SAN PABLO — Water here is so scarce that farmers habitually gaze up at the mountains surrounding their valley — where overpumping from aquifers may force 80,000 irrigated acres out of production.

As Rose Medina traversed her ancestral lands last week, scanning the Sangre de Cristos for the promise of a strong spring runoff, she saw barely a dusting of snow.

“Looks like we’ll need more,” Medina said.

Big spring snow could send water coursing down Culebra Creek and into her “lindero” boundaries — headgates controlled by an elected “mayordomo” steward — allowing growth of hay for her 16 cows and quenching her apple, plum and cherry trees. The ancient Moorish water-sharing methods adapted 400 years ago in southern Colorado ensure that, even in dry years, small family farmers survive.

But survival is far from ensured across the broader San Luis Valley, where leaders in an area that’s already among the poorest in the state are bracing for a major economic hit.

“Agriculture alone cannot sustain the economy of the San Luis Valley,” Colorado agriculture commissioner John Salazar recently told residents.

Unlike Medina’s 40-acre farm and others that rely on only surface water, the commercial agriculture that built up the valley is large-scale and competitive, and relies on center-pivot irrigation devices that pump heavily from underground aquifers. Commercial production of potatoes and hay — using 6,000 wells and 2,700 center-pivots to irrigate 120-acre crop circles — exploded after the 1950s.

The pumping has depleted aquifers by more than 1 million acre-feet since 1976 and now is affecting surface streams. One acre-foot approximately serves the needs of two families of four for a year.

By May, center-pivot farmers must activate a plan to reduce the water pulled from the aquifer by about 30,000 acre-feet a year.

“They’ve got to start to restore it,” state engineer Dick Wolfe said.

To avoid state shutdowns of wells — as happened in 2009 in northeastern Colorado — commercial farmers propose to pay to pump or purchase new surface-water rights and use these to offset pumping from aquifers.

While water may be conserved, the increased cost may cause 80,000 irrigated acres to come out of production.

That represents a substantial chunk of the agricultural base in a valley where 38 percent of children live in poverty. In one crucial subdistrict, the expected loss of 40,000 acres over the next five years represents nearly 25 percent of total farmland.

“These communities, and no doubt other communities around the world, are coming to the realization that business as usual has to change,” said Mike Gibson, manager of the San Luis Valley Water Conservancy District and chairman of the Rio Grande roundtable that participates in statewide planning.

“We’re looking at quite a lot of pain,” Gibson said. “We don’t know how much. The numbers of jobs we see at these solar energy facilities are relatively small.”

The tightening water regime is forcing agricultural leaders to align production with natural limits in a semi-arid environment. Colorado’s obligations under an interstate compact governing the Rio Grande constrain what farmers can draw from surface streams and the hydrologically connected aquifers.

In the past, Texas and New Mexico have sued Colorado for drawing too much water from the Rio Grande. Legal enforcers are especially vigilant today — with drought ravaging Texas and cities such as Albuquerque, where municipal water wells are depleted.

For many San Luis Valley farmers, the prospect of rising costs and cutting their acreage appears overwhelming.

“We’re going to farm less ground,” potato producer Brian Neufeld, 34, said. He grew up in the valley and, six years ago, began growing potatoes on two irrigated circles of his own. The fees he and others in his subdistrict pay probably will increase to $75 per acre-foot, from $45. That raises costs by as much as $20,000 for an irrigated crop circle.

“It might mean less employees,” Neufeld said. “It might mean layoffs in the area, less crops around the valley. That’s going to affect towns. Some guys think this is the start of a downward decline for their farms.”

But the time has come for commercial farms “to pay for the impacts they are causing to the river,” said Steve Vandiver, manager of the Rio Grande Water Conservation District and the leader of efforts to find water to replace water pumped from wells.

Rio Grande County Commissioner Karla Shriver, who in the 1990s led opposition to a Canadian developer’s push to plumb the valley’s deep aquifers and export water to booming Front Range suburbs, said mining jobs must make a comeback to help cushion the loss of irrigated acres.

Meanwhile, small-scale farmers like Medina, who hold long-established rights to surface water, are relatively unaffected and already have other sources of income. She works as a teaching assistant.

She counts only on snowpack to keep creek water flowing into gravity-based “acequia” ditches.

The communal People’s Ditch system in San Pablo, San Luis and neighboring Spanish land-grant communities, which dates to 1852, increasingly serves as a model of prudent agriculture. Interior Secretary Ken Salazar calls it “a perfect example of an important part of history that needs to be preserved.”

U.S. Department of Agriculture soil-conservation officials contribute about $300,000 a year to support its operation.

“It’s all about efficiency, giving your crops just what they need and then sharing the water with everybody else,” said Cerro Ditch Co. president Joseph Lobato.

Coloradans “ought to treat every drop of water as if it’s our last” and study efficiencies of systems like this, Gov. John Hickenlooper said.

However, it may make sense to line the ditches to improve flows for the future, he said. “Once you’ve done that,” he said, “you maintain a landscape you can talk about.”

Medina is weighing how much hay she should plant. New steel headgates reduce the water wasted when she diverts her allotments to crops.

She refers to her forefathers who settled the land — and looks down from the mountains at fat cows grazing beyond her trees. “There is no survival without agriculture.”