A new chapter in the decade-long lawsuit in the U.S. Supreme Court over Rio Grande water is set to begin.

After a close, 5-4 ruling from the Supreme Court dashed a proposed deal to end the litigation, the federal government and states of Colorado, New Mexico and Texas have been ordered back to mediation, which begins Tuesday in Washington D.C.

In addition to the parties, there will be attorneys for groups including farming interests, the cities of Albuquerque and Las Cruces, water utilities and irrigation districts, joining the talks.

In 2013, Texas sued New Mexico, alleging that groundwater pumping in southern New Mexico diverted water out of the Rio Grande owed to Texas violating the 86-year old agreement called the Rio Grande Compact. Signed in 1938, the compact divided use of the Rio Grande between Colorado, New Mexico and Texas.

Only the Supreme Court has the power to rule on disputes between states.

A dispute over baselines for groundwater

One of the core disagreements between the federal government and the three states is determining how much groundwater pumping needs to be cut along the Rio Grande in southern New Mexico. In the arid region, water is crucial for growing crops like chile and pecans, and both groundwater and water from the Rio Grande are used for irrigation.

In the rejected settlement agreement, the states requested a baseline adjusted to more groundwater pumping and drought conditions determined by an equation called the “D2 curve.”

The D2 curve was used as part of a 2008 settlement ending a fight between the irrigation districts and the U.S. Bureau of Reclamation over drought concerns.

Alternately, the federal government has previously asked for the states to adopt restrictions from when the compact was first signed.

The states will continue to argue for the D2 curve baseline, said James Grayson, the chief deputy for the New Mexico Department of Justice.

“In 1938, there was essentially no groundwater being used, and so the United States is essentially advocating to go back to that time and that way of using only surface water,” Grayson said.

The City of Las Cruces and the New Mexico Attorney General have urged federal officials in recent months to make a deal with the states before the start of Donald Trump’s presidency in January and compromise on its position to drastically limit groundwater pumping in southern New Mexico.

New Mexico’s congressional delegation asks feds to wrap up Rio Grande lawsuit

In a Nov. 14 letter, New Mexico Attorney General Raúl Torrez, a Democrat, appealed to U.S. Department of the Interior Secretary Deb Halaand to drop the objections.

“Time is running out,” Torrez wrote. “And I am pleading with you to resolve this issue for the benefit of all parties, but especially for the people of southern New Mexico, rather than leaving the matter to become a political bargaining chip for the next administration.”

In an October letter to U.S. Attorney General Merrick Garland, the City of Las Cruces stated that cutting pumping to a 1938 level would reduce the city’s groundwater use by 93%.

This action “would cripple farmers, families and communities in southern New Mexico,” and would require the state’s second-largest city to find a new source of water, requiring a $1 billion investment and take about 15 years to put into place.

In 2023 testimony before lawmakers, state officials said New Mexico would need to cut groundwater use in southern New Mexico by at least 17,000 acre-feet to meet the deal set by the D2 curve baseline, by reducing pecan and chile fields. If the 1938 standard was required, cuts would need to be in the hundreds of thousands of acre-feet.

How we got here

The contours of the dispute have changed since the case was first brought in 2013. Drought conditions in the early 2000s sparked a protracted series of water lawsuits in lower courts between the federal government, states, cities and counties and irrigation districts along the Rio Grande.

In 2019, the high court unanimously allowed the U.S. federal government to intervene as a party in the case, arguing that a series of federal dams, irrigation canals and ditches were threatened by New Mexico’s groundwater pumping. That federal infrastructure is used to deliver Rio Grande water to Mexico under a 1906 treaty and also meets agreements with two regional irrigation districts.

While the federal government initially sided with Texas in the lawsuit, a series of compromises eventually put the states in one camp and the federal government (and the regional irrigation districts) in another.

Colorado, New Mexico and Texas came to an eleventh-hour settlement in 2022, but the federal government objected and said that the deal couldn’t be made without their agreement.

In June, the U.S. Supreme Court sided with the federal government’s objections, rejecting the proposed deal.

Earlier this year, justices appointed a new special master, who oversees the progress of the case.

After an October hearing, Judge D. Brooks Smith ordered the parties into mediation, which starts Tuesday and will end Thursday, but could continue to be extended. If mediation talks break down entirely, the parties will resume going to trial.