Tribal rights to groundwater have not been legally established to the same extent as rights to other natural resources (e.g., surface water, timber, minerals). A March 2017 decision of the U.S. Court of Appeals for the Ninth Circuit upheld the rights of a California Indian tribe (the Agua Caliente band of Cahuilla Indians) to groundwater beneath the tribe's reservation in the Coachella Valley. In November 2017, the U.S. Supreme Court declined to review the decision; the case now proceeds to other phases that may partially shape this ruling's significance.

This is the first case in which an appellate court has held that a tribe's "reserved" rights under the Winters doctrine apply to groundwater.

Pursuant to Winters, when Congress reserves land for a reservation, it also reserves a right to water necessary to accomplish the purposes for which the reservation was created. For decades, tribal reserved rights for surface water have been established and have required settlements between the federal government (as authorized by Congress), tribes, and nonfederal users. Thus, some have raised concerns about the implications of interpreting Winters rights to include groundwater.

This Insight explores three aspects of groundwater relevant to Agua Caliente: quantity, quality, and "pore space." The legal outcome may affect how groundwater is allocated among tribal and nontribal interests and may influence groundwater allocations at other federal reservations.

The Supreme Court has not declared outright that groundwater is subject to the Winters doctrine, but declining to review the Ninth Circuit decision essentially upheld phase one of the three-phase case: a federally reserved right to the groundwater resource for the tribe. Phase two will address whether the tribe has the right to receive groundwater of a certain quality from the aquifer and whether it owns the pore space beneath tribal lands. Phase three will determine the quantity of groundwater to which the tribe is entitled, pursuant to its reserved rights. The outcome of phases two and three could have implications for expanding the scope of federal preemption of state water law. It also could affect how much groundwater is available for nontribal water agencies and water districts and those entities' ability to store water in an aquifer beneath federally reserved lands. Congress has authority over federal water law and could influence this legal framework.