Lake Mead - the West's largest reservoir - is running dry again and is on track to fall below a critical threshold in 2020, according to a new forecast by the Bureau of Reclamation.

In 2016, Lake Mead water levels drop to new record lows (since it was filled in the 1930s) leaving Las Vegas facing existential threats unless something is done. Las Vegas and its 2 million residents and 40 million tourists a year get almost all their drinking water from the Lake and at levels below 1075ft, the Interior Department will be forced to declare a "shortage," which will lead to significant cutbacks for Arizona and Nevada.

And now, two years later, the situation appears to be getting worse as The Wall Street Journal reports, in a prediction released Wednesday, the Bureau of Reclamation, a multistate agency that manages water and power in the West, said there is a 52% probability that water levels will fall below a threshold of 1,075 feet elevation by 2020.

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“The very big concern is the perception that water supplies are uncertain,” said Todd Reeve, chief executive officer of Business for Water Stewardship, a nonprofit group in Portland, Ore., that works with businesses on water use nationally. “So if a water shortage is declared, that would be a huge shot across the bow that, wow, water supplies could be uncertain.”

The Colorado River, which supplies water to 40 million people from Denver to Los Angeles, has been in long-term decline amid what bureau officials call the driest 19-year period in recorded history.

Lake Mead, which serves as the biggest reservoir of the river’s water, resumed its decline this year after the region returned to drought conditions. As of Wednesday, it stood at 1,078 feet, about 150 feet below its peak.

If Lake Mead’s water levels fall below the 1,075 feet threshold, it could trigger the first ever federal shortage declaration on the Colorado River - which experts say could undermine the Southwest’s economy.

Farmers in Arizona - which would be among the first states hit with cutbacks - are taking precautionary measures. Officials of the Maricopa Stanfield Irrigation and Drainage District, which could lose about half its Colorado River water if a shortage were declared, say they are working on alternatives such as digging more wells. The district, with 60,000 acres under cultivation between Phoenix and Tucson, might see as much as 15% of its planted fields left fallow under a shortage, said General Manager Brian Betcher.

“We’re not sure how much acreage will go out,” he said, “but we know there will be a hit.”

As one water research scientist warned, "this problem is not going away and it is likely to get worse, perhaps far worse, as climate change unfolds."