# ET and irrigation guide



## g-man

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

 USA only - https://digital.weather.gov/mobile/index.php click on the cloud thunder and pick Daily or Total Weekly FRET
 Some of Canada - https://farmwest.com/climate/calculators/evapotranspiration/
 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.

https://i.imgur.com/FRmQhiG.mp4​
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: https://casoilresource.lawr.ucdavis.edu/gmap/
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:
https://www.nrcs.usda.gov/Internet/FSE_DOCUMENTS/nrcs142p2_051279.pdf
https://nrcca.cals.cornell.edu/soil/CA2/CA0212.1-3.php
www.fao.org irrigation manual http://www.fao.org/3/a-ai593e.pdf
https://directives.sc.egov.usda.gov/OpenNonWebContent.aspx?content=17837.wba
https://turf.unl.edu/NebGuides/homelawnirrigation2011a.pdf

Licensed Attribution-NonCommercial-ShareAlike 4.0 International (CC BY-NC-SA 4.0)
https://creativecommons.org/licenses/by-nc-sa/4.0/


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## g-man

I wrote this article like 3 times. It was too complex and then not clear and it did not flow right. The last section needs some more work for clarity, but I wanted something published before the summer.


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## TSGarp007

Nice article, and explains why putting any more than 0.5 inches into my sand at a time is probably a waste.


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## SNOWBOB11

Excellent write up g-man. Thanks for taking the time to write and post this.


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## Babameca

Thank you @g-man . No 'training' can reach all audience, when audience is randomly skilled in turf care. IMO this is just the right amount of complexity.


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## davegravy

I haven't found live Et0 data available for my region, just historical.

To fellow Canadians /Ontarians : have you found this anywhere?

@g-man you mentioned it's calculated just from temperature. Is there a formula we can use to calculate our own Et0 using our personal weatherstation data?


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## Spammage

@davegravy if you go to the TexasEt Network website https://texaset.tamu.edu/, it will show you the measurements and the calculated ET from them. I don't know if this will help you figure out the formula or not.


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## g-man

@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.FArg for more info.

Select Ontario. then the airport and then pick the daily values (peak at 6mm/day)
https://farmwest.com/climate/et


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## Ohio Lawn

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.


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## occamsrzr

@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.


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## Bermuda_Triangle

@Ware excellent write up!


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## g-man

@Ohio Lawn @occamsrzr you can also pay the soil test lab(eg. Waypoint) to do a texture analysis. I forgot about that. I need to add it to the text.


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## Green

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.


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## Green

@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.


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## g-man

@Green be careful with 8-10 inches roots in your calculations. We need an effective root mass. A single root getting to 8-10inches is not going to sustain the turf. I set mine to 6in.


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## Ohio Lawn

I'm not understanding where the root depth comes into play in the calculation. My soil is clay loam (almost silt loam) so, my soil holds about 2 inches of water. 2 x 0.5 = 1.0 inch to replace 50%. How does root depth affect my deficit calculation?


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## g-man

@Ohio Lawn



> calculate how many inches of water is your capacity (root depth * soil type capacity per inches of depth).


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## Ohio Lawn

g-man said:


> @Ohio Lawn
> 
> 
> 
> 
> calculate how many inches of water is your capacity (root depth * soil type capacity per inches of depth).
Click to expand...

Thank you sir. So if my roots go to a depth of 4", my soil capacity would be 8?


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## g-man

2in/ft. A ft is 12in.

0.16in of water / in of soil.

0.16 * 4 =0.66


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## B-Rad

Thank you @g-man! Your write-ups are extremely helpful. :thumbup:


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## Green

@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?


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## g-man

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.


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## Green

@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?


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## g-man

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.


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## bernstem

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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## NJ-lawn

Wouldn't it be cool if there were probes or sensors in different sections of your lawn and they talk to the irrigation controller for the exact amount of water.


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## g-man

@NJ-lawn there are. Soil Station


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## NJ-lawn

g-man said:


> @NJ-lawn there are. Soil Station


That is so cool.....always wanted a PWS too. Amazing.....


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## Green

g-man said:


> 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.


