The water/plant figure is actually not a very good one to work with because there are too many variables in terms of plant size, climate, strain, soil, aspect (north/south) etc. After playing with a number of different formulas over the years and trying to find what fits, the best figure we’ve come up with is one gallon of water per day per pound of processed flower, i.e. a one-pound plant needs one gallon of water per day, whereas a five-pound plant needs five gallons per day, and a 10-pound plant needs 10 gallons per day.
One Acre-Foot = 325,851 U.S. gallons
The 1:1:1 ratio was determined by polling numerous cannabis farmers about their water usage. Emerald Growers Association and Mendocino Cannabis Policy Council conducted the polls.
Water Per Joint
Take one pound of cannabis and, for the sake of this example, roll it into 450 joints (approximately 1 gram/joint). Two people smoking a 1 gram joint are going to get an effect that makes them feel good and lasts several hours. Two people could even smoke half the joint and put it out, coming back later to enjoy again. One person could stretch a joint for a number of smoking sessions, achieving the same good feeling each time from the same gram of cannabis.
Two people can share a bottle of wine and get a good feeling that lasts several hours. They can also drink half the bottle and come back later. One person can make a bottle last several days… you get my drift.
So, my suggestion is that we compare the one gallon of water per day during the height of the growing season (say 100 days, which is when it’s driest and plants are largest) per pound of cannabis means that 100 gallons of water yields 450 joints with equivalent “human upliftment” to 450 bottles of wine. Wine uses about 400 gallons per bottle1.
That was my estimate, which is based on drip irrigation and careful usage. You could perhaps tack on another 10-20 gallons depended on length of season of plant. I haven’t discussed potential for over-watering or wasteful water practices, which we know occur. Providing education and access to resources for cannabis farmers will bring better industry standards. So, my revised estimate is:
- Low-end water usage estimate for one season: 100 gallons per pound of process flower.
- High-end water usage estimate for a season: 200 gallons per pound of processed flower.
- Most farmers fall in the 100-150 gallon range.
Water Per 1/8 Ounce
EGA and MCPC canvassing indicates that most cannabis farmers grow plants that average between 2 and 4 pounds. One-eighth acre (50 ft x 100 ft with 50 cannabis plants) would use 24,000 gallons per season (8 months = 240 days) to produce 50 two-pound plants (2 gal per day x 240 days x 50 plants) or 480 gallons per plant. A two pound plant divided into 1/8ths of an ounce yields 256 eighths. Thus the whole garden with 50 plants would produce 12,800 eighths of an ounce. (An eighth ounce is a standard retail unit like 1 pound of beef or 1 bottle of wine or 1 can of almonds.)
Each 1/8th ounce then requires 1.875 gallons (24,000 gal/12,800) of water to produce. It has been widely reported that to produce one pound of beef requires at least 1500 gallons of water. Wine uses about 400 gallons per bottle, as mentioned above, and almonds need one gallon per nut or about 100 gallons per can. Broccoli takes about five gallons per head.
Learn more about how much water it takes for all kinds of crops, foods and fuels at WaterFootprint.org.
But, wait! The same plant that produces two pounds of cured finished flower buds also produces at least a 1/2 pound of “little bud” or “smalls” that farmers and dispensaries often donate to needy patients, sell at a steep discount or consume themselves as everyday smoke. So, we need to include that each plant will also produce at least 1/2 pound of “trim shake.” The leftover smalls and the shake can then be processed to make concentrates, edibles, tinctures, salves, oils, etc. All that additional product, all that value added, with no additional water required.
1 (Vineyards produce about 2 to 10 tons per acre. 2 tons of grapes produces about 4 barrels of wine which is about 1440 bottles. Water use: I acre grapes (about 2400 plants) uses 25,000-35,000 gallons a week. 6 months = 24 weeks total x 25,000 gal = 600,000 gal/per season / per acre. Vineyard acreage in Calif. has increased by 63% since 1985, from 350,000 acres to 570,000. Which means that their water use has likewise expanded by 63%.)
Casey O’Neill and Swami Chaitanya both contributed to this article.