On a scorching afternoon in South Texas, Sonia Lambert looked out at an open-air canal that carries mud-green water from the Rio Grande to nearby towns and farmland, losing much of it to evaporation and seepage along the way.

“That will be someone else’s problem,” Lambert said, referring to her upcoming retirement as head of an irrigation district near the U.S.-Mexico border.

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In the Rio Grande Valley, a canal system designed more than a century ago for agriculture still delivers water to the region’s lush farmland and fast-growing towns and cities. Today, the canals lose as much as 40% of the water they carry, waste that experts say could contribute to steep water shortages in coming decades as the population grows and climate change intensifies droughts.

“As this region continues to become drier due to climate change, water supplies will be greatly reduced,” said Guy Fipps, a professor of irrigation engineering at Texas A&M University who has studied the water system since 1998.

State water officials predict that over the next 50 years, demand for water in the area’s cities and towns will double. For decades, McAllen developed at a dizzying pace, with newcomers drawn to a large free-trade zone and jobs in health care, education and retail. Between 1990 and 2020, McAllen and the neighboring cities of Edinburg and Mission grew sixfold to nearly 871,000 people, according to the U.S. Census Bureau. Similarly, the Mexican cities of Reynosa and Matamoros across the border mushroomed after U.S.-owned assembly plants were established in the mid-1990s.

Further complicating matters is a 1944 treaty between the U.S. and Mexico that defines how the countries share water from the Rio Grande. Mexico is supposed to route 350,000 acre-feet of water every year to the U.S. — enough to supply as many as 700,000 households. But it has periodically failed to meet those obligations, delaying deliveries because of drought, tight water supplies and a thirsty crop industry in northern Mexico.

The late deliveries are a source of frustration, but water managers and farmers in the U.S. are quick to acknowledge a major challenge at home too: the leaky canal system that has long been seen by local and state officials as too expensive to overhaul.

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The region’s more than 2,000 miles (3,219 kilometers) of pipelines and canals — some 100-feet (30 meters) wide — are meant for large, infrequent deliveries to farmland. Common fixes to modernize the waterways and make them more efficient — attempted by many districts to some degree — include lining earthen canals with concrete and more closely monitoring water use by farms with meters. Another option comes with a bigger price tag: replacing canals with underground pipelines, which lose far less water and are better suited to serving cities.

Converting a mile of open-air canal into underground pipelines costs between $250,000 and $1 million, said Lambert, the irrigation district manager for Cameron County, which remains mostly rural. Her district has only been able to bring about a fifth of its 250 miles (402 kilometers) of canals underground in the past two decades, she said.

“It just gets to be an amount that could not be supported by the farming community,” Lambert said.

Since the early 1900s, a network of about two dozen independent irrigation districts have served the area’s farmers, cities and towns. But as McAllen gobbled up much of the farmland surrounding it, some officials have wanted more control over a water district they say charges the city too much for water deliveries.

Yet the higher rates charged to city water utilities are often how irrigation districts pay for canal repairs, said Fipps. That has meant the water districts serving bigger cities have generally made more progress in bringing canals up to date.

Still, the Rio Grande Valley’s water utilities and farms are linked by the same aging system.

Since cities and farms get water from the same canals, hydrologists and water officials say the Rio Grande’s shrinking flows and low reservoir levels could eventually spell trouble for everyone in a prolonged drought. When there’s little water in a canal, a greater percentage gets lost to evaporation or seepage. And everyone’s share of water is threatened.

Already, experts say demand for water from the river exceeds supply.

Small towns, which get relatively small water deliveries, could be especially affected during a severe dry spell, and their irrigation districts are less likely to have the money to repair or replace canals.

“This is an unusual situation, that the agricultural canals are used to help deliver the municipal water,” Fipps said.

Over time, experts say the region’s farms will face deepening water shortages and be forced to make more difficult choices, a scenario already playing out across parts of the American West. Some irrigation districts over the years have received state or federal funding through grants administered by the Bureau of Reclamation for repairs, but water managers, farmers and hydrologists say the money has been insufficient for comprehensive fixes. The Texas Water Development Board predicts that by 2070, water used to irrigate farms in the Rio Grande Valley will fall by 36%, in large part because more farmland will be replaced by urban development.

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In rural Cameron County, Lambert is already seeing glimpses of that future. Earlier this year, before rain soaked the region, Lambert told sugarcane farmers in her district that they would only get one water delivery instead of two.

To salvage their thirsty crops, some farmers bought water from neighboring districts for tens of thousands of dollars. Others removed more than a hundred acres of the crop. A few weeks later, the skies opened up.

When farmers ask her how much water they can expect to receive the next season, Lambert says she often doesn’t have an answer.

“That’s the million dollar question asked by our farmers. And I have no earthly idea,” she said.