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 is 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.

Mexico, US share water

Under a 1944 water treaty, Mexico is obligated to deliver 1.75 million acre-feet (2.16 cubic kilometres) of water from its portion of the Rio Grande to the United States over a five-year cycle. The water is stored in the Falcon and Amistad reservoirs, which straddle the border of the two countries.

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 Quitmen 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 really 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.

Demand for water 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.

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