That doesn't make sense to me. Why would 2 inches be gone after 2 days, if the soil can hold 1.5 inches? Does the soil not actually hold the full 1.5 inches (for sandy loam, from the table in the reference)? It acts like it does, in my experience.

@bernstem, yes, I get your post fully. But I don't believe your example, 0.5 inch, was in reference to sandy loam, correct?


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## bernstem

0.5 inches was a randomly chosen number that worked well. It doesn't mean anything.

The thing to understand is that any water that moves below the root zone is not going to percolate back up, and water is lost from the top down. Lets assume your soil is bone dry and holds 1 inch in the root zone and you get 1.5 inches of rain. The first inch will fill the root zone fully. The half inch that follows that will also go into the root zone, but will push 1/2 inch out the bottom leaving 1 inch in the root zone and 1/2 inch below it. It then stops raining and the sun comes out. The grass then starts to pull water from the root zone and heat starts to evaporate water from the soil surface. The grass and evaporation won't pull any water from below the root zone and the water won't come back up from below. Once the grass and evaporation have used up the one inch of water in the root zone, there is no more left that it can get to. The half inch that is below the root zone is still there, but is effectively unavailable since there are no roots there. That is why anything added after the root zone is full is wasted.


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## g-man

^+1 and your soil holds 0.75in of water. The table list 1.5in for 12 inches of root zone, you only have 6in of root zone.


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## Green

g-man said:


> 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.
Click to expand...

Ok, I followed through the math and what it means several times now. So, we want to calculate how much water the main root zone area holds, not the entire 12 inch section...only the main root zone. I think that is what I was missing before.

But, can the roots suck up water from a couple of inches below where they reach by capillary action? Or can they really only access water to the depth at which a large percentage of the roots physically stop (e.g. 8 inches)?

And if most of the roots are present at 6 inches, some roots end at 8 in for most of the plants, less of them reach to 10 for some plants, and the occasional root of a plant in the lawn reaches to 12 in, do you still use 6 in because that is where most of then are?

Also, I assume this ET replacement at 50% depletion is for best turf quality. I guess the traditional 1 inch per week guideline (which we know is too little in Summer) was not for best turf quality. They must be expecting some root die off each time the soil dries to 0 and a deficit gets built with each cycle until the weather cools.


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## Green

bernstem said:


> 0.5 inches was a randomly chosen number that worked well. It doesn't mean anything.
> 
> The thing to understand is that any water that moves below the root zone is not going to percolate back up, and water is lost from the top down. Lets assume your soil is bone dry and holds 1 inch in the root zone and you get 1.5 inches of rain. The first inch will fill the root zone fully. The half inch that follows that will also go into the root zone, but will push 1/2 inch out the bottom leaving 1 inch in the root zone and 1/2 inch below it. It then stops raining and the sun comes out. The grass then starts to pull water from the root zone and heat starts to evaporate water from the soil surface. The grass and evaporation won't pull any water from below the root zone and the water won't come back up from below. Once the grass and evaporation have used up the one inch of water in the root zone, there is no more left that it can get to. The half inch that is below the root zone is still there, but is effectively unavailable since there are no roots there. That is why anything added after the root zone is full is wasted.


Makes sense.

Do we know for sure that the roots cannot create a concentration gradient that sucks some water from deeper levels (an inch or two below where they end)?

Or on the other hand: we always hear about root growth continuing deeper in response to water below the root zone. Any comments on that?


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## bernstem

@Green While they can probably suck some water from below the root zone it probably isn't a signifcant amount, and you shouldn't let it get that dry. It is better to refill at 50% or so depletion.

While it is nice to assume that the math works perfectly, it doesn't since this is biology rather than physics. The basic calculation is good and will get you close, but you may need to adjust depending on real world conditions. Six inches is a good starting point for root depth. That is where most of the roots are (yes, some are longer and some shorter) and seems to work well based on experience. The biggest benefit to using evapotranspiration is that it dynamically adjusts based on weather conditions day to day, and, if set up well, it can irrigate before stress shows.

In practice, some areas will need adjusting. Tree roots can pull a lot of water from the soil. In those areas loss rates will be higher than calculated. Grass roots may be limited by something and root zone size will be off. Maybe you have a lot of growth from a recent Nitrogen application and losses are higher than expected.


