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.

Avoid Water Runoff With Cycle and Soak

Have you ever seen water dribbling off a lawn onto the pavement or into a gutter? The sprinklers are likely still running, but the yard isn’t absorbing the water anymore. It’s frustrating, right? All that water being used, but it’s not going where it’s supposed to: the yard. This is called runoff, and it's a waste of water.

How do you avoid water runoff?

The best way to avoid runoff* is with a technique called cycle and soak!

Cycle and soak is a technique developed to avoid water runoff, making your water use more efficient, and making sure your yard gets the moisture it needs. The basic idea is to break up your irrigation into two or three parts in order to allow the dirt to better absorb the water. This has a few key benefits:

• It allows your grass or plants to properly absorb the water.
• It promotes root depth, which makes your yard more drought and disease resistant.

Using the same metaphor that we used in our MAD article, imagine that your yard is a dense sponge. When you pour water onto a sponge, it takes time for the water to work its way through the entire sponge. And you need the water to get through the entire sponge, because you want all of your roots to have water!

By breaking up an irrigation session into shorter parts, this lets the water work its way through the entire sponge, making room for the second or third cycle of water to follow. It also allows the water to get deeper into the soil.

How do you calculate how long each cycle should take?

The short answer is that you need to know the density of your sponge, and that depends primarily on what type of dirt you have. For example, sandy soil absorbs water very quickly, but clay soil does not, and there is a wide range of soil in between. An irrigation professional can easily tell you what type of soil you have, and it’s great to know on a zone by zone basis. Most professionals would be happy to do an audit of your landscape in order to determine this info, and make sure the rest of your irrigation system is optimized.

Your soil type, plus the type of plant being watered, plus a few other factors, determines how long the irrigation session should last. For example, one zone might need 45 minutes in one session. With cycle and soak, this would get broken into 2 or 3 different sessions, perhaps 15 minutes of watering followed by a 30 minute rest, then another 15 minutes of watering, then another 30 minute rest, then the final 15 minutes of watering. This allows the water to truly soak the landscape, without runoff.

If you would like to know how Rachio incorporates Cycle and Soak, and all the variables that come into play, please check out our support article on Smart Cycle.

* Assuming that your sprinkler heads are pointed where they are supposed to!

We're MAD For Irrigation

MAD. It’s all the rage.

But, what is MAD? I’m glad you asked.

MAD is an acronym for Management Allowable Depletion, the best and most recommended way to schedule your irrigation. But what is it?

To explain, let me use an analogy. Imagine your soil is a sponge, and imagine that sponge is as full of water as possible. As the grass or plants absorbs water from the sponge, or as it evaporates into the air, the amount of water in the sponge becomes less.

With an older style of irrigation, water would be added to the sponge on a fixed, regular schedule, regardless of how much water has been removed. For example, the sponge would have water added every three days for 30 minutes each day. This could result in over-watering, because the sponge is already full, or under-watering, because the sponge became completely dry before a fill up.

With MAD, the goal is to add water to the sponge when it reaches a certain amount of dryness, but before it is completely dry. This has a few key benefits:

• It encourages plants to develop deeper root structures, which creates stronger, healthier plants that are more resistant to drought.
• It requires less frequent waterings, because each watering is for a longer period of time with the goal of deeper watering. The overall effect is less water used.
• It allows less water to be lost to evaporation, because the water is deeper into the ground.

It is called Management Allowable Depletion because this irrigation schedule allows the amount of water to deplete to a fixed level before refill.

Compared to a fixed schedule, MAD would fill up the sponge only when required, which could be every two days, or five days, or even longer!

There are two key factors that need to be calculated for MAD style of irrigation: how big the sponge is, and how much water is in the sponge at any one time. To get to those two elements, there are quite a few data points that go into this:

• The type of plant
• The type of soil
• The amount of precipitation
• The amount of water the plant absorbs
• The amount of water that evaporates

The challenge in calculating MAD is ensuring that the inputs are correct, particularly because they are ongoing inputs. However, once you have these inputs, you can create an irrigation schedule that is tailor made for your soil and plants, while saving a large amount of water.

Whenever you install or update your irrigation system, this is a great opportunity to make sure that you know what type of soil you have, and what types of plants are in each zone. This will allow you to be as accurate as possible when creating your watering schedule.

If you’d like to know how Rachio incorporates MAD, please check out our support articles on Flex Schedules.