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What is Evapotranspiration?

 What is evapotranspiration and is it important for your irrigation system?

I'm sprinkling Andy, and you're watching another episode of Sprinkler TV™.

We are going to talk about evapotranspiration. It's a tricky word. It's a scientific word, and if you're in the industry, you've probably heard it before, and if you've just purchased a Smart Controller, You've also probably heard it before; smart controllers like the Hunter Hydrawise, Orbit B-Hyve, Rachio, etc.

All of these new smart controllers have a capability which utilizes something called evapotranspiration or ET. The ability to automatically adjust watering schedules based on the local weather/climate is what qualifies the controller to bve labeled as a "SMART" Controller. So let me explain it for you, and I'll try to make it really just as simple as possible. You could go get a book like the Irrigation 6th Edition, and you can get down into the details and the science of evapotranspiration and how it is calculated, but I'm going to explain it for you in simple terms.

There are many factors that go into calculating evapotranspiration, but what I want to do is stick to the basics in this video and really just tell you what it is.

Evapotranspiration is the combination of two words; EVAPORATION and TRANSPORATION. You put those two terms together and you get evapotranspiration, and essentially it is the process by which the plants consume water, or total water loss in the soil.

Whenn a plant drinks water and it pulls water up from the soil, that is when the plant is transpiring. Transpiration is the movement of water from the soil, up through the roots, into the plant and out through the leaves (stomates), that's transpiration. Then you have evaporation, which is simply water evaporating from the surface of the ground, from the soil itself.

Both of those terms, Transpiration and Evaporation are water loss from the soil. Evapotranspiration rate is often used to describe the total water loss in the soil due to evaporation and transpiration. Evapotranspiration is actually really important because it's the primary component of what you might call irrigation water requirement, or plant water requirement.

Evapotranspiration really determines the amount of water required to maintain healthy plants. In a perfect world, we would give our plants exactly the right amount of water, no more, no less, just the right amount of water to be as healthy as possible. And evapotranspiration is a scientific word used to describe that amount of water.

So let me give you an example. Let's say it is June, wherever you live, and the ET rate, or the evapotranspiration rate for today is a quarter inch, 0.25". In a perfect world, you would then reapply exactly 0.25" inches of water so that you are replenishing to the plant what the plant lost at that rate of evapotranspiration.

So the ET rate is what is used to determine this amount of water. That amount of water loss. Water loss from the plant and the soil.

One common question is, "what are the factors that are used to determine Evapotranspiration?" There are 4 factors used to determine the daily evapotranspiration. 

  1. Temperature. You can imagine that the warmer it is outside, the more the plant is going to consume and transpire, and the more water loss there will be from the surface of the soil due to evaporation. Right? When temperature goes up, there is more water loss. When  temperature goes down, there is less water loss. 
  2. Solar Radiation. The sunshine. The sunnier, it is outside the more water loss there will be through the plant and through the soil surface due to evaporation.
  3. Humidity. The drier it is, like in Tucson Arizona, the more water loss there will be through transpiration and evaporation.
  4. Wind speed.The windier it is outside, the more water loss there will be due to transpiration and evaporation. 
When you put all of those four factors together, that is the evapotranspiration. What you need to consider is that it's a variable number. It can change day-to-day, week to week, month to month, and there is something called a seasonal evapotranspiration curve, and that is really the basis of water conservation.

You want to match the seasonal ET or evapotranspiration curve from the spring, summer, and fall, and traditional irrigation control, like timers, clocks, call it whatever you want, run like clockwork. Monday, Wednesday, Friday, 7:00 AM for this same amount of time. However, the best way to water "SMART" is to match the  seasonal ET curve, because in the spring you need to water a lot less - in the summer you actually may need to water more - and then as you go into fall, you'll want to taper your watering off again. 

The best way to save water or where the biggest opportunity to save water is, is in the spring and the fall where we can match that evapotranspiration curve for the season.

Can your control evapotranspiration?

It's kind of a trick question because, no, you can't control evapotranspiration. It is based on environmental factors. However, if you were to plant a windscreen, then certainly there would be less wind on the plants on the other side of the windscreen, right?

If you planted more shade trees, there'd be more shade on the turf grass. So it is not possible to actually control evapotranspiration, but you can change. The microclimates of your site by changing the type of plants and how you plant. Otherwise, what you can control is how you water. And so you want to actually control your watering to match ET as best that as possible.

And I guess finally, it's important to remember that ET is a. Okay. It is not measuring real-time water loss in the soil right now. The best way to do that is to use a soil moisture sensor. It's actually the only tool you can use to measure real-time et loss as it's happening in the landscape. But evapotranspiration curves and calculations are great for estimating and forecasting purposes, and if you implement them right with your smart timer, you can save.

Water. So I hope that helps. If you have any more questions about smart controllers, evapotranspiration, how to schedule program set up your timer, feel free to reach out to us and any other technical questions you have about et dropping the comments down below. Love answering all of your questions and anything else that comes up, you can reach us by phone, chat, email, text message, and until the next Sprinkler supply store.

Technical review. Happy sprinkling. We'll see you then.

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