Tensions are rising in a border dispute between the United States and Mexico. But this conflict is not about migration; it’s about water.

Under an 80-year-old treaty, the United States and Mexico share waters from the Colorado River and the Rio Grande, respectively. But in the grip of severe drought and searing temperatures, Mexico has fallen far behind in deliveries, putting the country’s ability to meet its obligations in serious doubt.

Some politicians say they cannot give what they do not have.

It’s a tough argument to swallow for farmers in South Texas, also struggling with a dearth of rain. They say the lack of water from Mexico is propelling them into crisis, leaving the future of farming in the balance. Some Texas leaders have called on the Biden administration to withhold aid from Mexico until it makes good on the shortfall.

Both countries are staring down the prospect of another long, hot summer and many are pinning hopes on a storm to swell Mexico’s drought-stricken rivers. Yet experts say the pray-for-rain approach is a risky, short-term strategy in the face of a knotty long-term problem.

The conflict underscores the immense difficulties of navigating how to share shrinking water resources in a hotter, drier world.

A river in decline

Under a 1944 treaty, Mexico was required to send 1.75 million acre-feet of water to the US every five years from the Rio Grande, and the US to send 1.5 million acre-feet of water to Mexico from the Colorado River each year.

One acre-foot is enough water to flood one acre of land a foot deep. It adds up to an enormous amount of water exchanged between the two countries: around 490 billion gallons from the US annually and 570 billion from Mexico each five-year period.

Mexico is falling far behind in its obligations, said Maria Elena Giner, the US commissioner of the International Boundary and Water Commission, the bi-national body that oversees the treaty.

“We’ve only gotten about a year’s worth of water and we’re already well into our fourth year,” she told CNN. The current cycle ends in October 2025.

The Rio Grande — called the Río Bravo in Mexico — is one of North America’s longest rivers and flows roughly 1,900 miles from Colorado’s Rocky Mountains, weaving through three US and five Mexican states before ending its journey in the Gulf of Mexico.

Years of over-extraction to serve farmers and booming populations, along with climate change-fueled heat and drought, have taken a toll.

As heat drives snowpack loss in the mountains, the river’s flows are falling, said Alfonso Cortez Lara, a director at the College of the Northern Border.

Roughly 200 miles of the Rio Grande, stretching from Fort Quitman to Presidio, Texas, is known as the “forgotten reach,” where the riverbed is often bone-dry through the year. It is brought back to life further downstream by waters from the Rio Conchos in the Mexican state of Chihuahua, the Rio Grande’s largest tributary.

The river’s unpredictability is the reason Mexico’s commitments are based on five-year — rather than annual — cycles, Giner said. “There’ll be times of deficits and surplus.”

Shortfalls in one five-year cycle can be rolled over but have to be made up in the next, although the treaty has no enforcement mechanism.

During the first few decades of the treaty, all went well. But from the early 1990s, “something changed,” said Giner. There was less water coming into the river.

Just like the Colorado River deal between southwest US states, the Mexico-US treaty calculated water availability based on data from the first half of the 20th century. It foresaw short-term droughts, but not multi-year megadroughts.

Mexico ended two five-year cycles in deficit, from 1992 to 2002. “This is where the first time we had these heightened political tensions between the two (countries) regarding water,” said Vianey Rueda, a researcher at the University of Michigan who specializes in the 1944 water treaty.

Now, nearing another five-year cycle, Mexico is facing a similar situation. Only this time it’s more intense, Rueda said. “The water delivery system has stayed the same, but the water crisis has worsened.”

A confluence of factors has fed into this crisis.

Water demand shot up as development along the Rio Grande soared. The North American Free Trade Agreement, which went into effect in 1994, led to an explosion of farms and maquiladoras (factories) in Mexico, many growing and making products destined for US and Canadian markets. Both sides of the border urbanized and populations increased.

Underlying everything, the steady drumbeat of the climate crisis fuels more frequent and more prolonged heat and drought. “You have treaties that were meant for a stable climate, but now are trying to be enforced in a climate that is not stable,” Rueda said.

Pain in both countries

The water from Mexico goes to the Falcon and Amistad Reservoirs which straddle the border and provide water to homes as well as farms. Both reservoirs have slumped to historically low levels — in mid-June, Amistad was at less than 26% capacity and Falcon was at only 9.9%.

