How Much Water for My Landscape?

How Long Do I Run the Irrigation?  (Part 1 of 2)

By Lori Palmquist

It’s commonly stated that we over-water our landscapes by at least double. I’ve found that to be true, as I've assessed and/or worked on hundreds of irrigation systems in my landscaping career.

One of the biggest reasons that we over-water our landscapes, I believe, is because we really don’t know how much water plants or lawns even need. This isn’t because we’re ignorant or because we don’t want to know, or don’t care. It’s because the water needs of plants and lawns change daily (heck, even hourly). It’s totally a moving target. No wonder we’re so bad at scheduling irrigation and why smart controllers are so good at it.

So, now I’m curious about just how much water a garden needs during the non-rainy season, AKA irrigation season (traditionally May through October in California). I looked up the historical weather values (ETo) for my city of Richmond, CA in the Model Water-Efficient Landscape Ordinance (MWELO) document. Here are the monthly ETo values for May through October:

• May: 5.1 inches
• June: 5.3 inches
• July: 6.0 inches
• Aug: 5.5 inches
• Sep: 4.8 inches
• Oct: 3.1 inches

Total:   29.8 inches of ETo

So, it looks like my garden will need 29.8 inches in the coming irrigation season. How many gallons does that represent? Well, that depends on the water-use value of the garden and the size of the irrigated area.

Whoa! Wait! What the heck is ETo, and what does 29.8 inches mean? Way too fast here!

Okay, sorry about that!

Most life forms on earth need water to survive. In California, plants in our gardens get their water from rain during the rainy season (typically November through April), and from irrigation in the dry season (typically May through October).

ET stands for evapotranspiration. It represents the amount of water that has evaporated from the soil and that has transpired from the plants. It represents loss of water, and is measured in inches.

Basically, when it’s not raining, it’s “ET-ing.”  ET is like the anti-rain. In the winter, water is given to our gardens in the form of rain. In the summer, the water is taken away by ET. I also like to say that rainfall and irrigation are the deposits, and ET is the withdrawal. There are lots of analogies we could apply to this.

The graphic below shows how we need to provide supplemental water to our landscapes in California from May through October. We don’t have enough rainfall during late spring through early fall to provide plants with the water they require. The yellow line represents rainfall, and the red line represents ETo or the plant water need.

The amount of water depends, of course, on whether the landscape plants are adapted to the climate of the area or not. We have plenty of plants in California that are adapted to the dry summers, and, in fact, can practically go without supplemental water once established.

Let’s revisit the ETo numbers from above, to see how much water they represent.

May: 5.1 inches ETo
4 in. for high water plants (80% of ETo)
2.6 in. for moderate water plants (50% of ETo)
1 in. for low water plants (20% of ETo)

June: 5.3 inches ETo
4.2 in. for high water plants (80% of ETo)
2.7 in. for moderate water plants (50% of ETo)
1.1 in. for low water plants (20% of ETo)

July: 6.0 inches ETo
4.8 in. for high water plants (80% of ETo)
3 in. for moderate water plants (50% of ETo)
1.2 in. for low water plants (20% of ETo)

Aug: 5.5 inches ETo
4.4 in. for high-water plants (80% of ETo)
2.7 in. for moderate-water plants (50% of ETo)
1.1 in. for low-water plants (20% of ETo)

Sep:  4.8 inches ETo
3.8 in. for high-water plants (80% of ETo)
2.4 in. for moderate-water plants (50% of ETo)
.96 in. for low-water plants (20% of ETo)

Oct: 3.1 inches ETo
2.5 in. for high-water plants (80% of ETo)
1.6 in. for moderate-water plants (50% of ETo)
.62 in. for low-water plants (20% of ETo)

This breakdown is interesting enough, but it doesn’t address how much water we’re talking about. There are two very separate concerns when we want to know how much water is needed for the landscape. The first is how many gallons of water are we talking about? The second is how many minutes do we run the irrigation schedule for each zone in the landscape? I’ll address the first one here, and the second in part two of this post.

The above landscape plan is a simple example of how we can calculate how many gallons of water we'd need in each of the six non-rain months. The formula we use is:

(Inches of water need) x (area in square feet) x (0.623) = gallons of water per period

Looking at our numbers above, we see that in May, if the landscape is a high-water landscape, 4 inches of water is required. The total landscape area is 1,000 square feet (385 + 615).

So:
High water need in May: 4 inches x 1,000 sq. ft. x 0.623 = 2,492 gallons needed for 1,000 sq. ft. of high-water plants in May

Moderate water need in May: 2.6 inches x 1,000 sq. ft. x 0.623 = 1,620 gallons needed for 1,000 sq. ft. of moderate-water plants in May

Low water need in May: 1.0 inches x 1,000 sq. ft. x 0.623 = 623 gallons needed for 1,000 sq. ft. of low-water plants in May

We could calculate the rest of the months of the dry season the same way. Feel free to run the rest of the months through the same formula. If you do so and add them up, you'll know how many total gallons the landscape would need during the six dry months. 

Please see the next post that talks about the scheduling of irrigation systems. When we know the water need in inches, we can fairly easily determine how long to run the irrigation zone.

Graphics credit: ETo chart courtesy of QWEL (Qualified Water Efficient Landscaper training)