GEOFF BENNETT: In a social media post earlier this year, President Trump threatened tariffs and sanctions against Mexico, claiming the country violated a 1944 treaty and his stealing water from Texas farmers.

It's part of a long-running dispute over shared water in the Rio Grande River, a dispute that scientists say is made worse by higher temperatures, extreme weather and a greater demand for water.

Stephanie Sy reports from Texas for our series on the impact of climate change, Tipping Point.

STEPHANIE SY: From Texas State Highway 107 in Santa Rosa, the Rio Grande Valley Sugar Mill strikes an imposing figure.

TUDOR UHLHORN, Rio Grande Valley Sugar Growers, Inc.: We're in the sugar warehouse.

Sugar would fill this entire warehouse as its being processed.

STEPHANIE SY: But it's sat empty since the mill closed its doors last year.

TUDOR UHLHORN: We started disassembly pretty much almost immediately.

STEPHANIE SY: Tudor Uhlhorn is a former sugarcane grower and the mill's last chairman of the board.

TUDOR UHLHORN: I don't come out here like I used to with pride that I'm a sugar grower.

I come out and it's sad to see that this whole industry is gone now.

Symbolically, it is a reflection of nobody cares about us in the Rio Grande Valley.

STEPHANIE SY: The closure of Texas' last sugar mill, which once employed 500 people and processed about 160,000 tons of raw sugar annually, coincided with years of increasing water scarcity in the region.

The Rio Grande Valley is one of the few places in the U.S. where sugarcane will grow, but it takes a lot of water.

TUDOR UHLHORN: Farmers are plowing out cane because no one has any irrigation water.

We just got completely choked off by Mexico's failure to deliver the water.

STEPHANIE SY: That water is owed to farmers here in the Rio Grande Valley as part of a more-than-80-year-old treaty that requires Mexico to share some of the water that winds through its land.

Signed in 1944, the treaty establishes that the U.S. will release a minimum of nearly 500 billion gallons of water from the Colorado River to Mexico annually, providing water for agriculture in the Mexicali Valley and drinking water for large cities like Tijuana.

In return, Mexico will release a minimum of 114 billion gallons of water from the Rio Conchos and other tributaries to the Rio Grande annually, providing water for farmers and residents in the Rio Grande Valley.

The treaty stipulates that any water debt should carry over to the next five-year cycle in times of extraordinary drought.

But critics say drought is not the only reason Mexico hasn't upheld its end of the bargain.

SONNY HINOJOSA, Former Water District General Manager, Hidalgo County Irrigation District No.

2: Mexico is capturing the water that used to flow freely into the Rio Grande.

STEPHANIE SY: Sonny Hinojosa is a former water district general manager in Hidalgo County, Texas.

SONNY HINOJOSA: Since the treaty was signed in 1944, these other eight additional reservoirs were constructed.

STEPHANIE SY: He says Mexico's construction of additional reservoirs has allowed them to keep more of the water before it flows to Texas.

They have fallen short of their obligations to the U.S. five times since the 1990s, even when a tropical storm filled Mexico's reservoirs with over 700 billion gallons of water in 2022.

SONNY HINOJOSA: Mexico didn't release any of that water to comply with the terms of the treaty for the next couple of years.

STEPHANIE SY: The current five-year delivery cycle ends in October.

But, so far, Mexico has released less than 40 percent of what it owes.

SONNY HINOJOSA: Our farmers will once again be faced with shortages.

They either don't plant or they plant reduced acreage.

That's just the way it is.

STEPHANIE SY: Across the southern border, the dependence on the Rio Grande's main tributary, the Rio Conchos, has led to desperation.

Drought has brought this community in the state of Chihuahua out to a lakeside mass.

They pray for rain.

RAFAEL BETANCE, Volunteer Monitor, La Boquilla Dam (through translator): The situation is very critical.

So we need to ask God to send us the rain.

That's the only solution we have for everything.

STEPHANIE SY: For 30 years, Rafael Betance has been measuring the La Boquilla Dam, which stores water from the Rio Conchos.

He says the terms of the 80-year-old treaty are impossible to fulfill these days.

