Water Demands for Cannabis

Recently doing some legwork on a design request I found myself making incorrect assumptions about the water demands of Cannabis. The overall demand of the plant is a little misleading. While Cannabis does in fact require quite a bit of water it prefers the application be across a long period of time and evenly distributed above the root base. Fortunately for growers, application options are plentiful and automation has made the waiting game not quite as painful as it used to be. For the project at hand I decided to dig deeper…and put the findings on paper.

Starting from the top; Cannabis like any plant is constantly using valuable energy trying to grow a root base. To be sure there are no interruptions in growth we must provide the plant with a combination of nutrients and water. As the plant uses water to grow we are left with a depletion level that must be replenished. In simple terms the water used or burned up by the plant is the plant’s Crop Coefficient. Knowing the Crop Coefficient of many comparable species to Cannabis we’re given a value of 1.0. The Crop Coefficient combined with the environments, soon to be explained ET value, is going to help guide a grower down the path of efficiency.

The ET value of our growing environment is going to help determine how much water we should apply to refill our plants depletion. Figuring out the ET value of an indoor environment can be a bit tricky. Fortunately the same variables that apply to our outdoor environments also apply to our indoor environments. Along with a plants growth stage and maturity level; humidity, temperature and solar radiation are all things to consider when determining an ET value for your room. For reference it’s currently 72 degrees in San Diego with a nice cool breeze and…ok sorry, and the Daily ET is .12. Not too far removed from coastal SoCal in Phoenix it is a very exciting 116 degrees, if I had to guess it’s a dry air. Current ET in Phoenix? .33 This pattern tells us that with dryer, warmer air our plants need more water.

Understandably growers do their best to keep rooms cool and well ventilated but with most conventional lighting setups creating room temperatures between 70 and 80 degrees we will use .18 as our value. With the majority of humidity coming from irrigation and what the plants emit our ET value should be relatively low. In other words our Cannabis needs .18 inches of water per day to thrive.

“No way!”

– Naysayers

They are actually correct! Keep in mind, not all the water leaving your emission device is making it into the plant. Stay with me here; on average, drip irrigation has what we call a DU or Distribution Uniformity rate of .9, meaning 90% of the water leaving the device makes it where we need it to go. With environmental conditions eating away at 10% of our water and multiple lighting cycles expediting photosynthesis we’ll now need to apply 2 X our previously calculated .18 demand.

Here is our formula:

1.0  x .36 = .36 inches of water per day

1.0 is the Crop Coefficient of Cannabis

.18 x 2 = .36 ET Value (based on two 10 hour lighting cycles)

.36 is the plants daily watering demand

We’ve now determined that Cannabis needs approximately .36 inches of water per day to replenish what the plant has used for energy. We can now move on to the fun part, the application of water!

If you like this post and want more info like this please follow me on Twitter @MDSavesWater or on Instagram @jainsusa.


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13 Responses

  1. Michael, thanks for the article. It certainly gives all of us a starting point of how much to water and we can make adjustments based on our own environments. Thanks again for getting the discussion started on an important topic.

  2. Interesting article Michael, thank you. I believe this is going to become more important in the coming years because we are going to see more traditional agriculture moving indoors with the advancement of AgTech. More production with fewer inputs. The times are a changin…but this is all for the better.

    1. “More production with fewer inputs [indoors]”


      Water inputs
      When grown outdoors, conventional field-grown Cannabis needs 20-30 inches of water per ~4 month crop. That works out to just 0.20 inches per day, about HALF of what is being reported here for indoor grown plants. Furthermore, that value can be reduced by 30-50% through the use of mulch, drip irrigation, and increasing soil organic matter content. Now we’re down to 0.10-0.14 inches of water per day, roughly 1/3 the requirements of indoor grown plants.

      Energy inputs
      An indoor Cannabis factory using 400W LED grow lights requires about 19 kW per sq ft during an 8 week flowering period. Add in another 4 weeks (from clone to flowering) of minimum 18h light per day using the same efficient light source adds another 13 kW per sq ft. for a total of 32 kW per sq. ft per crop. An indoor facility using HID lights sees that value jump to 80 kW per sq ft. Thus, an indoor Cannabis production uses 1.4-3.5 GW of energy per acre JUST FOR LIGHTING. This doesn’t even account for energy use from HAF fans and air filtration, dehumidification, cooling, and heating! In contrast, an acre of sungrown Cannabis uses ZERO energy for lighting and environmental controls.

      Furthermore, outdoor grown plants tend to have much higher yields per unit of area than indoor grown plants (2-3X from my experience), This means that the water use efficiency of sungrown Cannabis is 3 to 9 times higher than indoor Cannabis while the energy efficiency of sungrown Cannabis is close to infinitely higher than indoor Cannabis.

      AgTech may be neat and fun, but it does not produce most crops more efficiently indoors than out.

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  4. Hi,

    Do you mind me asking what sources you used for getting a crop coefficient value of 1.0? Were there lysimeter studies done?

  5. That’s a solid article about growing cannabis, it’s important to understand the fundamentals of growing marijuana if you want to grow the super nice flowers, thank you for the hard work you put into gathering all of this great info. Have you heard of the new RSPEC Niemi Deep-Red grow lights are designed to enhance flower while still being a great full cycle grow light from seed to flower?

  6. USING the above
    .36in per sqft
    10in pot .5 sqft
    .18/12ft*.5sqft=.0075 cuft
    1cuft water = 7.5gals
    .0075cuft =.056gals
    or 212 ml per day

  7. Hello! Nice piece of information about cultivating cannabis, knowing the basics of cultivation is essential to producing exceptional weed. Many thanks for your efforts to compile all of this amazing information.

  8. Is this .36 inches cubed? That would mean it is a significantly small fraction of a gallon. I understand weather conditions affect this, but is the cubed inch calculation correct?

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