ET and Irrigation Guide
Water management on soils keeps evolving as technology allows moisture sensor to help drive the decision on when to soil needs water. But before we had moisture sensors, we used some prediction models to help determine the water needs. Most of the practices come from the farming/agricultural side. Sun, nitrogen and water are some of the most important aspects of growing plants. This article will focus on water.
tl;dr
1in water/week is not even close to accurate. Try to water your lawn to apply between 0.4-0.6in of water when it shows signs of wilting. This could be every 2-3days in the dog days of summer.
Evapotranspiration (ET)
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- from rachio support pages
- from rachio support pages
Wikipedia definition for Evapotranspiration is: "the sum of evaporation and plant transpiration from the Earth's land and ocean surface to the atmosphere." But I like to think of it in simpler words. Some of the water in the soil will evaporate to the air, some will be stored in the soil, some will move deeper into the soil to reach underground streams and some will be used by the plants. The amount of sun, wind, temperature, soil type, plant type and plant growth (driven by nitrogen); will all affect the ET.
Years ago some scientists decided to measure how much water/moisture changed as each of these factors changed minus the plant. That work evolved into a comprehensive controlled method to measure the ET and it is called the Reference ET, also called ET0. The equipment and setup gets complex, so there are models to approximate the ET0 using temperature, humidity and lat/long location. Many state climate offices or university publish the daily ET0 and the predicted for the next few days.
The ET0 gets multiplied by a crop factor. Each crop (eg. corn, soybeans, avocado tree) will use water from the soil at a different rate. The multiplication allows you to adjust the ET0 to your crop. In the case of turf, cool season grasses average is 0.80, while warm season average is 0.60. Yes, warm season lawns need less water. But this average does fluctuate in the season and it is influenced by the amount of nitrogen you use. The more nitrogen, the more water the plant will need. When the weather is really good, it grows more vs when it is cold, it will slow the use of water. But in general the 0.8 is a good approximation.
Why is all of this important? This information helps us know how much water your crop (lawn) uses each day. Unless you resupply it, or it rains, the lawn will suffer.
Sources for ET0
- USA only - NOAA National Weather Service Graphical Forecasts click on the cloud thunder and pick Daily or Total Weekly FRET
- Some of Canada - Evapotranspiration - Farmwest
- Your state climate / ag website
- Calculate using temps/location via Hargraves ETo Model (Hargreaves-Samani). The logfile I shared calculates it using this formula. It takes data via an API from your local weather.
Soil Moisture Balance
With the ET info we have, the easy answer would be, I'm going to irrigate to match the amount of the ET every day. Done. Easy. But this is not a good approach. First your turf leaf will get wet everyday, with heat, this leads to fungus. Second, you are constantly providing water to the top layer of the soil. The roots will chase the water (even the sound of water). You will end up with shallow roots.
A better approach is to treat the soil like a bank account. You want to maintain a balance in your account (to avoid bank fees). The main difference with soil balance is that there is a maximum amount of money (water) our bank account can hold. The best analogy for our soils is to compare them to a kitchen sponge. If you go on vacation and let it dry, it will become a solid, hard, rigid square. You won't be able to bend it or use it. That's what happens to our soils when we don't water them. It shrinks, cracks and it is very hard to work with. Most folks will say, I have bad clay soil, but in reality it is just bone dry soil. Going back to the sponge analogy, when we first hit that sponge with water, it will bounce off. It will repel the water. The structure is so tight, it wont let water get in. But eventually, water does get in and the sponge will expand and be malleable. If you continue adding water, it will reach a maximum and water will just flow because there is no more space for it.
The goal for our soils is to irrigate to get them to fully saturated state and then let it dry up to around 50% capacity. But why not to 0%? Once the moisture reaches too low, the roots will not be able to absorb it (wilting point) and we want some reserves (emergency funds in the bank account). The soil will normally dry from the top down, so we want to encourage the roots to go into the deeper layer to get that water. At the same time, we want to avoid it getting too dry damaging the roots. Lastly we want to avoid a hydrophobic soil (repel the water) to avoid wasting irrigation water that just runs off.
We want to maintain between 100% to 50% capacity, but how much capacity to hold water do I have? It depends on two factors: 1) how deep are your roots and 2)what is your soil type. Water below the reach of the roots, is not usable to the plant. You will need to grab a shovel/soil probe and determine the majority of your root depth.
And then the hard part; figuring out how much water your soil can hold. Sandy soils are composed of large particles with voids that let the water flow. This means it cannot hold too much water. On the opposite side, clay soils (real clay), are very fine small particles with barely any space in between them. But the small particle size also brings the ability to hold on to water, therefore clay can retain moisture. The main problem is that it takes time for water to flow through clay. Also, once it dries, it is like concrete and very hydrophobic. So we have the two extremes in soil types, then there is a third factor into the soil, organic matter. Organic matter improves a soil ability to hold on to nutrients and moisture too.
How to find out your soil type?
- Search these databases for native soils: SoilWeb: An Online Soil Survey Browser | California Soil Resource Lab
- Get it tested by a lab
- Do a mason jar soil composition test
Lastly use the table below or other online sources to determine how many inches of water your soil can hold. Calculate how many inches of water is your capacity (root depth * inches of water per inch of root depth).
Soil Type-----inches of water per inch of root depth
Coarse sand-------0.06
Fine sand----------0.08
Fine sandy loam----0.13
Silt loam-----------0.16
Clay loam----------0.18
Clay ---------------0.17
(converted from https://turf.unl.edu/NebGuides/homelawnirrigation2011a.pdf)
The end of this long article
If you are still awake and following all of this, now you should know how many inches of water your soil can hold. You also know how much water (ET) you are losing by day. Now the rest is simple math. Multiple your capacity by 50% (depletion) and everyday start subtracting the ET. Once it reaches zero, you need to irrigate. If you get a rain, add how many inches. If you get 3 inches of rain, you don't add 3 inches, you add until you reach the capacity. The goal is to irrigate only when you reach the depletion point.
You will find that sandy soils don't have a lot of capacity, you will need to irrigate more often and not as deep (or you end up wasting water), great loamy soil can hold water, so 1in irrigation every 5 days is good and silty clay soils (the vast majority in the midwest) can hold around 0.5in of irrigation for 3 days.
How to irrigate
You need to know how many inches of water you are applying. The best way is an irrigation audit. To do one, place a straight wall container (eg. tuna can) in the irrigation zones to see how long it takes to reach the target (eg. 0.5in). It doesn't matter what you use to irrigate, impact or in ground, the approach is the same.
An in ground system is great but it might not be designed/installed correctly and give you uneven distribution of water. You have to water long enough to get the worst spot the water it needs while over watering the good coverage areas. Above ground system can be a challenge to do all the zones in one day. But you dont need to do them all in one day. You can do the front one morning and the back the next morning. Also using a spigot 2-4 outlet timer will allow you to do a couple of areas without having to move too many hoses.
Final thoughts
While all this science stuff is great to understand and gain more knowledge, it is still an approximation. There are too many variables (amount of nitrogen, wind, sun, microclimates in your yard) to blindly follow this. If you see signs of wilting, throw the theory out the door and irrigate.
This article is only a very high level and simplified approach. Some further reading material:
Certified Crop Advisor study resources (Northeast region)
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