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Pot plants slurp up California's water supply

California cannabis growers may be making millions, but their thirsty plants are sucking up a priceless resource: water. Now scientists say that if no action is taken in the drought-wracked state, the consequences for fisheries and wildlife will be dire.

"If this activity continues on the trajectory it's on, we're looking at potentially streams going dry, streams that harbor endangered fish species like salmon, steelhead," said Scott Bauer of the California Department of Fish and Wildlife.

Studying aerial photographs of four watersheds within Northern California's so-called Emerald Triangle, Bauer found that the area under marijuana cultivation doubled between 2009 and 2012. It continues to grow, with increasing environmental consequences.

Bauer presented data to CNBC indicating that growers are drawing more than 156,000 gallons of water from a single tributary of the Eel River, in Mendocino County, every day.

The average marijuana plant needs about 6 gallons of water a day, depending on its size and whether it's grown inside or outside, according to a local report that cited research. Pot growers object to that number, saying that the actual water use of a pot plant is much less.

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Although the marijuana business has helped revive the local economy, residents may now be feeling the effects of living alongside growers. And, as growers—some legal, some not—face an ongoing, severe drought, local law enforcement officers expect the fight over natural resources to intensify.

"I never want to see crime increase, but I have a feeling it will, because of the commodities that are up here," said Humboldt County Sheriff Mike Downey. "When we get to the end of the grow season, which is August and September, the need for enhanced water availability is gonna be there, and I don't think the water's going to be there, so you're going to see people, I believe, having some conflict over water rights."

Stream water rules in California are the same for growers of marijuana as they are for growers of any crop: Growers should divert no more than 10 percent of a stream's flow, and they should halt diversion altogether during late summer, when fish are most vulnerable to low water levels. But Bauer pointed out that those rules apply to permit holders, and most marijuana growers haven't bothered to get permits.

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With so much of California's cannabis business operating in the more lucrative underground market, and with so many growers across the region (see the map below), the California Department of Fish and Wildlife and the Humboldt County Sheriff's office say they lack the resources to eradicate all offenders. So they target the most egregious.

In this map of Humboldt County, California, provided by law enforcement, each pin indicates a marijuana grow site that police have located. Yellow pins are discovered sites; red pins are sites that have been raided.
Source: Humboldt County Sheriff’s Office
In this map of Humboldt County, California, provided by law enforcement, each pin indicates a marijuana grow site that police have located. Yellow pins are discovered sites; red pins are sites that have been raided.

"We get those calls daily. People are upset. Somebody has dried up a stream, somebody is building a road across sensitive fish and wildlife habitat, so that is happening on a daily basis," Bauer said. "And we do our best with the personnel we have to respond to those calls."

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Sheriff Downey concurred with Bauer about the manpower challenge authorities face.

"We have a very active marijuana unit that is out there, especially during the grow season. But we have so many grows here that we have a hard time keeping up or making a valiant dent in the marijuana growing in the county," said Downey.

"With the increase in water usage and pressure upon that, that lucrative business becomes even more lucrative because the price of the marijuana, the value of it, goes up even though we've had a glut on the market the last few years," he added.

One increasingly popular solution among some growers is the collection of rain water during the wetter, winter months that they can use to water crops during the dry, summer season.

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"As long as cannabis farms remain small and decentralized, there's no reason why we can't grow everything we need to meet the state's demands using all stored rain water," says Hezekiah Allen, an environmental consultant and director of public affairs for the Emerald Growers Association.

And for some, it's a business opportunity.

"I've heard people shut down their grow operations, bought water trucks and have changed from growing to supplying waters to the other growers," said Chip Perry, a consultant for MC2, a service that helps people obtain medical marijuana cards.

—By CNBC's Harriet Taylor