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## Green

@bernstem, thanks. I know you've been doing the ET math for a number of years and also using a sophisticated (arguably one of the best at the time) preductive ET-based controllers. If I remember right, you have a rainbird ESP-me series model, and it even has a setting for root depth, I remember you saying in the past.

The thing that doesn't quite add up is how some grass areas can continue growing (slowing of course) for a few days or even weeks in a short term deficit or drought during times of high ET. If the ET predicts the water will only last 3 days for example, how does it go for, say, 6?

A take-off on this: my own grass kept going for about a week after the previous rain before wilting considerably. About 5 days in the sunniest areas. Some areas can go at least 9 days between waterings. How?


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## bernstem

Green said:


> The thing that doesn't quite add up is how some grass areas can continue growing (slowing of course) for a few days or even weeks in a short term deficit or drought during times of high ET. If the ET predicts the water will only last 3 days for example, how does it go for, say, 6?
> 
> A take-off on this: my own grass kept going for about a week after the previous rain before wilting considerably. About 5 days in the sunniest areas. Some areas can go at least 9 days between waterings. How?


If you can figure out the magic sauce to keep grass alive in 90 degree weather for 9 days without water, let us all know. :lol:

Seriously, though, I don't know. It is 100% true that in most people's lawns, the grass doesn't respond the same to drought across the stand. Certainly, there are differences in soil and grass within the lawn. That can lead to more water retention, longer roots, different levels of shade, etc.

The typical calculated watering amounts and intervals are designed to prevent stress. If you irrigate based on grass wilting, needling, etc., you are watering after stress. If you wait for browning, you are watering to prevent dormancy. Maybe someone can give a more definite point, but I don't think stress shows up until the root zone is almost depleted. Dormancy doesn't happen until some time after the available water is depleted.

If you wanted to set the controller to allow stress, you could move the watering threshold closer to 0%. The grass won't look as good, but you will likely use less water since rain will be more likely to come and fill all or part of the water balance.

Oh, I moved to a rachio to have online access to the controller. It is basically the same in calculating water needs, but uses online weather data rather than a local weather station.


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## NJ-lawn

bernstem said:


> 0.5 inches was a randomly chosen number that worked well. It doesn't mean anything.
> 
> The thing to understand is that any water that moves below the root zone is not going to percolate back up, and water is lost from the top down. Lets assume your soil is bone dry and holds 1 inch in the root zone and you get 1.5 inches of rain. The first inch will fill the root zone fully. The half inch that follows that will also go into the root zone, but will push 1/2 inch out the bottom leaving 1 inch in the root zone and 1/2 inch below it. It then stops raining and the sun comes out. The grass then starts to pull water from the root zone and heat starts to evaporate water from the soil surface. The grass and evaporation won't pull any water from below the root zone and the water won't come back up from below. Once the grass and evaporation have used up the one inch of water in the root zone, there is no more left that it can get to. The half inch that is below the root zone is still there, but is effectively unavailable since there are no roots there. That is why anything added after the root zone is full is wasted.


Makes sense.....Explains when you overwater it's such a waste


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## Aggrorider1

Green said:


> 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.


@Green where are you getting the ET? I am not finding anything local.


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## Green

Aggrorider1 said:


> Green where are you getting the ET? I am not finding anything local.


You can used my numbers. I'll post some data in my journal. Should be close enough for your part of the state, too. I need to redo the math to take into account root depth, which is mostly at 6" on average.


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## g-man

@Aggrorider1 I updated the article to describe some ET0 sources. I forgot to include that in the original version.


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## Justmatson

@g-man can this be used with newly established grass/ shallow roots?

If i did the math correctly and the ET for my area, I should technically be watering daily....
I've got clay loam and the ET for my area is .197"

I can't imagine the roots are any deeper then 2" . Its only 50 days old. (Could be wrong)

I'm watering every 3 days/ .5" Don't think I should be watering daily even with this younger grass...

Whats you opinion?


