources are at issue, (2) when the water or resource flows naturally between the states, and (3) when one state’s use of the resource can affect the resource in the other state.
Some argue that the decision in Mississippi v. Tennessee makes clear that interstate compacts offer states clear advantages over equitable apportionment. The high burden the Supreme Court set for proving injury in the case might encourage states to negotiate amongst themselves to share aquifers rather than immediately heading to court for damages. The burden is likely to be even harder to meet in the groundwater context, where substantial injury will be difficult to show unless wells are drying up and the cost of sinking them is exorbitantly high—at which point the aquifer has probably been thoroughly mined and there’s not much left to apportion. Additionally, going through the court system is costly, complex, and lengthy, and often results in unpredictable, sometimes unsatisfactory rulings.
However, by applying equitable apportionments’ flexible approach to the facts and circumstances in Mississippi v. Tennessee, the Supreme Court properly wielded a powerful tool to distribute a shared resource to the state. If the Court’s decision had favored Mississippi, it would have created uncertainty regarding shared natural resources and risked increasing tensions between neighboring states. States would likely have moved to hoard resources located inside their geographical boundaries, potentially creating a “tragedy of the commons” dilemma.
While Mississippi v. Tennessee is unlikely to spark a bold new era of interstate water compacts, a few states that share aquifers may take the opportunity now that the Court has made clear the background principles against which they negotiate. Combining surface water and ground water negotiations could allow states to deal with hydrologically connected ground water directly and to negotiate more creative tradeoffs across resources.
However, given the increasing scarcity of resources like water, it is critical that states work together to manage shared resources for the good of their citizens. To that end, the doctrine of equitable apportionment provides a tool to manage shared resources in a responsible way.
 Ted Poe, Whiskey is for drinking, water is for fighting, The Hill (May 5, 2018), https://thehill.com/blogs/congress-blog/foreign-policy/387545-whiskey-is-for-drinking-water-is-for-fighting (accrediting the quote to Mark Twain).
 Surface water refers to any body of fresh water that is above ground.
 Ground water refers to freshwater that is stored beneath the earth’s surface.
 State of Mississippi v. Tennessee, 142 S. Ct. 31 (2021).
 U.S. Const. art. III, § 2, cl. 1.
 Bernadette R. Nelson, Muddy Water Blues: How the Murky Doctrine of Equitable Apportionment Should Be Refined, 105 Iowa L. Rev. 1827, 1837 (2020).
 Id. at 1838.
 Id. at 1839.
 Robin Craig, In dispute over groundwater, court tells Mississippi it’s equitable apportionment or nothing, SCOTUSblog (Nov. 23, 2021), https://www.scotusblog.com/2021/11/in-dispute-over-groundwater-court-tells-mississippi-its-equitable-apportionment-or-nothing/ [https://perma.cc/6ASD-9825].