Water Inches

“Give your plants one inch of water each week in summer.”

“Almond orchards use an average of 4 acre feet of water every year.”

But what are water inches and acre feet? Let’s find out!

Watering tomatoes by hand

How much should I water my plants?

Sorry, but there is no single answer. Every situation is different. There are simply too many variables: soil structure, water holding capacity, sun exposure, plant species, age, size, and developmental stage, wind, rain… the list goes on. You can, however, generally keep your plants healthy by providing them with one inch of water each week in summer.

The term water inches is traditionally used in hydraulic mining and it refers to specific tube diameters, vertical surfaces, and pressure levels. We are not discussing those water inches, but there is some math involved.

Measuring water

Since irrigating plants often means the water is being absorbed into the soil as we water, it is practically impossible to know how much water your plants are getting without measuring it at the hose bib end. You can get a general idea of how much water is coming out of your garden hose by turning the spigot on to a set point and timing how long it takes a one-gallon bucket to fill up. If you counted to 15 while your bucket was filling up, you know that your hose puts out 4 gallons a minute, since 4x15 is 60.

Water math

Generally speaking, in the world of gardening, the phrase “one inch of water” refers to how much water it takes to cover one square foot of space with one inch of water. Since there are 12” in a foot, you can multiply 12”x12” for your “one square foot” to get 144. This means 144 square inches of water are needed per square foot of garden space. Of course, none of us have measuring cups or watering cans that are marked in square inches, so there is a little more math to do. Don’t worry, though. Once you get used to the numbers, as they apply to your garden space, you won’t have to repeat the calculations.

One gallon equals 231 cubic inches. If you divide your 144 sq. in. by 231, you get 0.6 or a little over half a gallon per square foot.

What about irrigating raised beds?

If you have heavily planted areas or raised beds, you can simply take the length and width measurements and multiply them, using the same steps. For example, say you have a 4’ x 6’ raised bed.

​First convert feet to inches:
    (4x12) x (6x12) = 48 x 72
Then calculate the area:
    48 x 72 = 3456
Since we now know one gallon equals 231 cubic inches, we divide 3456 by 231:
    3456 ÷ 231 = 14.9 gallons

This means that your 4’ x 6’ raised bed should be given an average of 15 gallons of water each week in summer.

What about watering container plants?

The math gets a little trickier with containers. Remember the joke about “pie are squared - pie are not squared, pie are round”? Well, this is where you actually get to use that equation. For those of you who need a little geometry refresher: 

• The measurement across a circle is its diameter.
• Half the diameter is called the radius.
• To calculate the area of a circle, you square the radius, which means you multiply it by itself, and then multiply that number by π (also known as pi, or 3.14).

For example, let’s say that you have a 10” planter pot. Since diameter is twice the length of the radius, we would create this formula:
    = (3.14)(10÷2)(10÷2) 
    = (3.14)(5)(5)
    = (3.14)(25)
    = 78.5 

That may sound like a lot, but it ends up that 78.5 square inches of water equals a little over one-third of a gallon. [78.5 ÷ 231 = 0.34]

Acre feet

If all this math hasn’t made you crazy, let me just tell you that an acre foot equals the amount of water it would take to cover one acre of land with one foot of water. Without going through all the numbers, one U.S. acre foot equals 325,850 gallons of water. In 2018, it was predicted that the average acre of almond orchard would produce 2,150 pounds of almonds. That works out to over 150 gallons of water per pound of almonds.

Watering your plants properly can make or break your garden. Getting a more accurate idea of how much water you are giving your plants can improve their health and reduce water waste. And remember, the “weekly water inch” is just a recommended average for summer. You should always monitor your plants for overall health. If they start wilting and the soil is dry, water them. If they start wilting and the soil is moist, do not add water. Instead, check for root feeding grubs, gopher holes, and hardpan.

Did you know that the amount of water in an Olympic-sized swimming pool weighs over 5.5 million pounds? I didn’t either.