Evapotranspiration (ET)

A very commonly used term when discussing irrigation or landscape is evapotranspiration (or ET for short).

What is evapotranspiration?
Evapotranspiration is the sum of water lost from the soil surface and plant foliage (evaporation) and water used by plants (transpiration). There are a number of factors that affect evapotranspiration including plant species, weather factors and quality of water available to the plant.

Reference Evapotranspiration (ETo)
A helpful way to quantify weather based data is the calculation of reference evapotranspiration or ET. The best sources for live or real-time ET data are local sources (local weather stations), Cooperative Extension Service, National Weather Service and water purveyors.

Evapotranspiration can be based on alfalfa ET(r) or if turf, ET(o).

ET(o) is defined as the ET rate of healthy turf, completely covering the ground to a uniform height of 4 -6 inches and having an adequate supply of water with no microclimate factors influencing it.

The reference ET, commonly called ET(o), is expressed in terms of a depth of water per some unit of time. Example: inches per: day, week, month or year. The following table show ETo (monthly) for Colusa and Santa Monica. (courtesy of CIMIS). As you can see, the ET values change monthly based upon how much water the plant (in this case turf) uses in a month. The hotter the month, the more water the plant will use.

Why is ET important?
Evapotranspiration or reference ETo, is an important reference point for irrigation water use calculations. It represents a specific rate of use in response to local weather conditions.

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Example of ET in use.

Determine the Plant Water Requirement for Acacia Redolens planted at 15" spacing on center.

Location is a garden setting surrounded on three sides by four story concrete buildings in Los Angeles

The objective of a accurate irrigation schedule is to supply the correct amount of water to the plant without over watering. (because we don't want to waste money with unnecessary watering nor do want to kill the plant by over watering)

So how do we find this specific amount of water?

To calculate the ET rate of a specific plant we need the following two factors; the landscape coefficient K(L) and the reference evapotranspiration (ETo).

The specific ET for a plant is ET (L), the "L" = Landscape.

ET(L) = K(L) x ET (o)

K(L) = Landscape coefficient

ET(o) = Reference evapotranspiration

The ET for landscape ET(L) will vary as the mix of plant species is used, the density of planting and the planting location microclimate. After we evaluate each factor and assign a numerical value to each, we then can estimate the overall K(L).

K(L) = K(s) x K(d) x K(mc)


K(L) = Landscape coefficient

K(s) = Species Factor

K(d) = Density Factor

K(mc) = Microclimate Factor

Once we calculate K(L) we simply multiply it against our ET(0) to determine our landscape water requirement.

Species Factor K(s)
Species Factor K(s) will vary considerably in their transpiration rates. Values for species factor are available through local sources such as Cooperative Extension Service.

Because there is such a range of water usage among different plants, the water use is divided into three groups: low, medium and high. This is shown in the table below:

landscape species factor

Density Factor K(d)
Landscapes can also vary in density. Leaf surface is often less in newly planted landscape than mature landscapes and will use less water.The total water loss from dense planting most likely will be higher due to the greater total leaf area for the site. We then assign a higher density factor to these areas.

landscape density factor

Microclimate Factor K(mc)
The environment may also play a role in landscape water use as well. A medium microclimate area is where buildings, pavement, slopes, shade and reflection do not influence the site. In these conditions K(mc) = 1. A high microclimate conation is where the landscape is surrounded (or affected) by heat-absorbing surfaces, reflecting or high wind conditions. In these cases, K(mc) could be as high as 1.4. A low microclimate is where a planting area is in shade or protected from the wind. This area will have a K(mc) = .5.

landscape microclimate table

Lets plug the numbers in and find our answer:

From looking at our reference guide, Estimating Irrigation Water Needs of Landscape Plantings in California, (WUCOLs ), we can quickly determine that the species factor K(s) is "Low" (for zone 3-Los Angeles).

Therefore, K(s) = .2

Next we need to determine the Density factor. From the problem we can see that these shrubs are planted 15' o.c., hence they density is quite low.

Therefore, K(d) = .5

Next we need to determine a microclimate factor. The planting area is surrounded by concrete buildings which will reflect heat. But the the areas is also being protected from the sun, and wind. Hence since these conditions offer a trade off , we will use a value of 1.0.

K(mc) = 1.0


K(L) = K(s) x K(d) x K(mc)

K(L) = .2 x .5 x 1.0

K(L) = .1

Now lets multiply our ET(o) x K(L)

From our ET reference table (above) we see that July is the hottest month of the year with an ET of 5.4. Since this is worst case, lets use that value.

ET(L) = ET(o) x K(L)

ET(L) = 5.4 x .1

ET(L) = .54 inches of water for the month of July!

How cool is that? We just determined that Acacia Redolens, for this particular setting and condtion will only need .54 inches of water for the entire month of July! Now mind you, this is just an estimate, (your mileage may vary..) but you can cleary see how using ET values can help you determine plant water needs.

Next up, we'll determine Irrigation Water Requirement.

Whether you need to irrigate a commercial shopping center, municipal building, sports field, parking lot or residence, Atomic Irrigation can help. We’re up to speed on all the latest in technology from low-water spray heads, drip irrigation products and the latest ET Controllers. We can help.


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