Ciudad Acuna, Mexico — In 1945, President Harry S. Truman signed a treaty intended to bring fair play to the fight for water in the parched deserts of the US-Mexico borderlands.

Nearly 70 years later, engineer Roberto Enriquez de la Garza stood on the lip of the Amistad Dam — vultures circling overhead, grassy islands poking out of the depleted reservoir below — and explained why Mexico can’t hold up its side of the bargain.

"The US gets angry: Why aren’t you giving us water? Well, how can we when there is no water?" he asked. "I can’t do anything. It’s not raining."

The historic drought across the western United States that has drained the water table in California and devastated rivers and reservoirs in Arizona has intensified a diplomatic dispute here along the Texas border. Under the terms of the treaty, the United States is obliged to give Mexico water from the Colorado River, while Mexico must transfer water from the Rio Grande and its tributaries.

But in recent years, Mexico has fallen behind on its obligation. The accounting for the water sharing is tallied in five-year cycles. And at this point, in the fourth year of the present cycle, Mexico owes the United States 380,000 acre-feet of water, more than all the water consumed in a year by the 1.5 million residents of the Lower Rio Grande Valley in South Texas.

"This issue is life or death for some of our farmers, their ability to support their families and make a living," said Texas state Representative Eddie Lucio III, who has been leading the charge to make Mexico give from its rivers. "We’ve been good neighbors. We just want to share and share alike."

The Amistad Dam, with its one-quarter-full, border-straddling reservoir, is ground zero in the dispute. Finished during the Nixon administration in 1969, the dam is jointly administered by the two countries: Of the 16 floodgates, eight are maintained by Mexico and eight by the United States. Each country operates a hydroelectric power plant at the dam, and water levels and releases are calculated and coordinated by engineers from both countries. In the Cold War-era control rooms, clocks show both Mexican and US time zones.

Mexico doesn’t dispute its water debt but says that its own shortages make it impossible, at this point, to supply the annual 350,000 acre-feet that it should be giving to the US.

"We have had a prolonged drought since 1994 until now. It has been difficult for Mexico to give this water," said Ignacio Peña Treviño, Mexico’s representative here on the International Boundary and Water Commission. "There isn’t rain like there was in the past."

The treaty, which was agreed on in 1944 and ratified by the US Senate the following year, stipulates that in the event of a dam failure or "extraordinary drought," either side can make up its shortage in the next five-year cycle. But water officials in Texas don’t think Mexico’s weather conditions meet that standard.

"They haven’t been in any sort of significant drought conditions since March of 2012," said Carlos Rubinstein, chairman of the Texas Water Development Board. "That excuse, pardon the pun, doesn’t hold water."

Texas’s water supply has improved since its most acute drought a few years ago — in 2011, all 254 counties were suffering drought conditions. But pockets of the state are still suffering. Cities such as Raymondville and Rio Hondo have had to purchase water from other jurisdictions.

A Texas A&M University study estimated that Mexico’s failure to share water was causing a loss of nearly 5,000 jobs and $229 million in revenue from crops such as cotton, corn, sorghum and citrus fruits.

Texas officials of all ranks clamor for relief. Governor Rick Perry wrote to President Obama last year about the problem. Federal and state legislators are demanding that the State Department take further action to pressure Mexico to comply with the treaty. US and Mexican technical officials regularly discuss the issue, Rubinstein said, but "without State Department support, none of it has translated into a meaningful agreement."

In my mind," he said of the State Department’s efforts, "they have failed."

The diplomats insist they are engaged. Roberta Jacobson, the State Department’s top official for Latin America, wrote in a letter last year that she and her colleagues raise the water issue "everywhere we encounter Mexican leaders." She added that "we will persist in our advocacy until we achieve our goal of securing sufficient water deliveries from Mexico to relieve the hardship of south Texas communities." A US official who spoke on the condition of anonymity because he was not authorized to comment publicly on the issue said US officials are "continuing to stress to the government of Mexico the importance we place on reaching agreement on a durable solution quickly."

Whether the drought on the Mexican side qualifies as "extraordinary" or not, the signs of a dwindling water supply are everywhere. Grim little tributaries seem to barely clear the rocks before they reach the Rio Grande. Even if the floodgates were opened at the Amistad Dam, the water level wouldn’t reach them.

The United States may not be getting its full legal share, but Mexican officials point out that their northern neighbor still draws far more out of the reservoir than they do. Of the 23 cubic meters per second flowing out of the reservoir on an average day, the United States is using 20 of them. Texas officials argue that if Mexico managed its tributaries better, there would be more water for both sides.

At the same time, demand for water in Mexico has also grown sharply. Many people have moved to Ciudad Acuna, the town closest to the dam, to fill factory jobs making parts for Mexico’s booming auto industry. The population in the past 20 years has nearly doubled.

In these dry times, Mexican officials argue, the need to drink and irrigate crops trumps Truman’s treaty.

"You can’t leave people without water," Enriquez said.