“Farmers in the Rio Grande Valley of Texas are either out of water or running out of water quickly,” said Brian Jones, a farmer who grows irrigated cotton, corn, sorghum and soybeans in Hidalgo County, Texas, and a board member of the Texas Farm Bureau.

Low water deliveries from Mexico, combined with a dearth of rain in the region, are threatening the state’s citrus industry, Jones told CNN, but the situation is even worse for sugar.

“The sugar industry is lost to Texas and will never return,” he said.

The state’s only sugar mill, which employed more than 500 full-time and seasonal workers, shut in February after more than 50 years of operation.

The mill’s owners, the Rio Grande Valley Sugar Growers, blamed Mexico. “For over 30 years, farmers in South Texas have been battling with Mexico’s failure to comply with the provisions of the 1944 Water Treaty,” it said in a news release announcing the closure.

Some state leaders have demanded punitive measures. “Mexico’s lack of timely water deliveries puts all Texas agriculture at risk,” Texas Rep. Monica De La Cruz, a Republican, said in February, calling on the Biden administration to “hold Mexico’s feet to the fire.”

De La Cruz, who also has the support of Ted Cruz and John Cornyn — both Republicans representing Texas in the US Senate — added language to the House of Representatives 2025 budget bill that would withhold aid for Mexico until it agrees to deliver on the water treaty. The bill cleared a procedural hurdle on Wednesday, although it’s uncertain whether it could amass enough votes to pass through Congress.

The pain of water scarcity is not one way. South of the border, people are also suffering.

Mexico is enduring its most expansive and severe drought since 2011, affecting nearly 90%of the country. Water has become an increasingly fraught topic, with fears cities — including Mexico City — could be barreling toward a “day zero,” on which water runs dry.

The situation is particularly dire in northern Mexico. “The impact is reflected in very low levels of many of the dams in northern Mexico and even in the groundwater levels,” said Victor Magaña Rueda, a climate scientist at the National Autonomous University of Mexico.

The entire state of Chihuahua has been in drought since February, with data from the end of May showing nearly 40% is in “exceptional drought,” the most severe designation.

“Not a single drop of rain has fallen in more than eight months,” said Salvador Alcántar, a congressman in Chihuahua. “Climate change is here to stay, we have to learn to deal with it.” River water and groundwater supplies are fast diminishing and farmers have been suffering for years, he told CNN.

Tensions reached a boiling point in 2020 after the Mexican government decided to release water from one of Chihuahua’s dams to fulfill its water-sharing obligations. Farmers protested in a confrontation that turned deadly when a woman was shot by the National Guard.

Again Chihuahua is struggling. “If there is no water, what can we realistically be expected to pay with? No one can be forced to give away what they don’t have,” Alcántar said.

A conflict with no winners

The difficulty of reshaping 80-year-old water-sharing agreements is that they’ve created reliance.

People come to depend on water rights to develop industry, grow agriculture and build towns, said Sarah Porter, director of the Kyl Center for Water Policy at Arizona State University. “And once you have that reliance, it becomes extremely painful to change,” she told CNN.

A full renegotiation of the treaty is unlikely. Instead, amendments are agreed between the countries through a “minute” process. Minutes can encompass issues from data-sharing to water delivery changes.

Discussions for a new minute aimed at making Mexico’s water deliveries more reliable stalled at the end of last year, as Mexico focused on elections. Now that they’re over, with climate scientist Claudia Sheinbaum set to take office in October, negotiations are ramping up again, said Giner. “We’ve asked Mexico for a plan on how they’re going to meet their deficit right now.”

Some hopes lie in a tropical storm or active hurricane season. But it’s hard to predict when and where storms will hit and rain will land. To rely only on storms to reduce water scarcity would make Mexico “totally exposed to what nature decides about our water future,” UNAM’s Magnaña Rueda said.

In the short term, “if there’s no water to distribute, there’s nothing we can do,” Giner said, but she remains positive. She is pushing for tools to build drought resilience and promote water conservation and efficiency.

Sheinbaum, the president-elect, has committed to prioritizing water issues. But the US could also have a new president when the current five-year cycle ends in 2025, which could further complicate relations.

Ultimately, there needs to be a recognition that water-sharing agreements must adapt to a changing climate, Rueda said.

Instead of seeing water as a zero-sum game, where one party’s gain is contingent on the other’s loss, both sides should realize they are “suffering the same thing because of climate change,” she said.

“Then you start eliminating that zero-sum game, you start saying we’re both losing essentially. Nobody’s winning.”