*Clarification appended

A top Texas lawmaker is asking President Donald Trump to renegotiate a long-standing agreement between the United States and Mexico that dictates how to divvy up water in the Rio Grande and Colorado rivers amid the president's push to re-evaluate international treaties. The key message for the administration: Don't forget about the water.

Concerned about the impact to Texas agriculture in the fertile Rio Grande Valley, state lawmakers have long tried to get the federal government to better enforce a 1944 water treaty with Mexico that entitles the United States to one-third of the water from the Rio Grande — an agreement they say is all the more vital with drought gripping much of Texas.

Earlier this month, state Rep. Lyle Larson— a Republican from San Antonio who heads the House Natural Resources Committee — sent letters to Trump and U.S. Secretary of State Mike Pompeo asking that the administration address what he described as Mexico's repeated failure to hold up their end of the agreement. As the two countries discuss challenges like immigration and trade, Larson wrote, water obligations should also be grounds for discussion.

The Texas Tribune thanks its sponsors. Become one.

The International Boundary and Water Commission — the bi-national agency that monitors and enforces the treaty — currently lacks a permanent U.S. commissioner. Larson's letter also asks that whoever is appointed better represent the nation's interests regarding the treaty.

Current IBWC leadership say Larson's calls for renegotiation are unnecessary, pointing to the fact that Mexico repaid its water debt in full in 2016, and is currently ahead in its scheduled water deliveries. But to Larson, that's not enough.

"We need consistency," Larson said. "Of the 30 years, they've been out of compliance something like 25. That is killing the economy of the Rio Grande Valley and it's unfair to the United States. We're supposed to congratulate someone for meeting the conditions of the treaty? I think it's ludicrous and it shows how out of touch the IBWC is with their job."

The treaty establishes how the two countries share surface water from the Rio Grande and Colorado River (not the Colorado River that flows through Texas.). Under the 74-year-old agreement, Mexico delivers water to the United States from six tributaries that feed into the Rio Grande and, in exchange, receives water from the Colorado.

The treaty stipulates that the United States must meet its water obligation annually, releasing 1.5 million acre-feet of water down the Colorado River to Mexico each year. In contrast, Mexico must deliver an annual average of 350,000 acre-feet of water every five years.

Half a decade to comply has allowed Mexico to habitually under-deliver since about 1992, Larson said. Provisions in the treaty allow reduced deliveries in the case of “extraordinary drought,” which Larson said Mexico has frequently abused.

The Texas Tribune thanks its sponsors. Become one.

“[The United States is] releasing those waters to Mexico with 100 percent compliance for the past five decades,” Larson said. “To contrast that with what Mexico does with the United States, they’re seldom ever in compliance on an annual basis. The federal government has to get involved. We're rewarding bad behavior.”

U.S. lawmakers calling for action is nothing new — legislators have sent similar letters to past secretaries of state and have frequently urged Mexico to comply with the treaty. The IBWC has been the target of calls to action from lawmakers before.

But while Mexico has fallen behind in scheduled water payments in the past, more recently, the country has been more consistent with its obligations, said Sally Spener, foreign affairs officer for the IBWC. As of July 7, Mexico is actually ahead of schedule with its water deliveries to the states by about 46,000 acre-feet. In 2016, Mexico fully paid off its Rio Grande water debt from the previous five-year cycle.

And Spener said Mexico isn’t the only country to decrease deliveries because of drought. Under other water treaties, like the Convention of 1906, she said both Mexico and the United States have voluntarily accepted reduced deliveries in an effort to maintain and boost elevations at Lake Mead on the Colorado River in times of drought.

“Everybody understands that there is not sufficient water to allocate a full supply to users in the U.S. or Mexico," she said.

The 1944 treaty does not include a specific definition of extraordinary drought, Spener said. Larson thinks that’s something the Mexican government takes advantage of.

But while Texas does have legitimate water concerns, the treaty is more complicated than what Larson lays out, said Stephen Mumme, a political science professor at Colorado State University.

For one thing, times of heavy rainfall can cancel out water debt. When lakes Amistad and Falcon, the two international reservoirs, reach capacity, water debts are cancelled and a new five-year period begins.

The Texas Tribune thanks its sponsors. Become one.

"That's very frustrating for people who aren't familiar with the treaty," Mumme said. "It sort of seems like things are indeterminate and difficult to predict, and they are, because you're depending on Mother Nature to help compensate for deficiencies that may have occurred in preceding years."

Still, Larson said the unpredictability of water deliveries puts pressure on farmers, both in the West and Rio Grande Valley. As farmers deal with widespread drought and lowering aquifers, the need for consistent water from Mexico becomes even more dire, he said.

Mexico’s biggest water deliveries come after heavy rain periods, Larson said, and often in seasons when western farmers don’t need as much irrigation.

“This treaty is one that’s costing hundreds of millions of dollars to agricultural interests in Texas,” Larson said. “You can talk to any farmer in the Rio Grande Valley, people are just besides themselves frustrated with the people in Mexico abusing this treaty.”

According to the Texas A&M AgriLife Extension, Texas suffered from $203.5 million in crop revenue loss due to irrigation shortages in 2016.

Spener said the commission is currently developing a model for Rio Grande management that should help combat unpredictability in water deliveries. The bi-national model should aid in better decision-making and management of water resources, she said. In the past there have been updates to the treaty, but she said she is not aware of any initiatives from either country to renegotiate water management.

Larson said he still thinks Mexico should be required to deliver a set amount of water on an annual basis, as the United States is. And he ​​​​​​​said he hopes whoever is selected by the president as the new commissioner takes a “hard line on compliance with the people in Mexico.”

“In any negotiation that the United States has with Mexico, water needs to be brought up," Larson said. "It can’t wait three or four years when that has a devastating impact on the agricultural economies of the Rio Grande Valley.”

Clarification: This story was updated to clarify that Mexico and the United States have decreased water deliveries to each other during drought under a 1906 water treaty.