On December 14, 2020, the U.S. Supreme Court decided Texas v. New Mexico, holding that New Mexico was entitled to delivery credit under the Pecos River Compact for water stored at the request of Texas that evaporated during storage.

The Pecos River originates in New Mexico and flows south through Texas into the Rio Grande River. Texas and New Mexico are parties to the Pecos River Compact, which provides for “the equitable division and apportionment of the use of the waters of the Pecos River” between the two states. In 1987, after several disputes under the Compact, the Supreme Court appointed a “River Master” to calculate New Mexico’s annual delivery obligation to Texas.

In the fall of 2014, a tropical storm caused heavy rainfall in the Pecos River Basin. To prevent flooding, Texas’s Pecos River Commissioner requested that some of the river’s water be stored in New Mexico. New Mexico’s River Commissioner agreed. Several months later, the water was released. But a significant amount of water had evaporated while the water was held in New Mexico. Upon a motion by New Mexico, the River Master granted New Mexico delivery credit for the evaporated water. Texas filed a motion for review with the Supreme Court, invoking the Court’s original jurisdiction.

The Supreme Court denied Texas’s motion for review, leaving in place the River Master’s determination. First, the Supreme Court held that New Mexico’s motion for credit was timely, because both states agreed to postpone the River Master’s resolution of the evaporated-water issue while they tried to negotiate an agreement. Second, the Court upheld the River Master’s merits determination that New Mexico was entitled to credit for the evaporated water. Under the River Master’s Manual, which the Court adopted by decree in 1988, “[i]f a quantity of the Texas allocation is stored in facilities constructed in New Mexico at the request of Texas, then … this quantity will be reduced by the amount of reservoir losses attributable to its storage.” This text and the record evidence of the states’ “establish that New Mexico is entitled to delivery credit for the water that evaporated while New Mexico was storing the water at Texas’s request.”

Justice Kavanuagh delivered the opinion of the Court, in which Chief Justice Roberts and Justices Thomas, Breyer, Sotomayor, Kagan, and Gorsuch joined. Justice Alito filed an opinion concurring in the judgment in part and dissenting in part. Justice Barrett took no part in the consideration or decision of the case.