Christopher Ingraham has a fascinating piece in Washington Post on the water demands of marijuana cultivation. Drawing on a study in the journal Bioscience, Christopher notes that growing an acre of weed consumes more water than growing an acre of wine grapes and about as much water as an acre of (notoriously thirsty) almonds.

An environmental catastrophe in the making? I doubt it.

The amount of water required for an agricultural industry is a function of two variables: (1) The amount of water per unit of land (acre, square mile whatever) and (2) The amount of units of land needed to meet market demand. The Bioscience paper places great emphasis on the first of these variables but in my view, gives short shrift to the second.

The latest USDA data show that 936,000 acres are devoted to growing almonds. In contrast, the marijuana consumption of the entire U.S. population could be cultivated using only 1% of that acreage. Because it takes so little land to grow a large amount of marijuana, weed can be as thirsty as almonds per unit of land but have only a bare fraction of the almond industry’s impact on the water supply.

Drug policy maven Jonathan Caulkins offered me a rough formula that makes this point concrete:

A heavy daily user of marijuana might consume one pound of marijuana a year, which is roughly equal to what a single outdoor plant can yield. Thus, in round terms, that is 1 plant per heavy daily user. So that is 5 months of growing season * 30 days per month * 22 liters per day * 0.26 gallons per liter = 858 gallons, or about 143 toilet flushes. So even a heavy user going to the bathroom only once a day over those 150 days will consume more water flushing the toilet than via the marijuana they consume.

This doesn’t mean of course that marijuana cultivation isn’t straining water supplies in drought-stricken regions of California (e.g. Humboldt County). It is. But from a national viewpoint, the demand that marijuana cultivation imposes on the water supply is rather trivial.

[Cross-posted at The Reality-Based Community]

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Keith Humphreys is a Professor of Psychiatry at Stanford University and served as Senior Policy Advisor in the White House Office of National Drug Control Policy in the Obama Administration. @KeithNHumphreys