Fighting the Water Wars : Along the Rio Grande, competition for a scarce resource.

Dale Murden ("like murder, except with an n," he explains) was driving his pickup through the 100-degree Texas heat on a recent afternoon when his mobile phone rang. It was another farmer, who began the conversation by saying, "Mexico sucks." It's a sentiment shared by anybody whose ability to farm the sun-scorched dirt of southeast Texas depends on the Rio Grande, the 1,885-mile river that divides the United States from its southern neighbor. During a decade of sporadic drought, Mexico has flouted a 1944 treaty that requires it to release 350,000 acre-feet of water a year from Mexican tributaries into the river for American farmers downstream. Murden and his friends fear economic ruin if Mexico doesn't open its dams soon.

It is hard to imagine draining a river that once flowed as mightily as the Rio Grande. Before the early 1990s, rains provided enough water for Mexico to meet its treaty obligation. With the drought, the river has become a long lake, ending in a sandbar where it once flowed into the Gulf of Mexico. In a war over water, the worst thing is to be downstream. The next worst thing is to be politically unimportant. The farmers of southeast Texas are both--which explains why they must make do with 40 percent less water than they use in a normal year.

That is a sharp contrast to the Mexican state of Chihuahua, 1,000 miles upstream. With two dams, Chihuahua controls the flow of the Rio Conchos, the most important feeder river of the Rio Grande. There, officials claim that the drought has simply made it impossible to comply with the treaty. But it's not so simple, according to a pair of recent studies. One suggests that instead of releasing the water into the river, Chihuahua has been maintaining normal levels of irrigation inside its borders; proof is a series of satellite images collected by the University of Texas, showing wide swaths of green in the Rio Conchos basin. The second study concludes that not only has Chihuahua increased agricultural production over the past several years, but it has switched to thirstier crops such as pecans and alfalfa. Mexican officials dispute the findings. "The dams are at their lowest levels in 30 or 40 years," says Jose Luis Garcia Mayagoitia, the Chihuahua state secretary of agriculture. "We can't deliver water we don't have."

On both sides of the border, the debate boils down to competition between local and national interests. Congressional elections are scheduled for ...