They start from the assumption that in the North Coast region of California -- which encompasses California's famed "Emerald Triangle," where much of the state's marijuana is grown -- "an estimated 22 liters (L) of water or more per plant per day are applied during the June–October outdoor growing season." Multiply up by the average outdoor marijuana planting density of 130,000 plants per square kilometer, and you arrive at something like 430 million liters of water per square kilometer of outdoor marijuana crop during the typical growing season.
This is a huge number! But let's convert it to something more manageable. We'll lean on our trusty old friend the "acre-foot," which is defined as the amount of water it takes to cover 1 square acre of land with 1 foot of water. Here's how one acre-foot of water compares to one six-foot tall person:
And so that 430 million liters of water per square kilometer translates into something like 1.4 acre-feet of water per acre of crop. Which is a lot! As the study authors point out, that makes an acre of pot plants about twice as "thirsty" as an acre of grape vines. And roughly as thirsty as that infamous villain of the California water crisis, the almond.
But it works out to quite a bit less water per acre than some other common crops, like corn, potatoes, and various tree fruits. And it's less than half as much water as alfalfa requires.
So there is an argument to be made that illegal marijuana operations are particularly worrisome in the context of California's drought. But on the other hand, the plant's water needs place it well within the range of other crops that California farmers are growing.
Still, there's little question that illegal, unregulated marijuana grows can wreak havoc on a local ecosystem. Much of the water used to irrigate illegal marijuana operations gets diverted from small streams and wetlands, which are already under tremendous stress due to the ongoing drought. And this doesn't even begin to touch on all the other aspects of outdoor pot's environmental footprint, which includes things like forest-clearing and pesticide application.
But, it's worth pointing out that many of these ecological headaches arise at least in part due to pot's illegal and unregulated status. A clandestine agricultural operation on a forest hillside is far more environmentally disruptive than a well-tended plot in a field somewhere, maintained under the watchful eye of federal and state regulators.
This is why many experts maintain that legalizing and regulating marijuana production will lead to considerably less environmental harm than we're dealing with today.