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## g-man

You can use it. You will realize that you need to keep watering daily for bit. With a new lawn, I don't take risks to save on water. 50days can have 2+ in of root depth (go pull one to see), but it is still young and thin. The difference between wilt point and dead is narrow.

With a spring lawn, in my opinion I would irrigate to match the ET at 100%crop factor until at least 3 months or until the temps drop to 80F.


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## Ohio Lawn

g-man said:


> @Aggrorider1 I updated the article to describe some ET0 sources. I forgot to include that in the original version.


Thank you for making this update @g-man . That cleared things up a ton in my opinion. Thanks for doing that!

If we are watering when the root zone gets to 50% depletion (and not 0%) will roots still be encouraged to drive deeper into the soil?
Thanks.


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## Justmatson

g-man said:


> You can use it. You will realize that you need to keep watering daily for bit. With a new lawn, I don't take risks to save on water. 50days can have 2+ in of root depth (go pull one to see), but it is still young and thin. The difference between wilt point and dead is narrow.
> 
> With a spring lawn, in my opinion I would irrigate to match the ET at 100%crop factor until at least 3 months or until the temps drop to 80F.


Thanks g-man! 
I'll give that a try. Will also see how deep these roots are too.
I don't pay for water so watering isn't an issue.

My only concern with daily watering is the chance for fungus. Being in Canada we are very limited on fungicide unless we order from the States that can take 30 days to arrive. I do have myclobutanil on hand but thats it.


----------



## g-man

Really good info from USU. Thanks @osuturfman for the link.

#wiltpoint


----------



## osuturfman

@g-man USU?? Haha.

Go Bucks!


----------



## Ohio Lawn

The current Total Weekly FRET in my area is 1.6" Is it reasonable to water every other day right now? We've had zero rain in the past 10 days.


----------



## bernstem

@Ohio Lawn It depends on root depth and soil water holding. At 1.6 inches of water loss per week, I would be watering every 2-3 days. If you have deeper roots, you can water less often, but apply more water each watering. If you have sandy soil that doesn't hold much water, you will need to water less more often.

Without knowing the details of your root depth and soil type, it is impossible to really answer the question, but... every 2-3 days is probably reasonable. What I can say with certainty is if your water loss is 1.6 inches over a week, you will need to apply 1.6 inches that week to replace it. If you water three times, you will need to apply 0.53 inches each time. If you water twice, you will need to apply 0.8 inches each time.


----------



## Ohio Lawn

bernstem said:


> @Ohio Lawn It depends on root depth and soil water holding. At 1.6 inches of water loss per week, I would be watering every 2-3 days. If you have deeper roots, you can water less often, but apply more water each watering. If you have sandy soil that doesn't hold much water, you will need to water less more often.
> 
> Without knowing the details of your root depth and soil type, it is impossible to really answer the question, but... every 2-3 days is probably reasonable. What I can say with certainty is if your water loss is 1.6 inches over a week, you will need to apply 1.6 inches that week to replace it. If you water three times, you will need to apply 0.53 inches each time. If you water twice, you will need to apply 0.8 inches each time.


Thank you for the information. From reading this ET guide, I thought I couldn't water more than what my soil can hold at one time. I have clay loan and 4" roots. That comes to .72" or water holding. 50% of that would be .36". So I thought I shouldn't water more than .36" at one time?
Am I reading this wrong?


----------



## davegravy

bernstem said:


> What I can say with certainty is if your water loss is 1.6 inches over a week, you will need to apply 1.6 inches that week to replace it. If you water three times, you will need to apply 0.53 inches each time. If you water twice, you will need to apply 0.8 inches each time.


... In order to always have some moisture in the soil, correct? I assume the following (please correct if wrong):

You can back off that a bit if you're willing to accept some stress / reduced growth. It's not like 1.59"/week will lead to instant dormancy or death. The grass can survive off its reserves for some period of time.
If you're battling fungus, applying less than 1.6"/week for a while might be a wise choice.


----------



## bernstem

Ohio Lawn said:


> Thank you for the information. From reading this ET guide, I thought I couldn't water more than what my soil can hold at one time. I have clay loan and 4" roots. That comes to .72" or water holding. 50% of that would be .36". So I thought I shouldn't water more than .36" at one time?
> Am I reading this wrong?


