ScottB asked how much water pot takes, and frankly, I have no idea. But I know how to guess.
First, I hoped for an ETcrop (evapotranspiration is how water moves through the plant into the atmosphere). That’s a co-efficient that researchers have measured for all the major crops. You multiply it against a reference ET, which is how much water a reference crop uses. The California Irrigation Management Information System uses grass for its reference crop. Other systems use alfalfa. If you wanted, you could go to CIMIS and find out how much water a patch of grass transpired under each day’s weather conditions. Then you multiply that by the coefficient for your crop, and figure out how much water you should put on to re-fill the soil profile.
I didn’t hope to find an ETcrop for pot, but I did find one for a type of hemp. Under well-watered conditions, the seasonal average crop coefficient for Sunn hemp was 1. So Sunn hemp needs the same amount of water as a patch of grass. If pot and Sunn hemp are similar, pot has the same water demands as the reference grass for CIMIS.
All of that is thinking too hard. If you don’t know, guess 3.5 feet per year. Nod sagely and squint at the mountains in the distance. If someone gives you grief, say “but did you consider your salt flushing requirement?” Distract them with questions about their system’s filter capacity and ask when they last backflushed their filters. Tell them the manufacturer specs require more frequent backflushing and then quit the field victorious.
On the public record 
Yeah, but everyone knows the best stuff comes from Humboldt County….:
“They say Humboldt could become the new Napa Valley of cannabis. The AP reported that “a dreadlocked younger grower … objected that no one could replicate the quality of the region’s” cannabis. “Humboldt nuggets — that was like the holy grail,” he said. “Anyone can grow marijuana. But not everyone can grow the super-heavies, the holy bud.”
(http://www.eastbayexpress.com/LegalizationNation/archives/2010/04/06/humboldt-growers-freak-out-over-legalization)
I imagine grass (of all kinds) doesn’t take as much water in Humboldt County as it would in the Central Valley….
Legalizing dope would do interesting things to water politics. Imagine the chances of chinook salmon etc if Westlands had been talking good shit instead of almonds.
Anyone know what the state’s ag economy produces when expanded to include NW California’s non-infrastructure-dependent agriculture?
Anyone care to estimate what tax contribution that industry would make to schools, health care, parks, women’s shelters, etc., if legal?
Anyone want to list yet again the names of the many establishment figures who figure the cost-benefit of legalization is obvious?
3.5 feet seems about right. Probably a little less if you use drip. The newer cultivars have been bred to produce an abundance of sex organs (i.e. “buds”), and have a pretty compact structure, not unlike the modern varieties of cotton, so conversion rate the water applied to economic value is quite good. Unlike cotton, however, dope has a fairly inefficient, vegetative root system, so reliability of water supply is very important.
Another important argument in favor of legalization is public safety. We have decided to give this business over to nasty gangsters (at least on the commercial level) whom I doubt think that obeying pesticide residue regulations and worker safety laws are very important.
There was a funny string of stories last summer out of Mendocino about suspicions that pot growers were rustling water and drawing down wells. While for a grower looking up the local ET rate of hemp is logical, following down this line might be more fun, provided you got to the rustlers. http://www.pressdemocrat.com/article/20090908/ARTICLES/909089940