In drought-ravaged California, the vast freshwater aquifer beneath the Coachella Valley is a rare bright spot.
The U.S. Geological Survey once tried to measure how much water it held but gave up because the supply was so plentiful.
But there is growing concern by some that local water agencies are drawing too much out of the aquifer, which supplies water for more than 260,000 people. The two water providers that control the aquifer, the Coachella Valley Water District and the Desert Water Agency, acknowledge that they have drawn down the water supply but say they replace some of it with water from the Colorado River.
That's not enough for some critics, including leaders of an Indian tribe that is now suing to wrestle water rights from the districts.
It's one of several legal disputes over water being fought across California, fueled by a drought that is making groundwater a more precious resource than ever.
"No one cares about water rights except when there's a shortage — and then people care a whole lot in a hurry," said water law expert Eric L. Garner. "If there's a shortage, you tend to get some finger-pointing. And, frankly, this drought is scary."
The Coachella Valley's aquifer sits partly under tribal land controlled by the Agua Caliente Band of Cahuilla Indians.
Tribal chairman Jeff L. Grubbe passed his childhood days playing tag in these canyons, where majestic rocks and wavy palm trees shaded him from the relentless desert sun. He didn't think twice about slurping up the reservation's clean tap water after a day tramping through the dirt.
But these days, Grubbe says, the tap water doesn't taste as good, because the Colorado River water being pumped into aquifers is not as pure as the groundwater. He and others worry the water districts are using too much water, creating the possibility of shortages for future generations.
Grubbe and his tribe filed suit against the two districts in 2013, asking a court to recognize its water rights and give its members a say in how the water is managed.
In court documents, the tribe argues that because the reservation was established by executive order in 1876, it has rights to groundwater "sufficient to accomplish the purposes of the Reservation." The tribe contends it should get a formal say on decisions about how to distribute and protect the dwindling resource.
The water agencies dispute that the tribe has or needs "reserved rights," and in legal documents they deny that they are wasting or contaminating the water.
Earlier this month, a U.S. district judge ruled on the first part of the tribe's lawsuit, declaring that it has reserved rights to the groundwater. In coming months, the court will also decide whether to stop the local water agencies from taking more water from the aquifer than they put back in and whether to prevent them from recharging it with Colorado River water, as the tribe demands.
The U.S. Department of Justice had filed a complaint supporting the tribe, requesting that the court acknowledge its water rights and prevent the water agencies from overdrafting.
The water agencies Monday filed a petition for permission to appeal the court's initial ruling. They say that Colorado River water meets drinking standards and that treating it would drive up customers' bills by as much as $450 annually.
There is much dispute about whether the districts are pumping out too much water, which supplies Coachella Valley. The water goes to 120 golf courses that suck up 37 billion gallons annually.
The Coachella Valley Water District has taken out more water than it has put back in from the Colorado River all but five years since 1990, according to district data.
But officials say that's not a major problem because the water supply is so vast.
"We're not in a situation where the groundwater supply is in any immediate jeopardy," said district spokesman Jack Porrelli. "But that doesn't mean we don't take it seriously."
Officials also note that the rate of annual overdraft is slowing, in part, because water use is decreasing. In a 2014 report, Coachella Valley Water District officials predicted that overdrafting could be eliminated by 2021 if reductions continue.
The tribe, citing Coachella Valley Water District statistics, says the aquifer lost about 35 billion gallons of water each year between 2000 and 2009.
The tribe is among the water districts' customers. About 70% of its more than 450 members still live on the reservation. The tribe also employs about 2,500 people who help run casinos, golf courses and other businesses. A breakdown in communication and concern about water scarcity prompted the tribe to file suit, Grubbe said.
"The water agencies didn't respect the tribe's claims to water rights at all," he said. "We wanted to have a voice."
At times, the dispute has become testy.
In one Coachella Valley Water District news release, an official said the tribe used "scare tactics and makes false accusations in an attempt to take away the public's water rights." In a tribe statement, Grubbe called the agency arrogant and said its statements were "rife with fabrications, lies and misstatements."
Legal water warfare is being waged in courtrooms elsewhere in Southern California too.
A battle over an unregulated Antelope Valley basin wound through the courts for 15 years until water providers announced a settlement this month outlining how the aquifer will be managed.
In San Bernardino, water agencies have been fighting over the Rialto-Colton groundwater basin since 2013. A decades-old court decree limits pumping from the basin during times of drought, and a group of local agencies sued Fontana Water Co., alleging it has been pumping more water than the decree allowed.
New groundwater laws passed last year could bring yet another wave of water litigation.
The laws, which require local agencies to create groundwater management plans, apply to hundreds of groundwater basins that are not already subject to court orders, experts said. And in Central and Northern California, where far fewer aquifers have been divvied up, the new rules are likely to ignite additional fights.
It will take years for new plans to develop, but when they do, some parties are likely to feel slighted. There will be arguments over pumping limits, over which local agencies will make decisions, and over who can impose fees and how much, said Henry S. Weinstock, an attorney representing the Fontana Water Co. in its case.
"Things are heating up," Weinstock said. "Water lawyers are already more busy than they were a few years ago."