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What is evapotranspiration

Evapotranspiration. If you can pronounce that, congratulations, you’ve reached another level in your irrigation life.

Evapotranspiration (ET) is a fancy way of referring to the amount of water that leaves the ground, through two methods:

  1. Good old evaporation, when water turns into vapor
  2. Plant transpiration, when plants ‘breath’ and release moisture into the air


In the real world, there is no good way to distinguish between evaporation and transpiration, so ET is commonly referred to as one process. Furthermore, it is just one part of the water cycle, which describes how water moves throughout our environment. This includes such fun terms as condensation, sublimation, seepage, and (my personal favorite) infiltration. However, for our purposes, we only really care about ET.

Knowing how much water leaves the ground at any point is completely vital to efficient water management for one reason: it allows us to know how much water we need to add back to the ground.

There are only two main ways that water gets returned back into the ground: precipitation and irrigation. To successfully calculate how much water needs to be returned to the ground by irrigation, you must know about precipitation, but that is relatively easy to track.

That being said, measuring ET can be a challenge. The amount of evapotranspiration fluctuates throughout the year, primarily because of temperature. ET is higher with warmer temperatures and lower with cooler temperatures. This is the main reason why the amount of water your landscape needs increases in the summer and decreases in the spring and fall.


* Image from the Metropolitan Water District of Southern California

However, there are many variables that go into its calculation like soil type, plant type, temperature, direct or indirect sunlight, wind, ambient humidity, etc. Accurate data for all of these variables is imperative to calculate the most correct ET value. Without ET, it’s impossible to correctly calculate how much water your landscape requires.

If you’d like to understand how Rachio incorporates evapotranspiration, check out this excellent support article.