RAFAEL BETANCE (through translator): The commitments they made were when there were surpluses of the dams, when it spilled over, when it rained a lot.

Right now, where do we give them water from?

It hasn't rained for two or three years.

That's the problem.

STEPHANIE SY: The scarcity of water isn't merely due to a lack of rain.

Farms in the region have significantly expanded their acreage in the last few years.

An aging irrigation infrastructure means farmers there are wasting water.

Miles from the dam, farmer Fidel Hidalgo Tarano waters his five-acre walnut grove for the first time in a month-and-a-half by flooding the field, one of the least efficient irrigation techniques.

FIDEL HIDALGO TARANO, Farmer (through translator): I think all of us, those of us who farm, are aware that we are wasting a lot of water.

If there were another watering system, a sprinkler irrigation system or belt system, it takes a lot of money, and it's expensive per hectare.

STEPHANIE SY: Chihuahua is a major producer of walnuts, a water-intensive crop that demands even more irrigation on hotter, drier days.

ALEX RACELIS, University of Texas Rio Grande Valley: It has gotten hotter.

The annual average temperature has changed, increased by almost three degrees.

The average annual number of days over 100 has increased fivefold since 1950.

STEPHANIE SY: Alexis Racelis is an environmental scientist at the University of Texas, Rio Grande Valley.

He says the impacts of climate change, higher temperatures coupled with less predictable rains, have strained water resources, as has an increase in demand.

ALEXIS RACELIS: The U.S. population has increased by three times since 1940 and Mexico by seven times.

So you just have a ton of users, less consistent input of water, which makes it very contentious.

STEPHANIE SY: Racelis works directly with local farmers in the Rio Grande Valley to adapt to a new climate reality, one with less water.

ALEXIS RACELIS: When you look at climate change predictions, where it's only predicted to get hotter and that the periods of drought are predicted to get even more prolonged, and you combine that with the fact that the population is expected to increase, it's pretty grim for agriculture.

STEPHANIE SY: At Hub of Prosperity, a small off-campus farm operated by the University of Texas, Rio Grande Valley, he demonstrates water-efficient farming techniques and promotes a variety of vegetables that can grow in a hotter climate.

ALEXIS RACELIS: Farmers are very resilient.

They're very innovative.

They're coming up with their own techniques that we're helping them explore.

DALE MURDEN, President, Texas Citrus Mutual: That across the street was an orchard.

We were starting to hit that uncertainty of water.

STEPHANIE SY: For most of his life, Dale Murden has been a citrus grower.

He's also president of Texas Citrus Mutual, representing hundreds of other farmers in the state.

DALE MURDEN: This is really not what you want a grove to look like.

STEPHANIE SY: The rough shape of this grower grapefruit grove illustrates the difficult choices growers are having to make in an era of scarcity.

DALE MURDEN: This grove should have already been watered a couple of times this year, and we probably should have had two -- at least two waterings more on this than it has, but that's where we're at with limited supply of water or no water.

And so you kind of tend to let things go like we control.

STEPHANIE SY: Murden watched as the sugarcane industry in the Rio Grande Valley folded, wondering if citrus was next.

DALE MURDEN: What does the future hold?

If I can't even count on my neighbor giving me the annual payment like we do on the Colorado side, man, it renders me really, really incapable of the ability to plan for long range.

STEPHANIE SY: Calls from Texas lawmakers to force Mexico to deliver water more reliably have been echoed by the president.

In April, Trump raised the prospect of escalating tariffs and possible sanctions -- quote -- "until Mexico honors the treaty."

In response, Mexican President Claudia Sheinbaum acknowledged her country's shortcomings and released roughly 18 billion gallons of water.

DALE MURDEN: It's definitely on this administration's radar, for whatever reason.

There's chips in every trade negotiation.

If I'm being used as a chip, so be it.

At least we're getting the attention.

STEPHANIE SY: But the attention may be too little, too late.

That's certainly the case for Texas' sugarcane industry, the first, but likely not the last casualty of this water battle.

For the "PBS News Hour," I'm Stephanie Sy in the Rio Grande Valley.