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ET and irrigation guide

46K views 132 replies 44 participants last post by  tommydearest 
#1 ·
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)


- 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

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?

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:
www.fao.org irrigation manual http://www.fao.org/3/a-ai593e.pdf

Licensed Attribution-NonCommercial-ShareAlike 4.0 International (CC BY-NC-SA 4.0)
 
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#8 ·
@davegravy Yes, the Hargraves ETo Model (Hargreaves-Samani) uses your temperatures, humidity and your lat/long to provide an approximation (does not account for wind). Check the www.FAo_Org for more info.

Select Ontario. then the airport and then pick the daily values (peak at 6mm/day)
https://farmwest.com/climate/et
 
#9 ·
Great write up, thank you. Does soil CEC have any correlation regarding water holding capacity and can it be used to help determine how many inches of water your soil can hold?
For example, I have clay loam soil with a CEC of 11.6 and I'm trying to calculate how many inches of water mine can hold.
Thanks again.
 
#10 ·
@Ohio Lawn - what kind of soil do you have using this map? https://casoilresource.lawr.ucdavis.edu/gmap/

It seems like CEC would have a correlation with water holding capacity. I have about the same CEC (11.7me/100g) with sandy loamy soil and organic matter of 4.3%. Seems like all of these would correlate to water holding capacity. Not sure if we could use these as inputs to get water holding capacity or if it's more complicated than that.
 
#13 ·
Great article. I guess I have no excuse now for not doing the math. So, my soil holds about 1.5 inches of water. Roots go 8-10 inches or so.

1.5 x 0.5 = 0.75 inch to replace 50%.

ET is currently about 0.212 inch per day in full sun. So 3.5 days to replace 50%.

That means what I'm able to right now, every 5 days or so, could create a deficit over time if there's no rain. And I'm already below 50%, I believe, before I started watering (due to wilting).

So, it is being pushed to or even past the limit and relying on the drought avoidance of my grass.

------------------
Last year in July, I did a half inch 3 times per week. I will calculate that situation next. So, say 0.25 inch of ET per day when it's above 90F and sunny. It's rarely higher than 0.25 or so I've found.

7 x 0.25 = 1.75 inches used per week when it's above 90F. If I'm putting down 1.5 inches, I'm almost keeping up, but could still incur a deficit that could build up over time, at 0.25 inch per week.
 
#14 ·
@g-man

So, if the water is down to 0%, should we really not ever water more than the soil's capacity because it just gets wasted? (E.g. for my sandy loam, never put down more than 1.5 inches--which takes forever due to both infiltration rate and sprinkler gpm, anyway--in a session.)

This is not hypothetical, either...I literally watered an area with 1.5 inches over the last 24 hours because it was down to near 0% (bad wilting). I was going by intuition and experience. But then after I did it and read this post, I saw that the article you used as a reference confirmed that 1.5 in is about the max my soil can hold.
 
#21 ·
@g-man, I'm not following the root depth thing, either.

Sandy loam holds 1.5 inch of water.

Say most of the roots hit 6 inch depth.

Now, what are we doing with these numbers?
 
#22 ·
Green said:
@g-man, I'm not following the root depth thing, either.

Sandy loam holds 1.5 inch of water.

Say most of the roots hit 6 inch depth.

Now, what are we doing with these numbers?
It holds 1.5in of water per foot of soil. You don't have roots to 12in, so you need to convert the 1.5in/ft to in/in. That's 0.125in of water / in of soil

If your roots are at 6in, then 0.125 * 6 = 0.75in of water. Using a 50% depletion, that's
0.375in.

You should water 0.37in when you water the lawn. You said your ET was 0.2/day, so every 2 days 0.37in of rain or irrigation.
 
#23 ·
@g-man, just looking quickly but think I'm following now.

And this actually lines up exactly with what I did last July, just going by experience and looking for wilt. I aimed for 0.5 in with the irrigation system 3x oer week, but some areas got 0.3 in and others got 0.8 because coverage can only be so even.

The thing I'm not following: does 0.37 inch of water penetrate as far as, say 1.5 inch, if allowed enough time to soak in (assume you just have soil with no roots and do a controlled experiment). Does amount of water determine how far down it gets? Intuition says yes, but what did you find in your research for this write-up?

Another way: Will 0.37 in soak the whole root zone beyond the average 6 in when replacing 50-60% of the soil water?

Also, how does current soil water depletion percentage affect how far it soaks in?
 
#24 ·
Im not sure I understand. After 2 days of 0.2 ET, the 0.37in or 2in of rain, are all gone. They are not available to your turf. They no longer matter. Look at the first picture, some evaporates, some goes to the plant and some goes to ground aquifers. What matter is the moisture in your root zone.
 
#25 ·
Once the root zone is filled, the extra water is wasted. It either runs off or moves below the roots where they can't reach it. If your soil holds 0.5 inches of water in the root zone and you get 2 inches of rain, the root zone will be half depleted after 0.25 inches of evapotranspiration. After another 0.25 inches of evapotranspiration and the root zone is empty.

Think of it like filling a beer stein. Once full, the extra runs over and is wasted beer.
 
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