"This desert groundwater is very different than what we'd observe when we analyze the few millimeters of rainwater that fall here each year," Jobbagy said.
The desert water contains molecules that must have originated in the Andes, he said, and that match the molecular structure of water formed in clouds that form between the Pacific and the Andes.
The Atlantic moisture arrives after flowing west from the humid Pampas towards dry regions right below the Andes.
The Pampas is a vast, fertile flatland and the source for much of South America's agricultural production.
A Radical Decision
The loss of wetlands and a wave of European immigration at the turn of the 20th century forced the indigenous Huarpes and Criollos peoples—who once numbered in the tens of thousands and are known today as Lavallinos—to build communities elsewhere.
They made a radical decision: Rather than stream into Argentina's cities, they chose to adapt to the one environment Argentina's immigrants shunned—the harsh 15,440 square-mile (40,000-square-kilometer) Monte Desert.
Now the modern-day heirs to that culture depend on a reliable flow of water from the Andes to the desert.
The water descends a topographic staircase: At the top are the Andes, where melting snow begins its descent into the Mendoza Valley. Below the valley, the water flows into the Monte Desert.
Around 2,000 Huarpes and Criollos of mixed Spanish and indigenous descent live in Monte Desert. They used to live in the Pampas wetlands, but this region has become drier since the onset of large-scale irrigation about 60 years ago.
Native plants and animals also rely on the groundwater to survive.
The Algarrobo tree grows pods that contain many of the same nutrients found in soy. People eat the pods or feed them to their goats and cattle, Jobaggy said.
"It amazes me because these people can harness the resources of the desert that most people could not," he said.
Though dry at first blush, the desert is a deceptively productive environment, Jobbagy said, but only because of its geographic location below the Andes.
Fierce National Pride
The story now is a classic supply-and demand scenario. Argentina suffered the worst monetary default in history in 2002, and since its collapse the country's wine industry has proven to be one of the few profitable large-scale enterprises.
The industry provides jobs, a market for technological innovation in grape processing, and in a nation seared by the economic hardship—a sense of national pride.
Juan Vincelli and Eduardo Levesque head operations in neighboring vineyards 40 miles (64 kilometers) from the desert.
With the snow-capped peaks of the Andes as a backdrop, both say their operations need more water if their industry is to remain profitable.
"I am not honestly aware that what we are doing here with our irrigation systems is hurting anyone," said Levesque, a first-generation Argentine and a leading winemaker whose family sold wine in France before arriving in the Mendoza Valley 50 years ago.
"If our industry is hurting indigenous Argentines, I am willing to discuss solutions," he said. "But I can say that our indigenous culture makes me, as an Argentine, proud."
The greater problem, Levesque said, is that the provincial government is not spending the money to make irrigation systems more efficient.
"You sometimes see dry channels that could be full if blockages from rockslides in the Andes were removed," he added.
Vincelli and Levesque are also among a group of growers and vineyard workers who have made their fields more fertile by buying animal dung from the Huarpes people.
Head Above Water
Cecilia Villegas, a Huerpe, and her nine-year-old grandson live in a square charcoal-gray-colored dwelling called a rancho. It's constructed with wood, stone, metal strips, and bricks made from sun-dried mud. The home is set on a flat patch of sand near a 22-foot (7-meter) deep well.
They are completely self-sufficient in an environment few Argentines ever visit.
Villegas hasn't noticed anything beyond the usual ebb and flow of the water level, which she believes is tied to the changing seasons. The level is typically slightly higher during the Southern Hemisphere's winter.
But she says others have moved to lower ground within the Monte Desert, in valleys several meters closer to the aquifer.
"If the water level fell here," Villegas said, "We would have no choice but to move. And that's a choice we don't want to ever be faced with."
(Related: "Water Pressure" in National Geographic Magazine.)
Agronomist Jobbagy tries to raise awareness of the consequences of unfettered water use.
Most vineyards in the Mendoza Valley use the mantle irrigation system, in which an entire field is flooded. However the larger, more profitable, vineyards use drip irrigation, which targets specific rows of trees.
A rubber lining is placed along a line of grape trees and water is pumped through holes in the lining.
"Our society has a choice to make," Jobbagy said. "Do we want to expand irrigation, or do we want to do it without affecting systems like the one indigenous people depend on?"
Admittedly, he said, it only affects hundreds of people, as opposed to the millions who need irrigation for their farms and vineyards.
"But these [indigenous] people are some of the last pieces of a rural culture that makes most Argentines proud."
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