You are correct. You should be watering no more than 0.36 inches if you want to use a 50% depletion threshold. At 0.22 inches of loss per day, that will be watering every 1-2 days. You can push depletion past 50% and water less often, but would need more watering per cycle. If you allow 75% depletion, you would only water at every 2 days.


----------



## bernstem

davegravy said:


> bernstem said:
> 
> 
> 
> What I can say with certainty is if your water loss is 1.6 inches over a week, you will need to apply 1.6 inches that week to replace it. If you water three times, you will need to apply 0.53 inches each time. If you water twice, you will need to apply 0.8 inches each time.
> 
> 
> 
> ... In order to always have some moisture in the soil, correct? I assume the following (please correct if wrong):
> 
> You can back off that a bit if you're willing to accept some stress / reduced growth. It's not like 1.59"/week will lead to instant dormancy or death. The grass can survive off its reserves for some period of time.
> If you're battling fungus, applying less than 1.6"/week for a while might be a wise choice.
Click to expand...

Yes, you can allow the soil profile to dry more or even completely. That will cause some stress to the turf.

Whether to allow stress if you have fungus is an open question. In my opinion, and others may differ, I would water to prevent stress. Diseased turf will be less tolerant of drought. Drought stressed grass will be less tolerant of disease. The healthier and deeper your root system is going into summer, the less often you will need to water. The healthier your grass is going into summer, the more resistant to disease it will be. The time to work on summer performance is spring and fall. How the grass does in the heat is less about how you are managing it now, and more about what you did 6-8 weeks ago.


----------



## Ohio Lawn

bernstem said:


> Ohio Lawn said:
> 
> 
> 
> Thank you for the information. From reading this ET guide, I thought I couldn't water more than what my soil can hold at one time. I have clay loan and 4" roots. That comes to .72" or water holding. 50% of that would be .36". So I thought I shouldn't water more than .36" at one time?
> Am I reading this wrong?
> 
> 
> 
> You are correct. You should be watering no more than 0.36 inches if you want to use a 50% depletion threshold. At 0.22 inches of loss per day, that will be watering every 1-2 days. You can push depletion past 50% and water less often, but would need more watering per cycle. If you allow 75% depletion, you would only water at every 2 days.
Click to expand...

Ok thank you for your help. I appreciate it. Last thing, is there any correlation between the moisture depletion percentage and encouraging roots to drive deeper?
If I constantly maintain a 50% depletion, will the roots just stay at their current depth? Does it need to go to 0% to encourage roots to search deeper?


----------



## bernstem

Ohio Lawn said:


> Ok thank you for your help. I appreciate it. Last thing, is there any correlation between the moisture depletion percentage and encouraging roots to drive deeper?
> If I constantly maintain a 50% depletion, will the roots just stay at their current depth? Does it need to go to 0% to encourage roots to search deeper?


Good question. I don't have a definitive answer, though someone on this site probably does. Traditional thinking is that the less often you water, the deeper the roots will go since that is where the moisture is. That seems very reasonable, but... that doesn't mean it is true. Assuming it is true, I have no idea what percentage of depletion is ideal. At some point of depletion roots will start to die. I can say moisture depletion certainly isn't the only factor in root depth.

I would also point out that total water needed is only dependent on evapotranspiration. Deeper roots help to extend the time before you need to irrigate which can decrease water use if it rains before irrigation is needed, but the total amount of water needed has nothing to do with how deep the roots are or how much water your soil holds.


----------



## TSGarp007

bernstem said:


> Ohio Lawn said:
> 
> 
> 
> Ok thank you for your help. I appreciate it. Last thing, is there any correlation between the moisture depletion percentage and encouraging roots to drive deeper?
> If I constantly maintain a 50% depletion, will the roots just stay at their current depth? Does it need to go to 0% to encourage roots to search deeper?
> 
> 
> 
> Good question. I don't have a definitive answer, though someone on this site probably does. Traditional thinking is that the less often you water, the deeper the roots will go since that is where the moisture is. That seems very reasonable, but... that doesn't mean it is true. Assuming it is true, I have no idea what percentage of depletion is ideal. At some point of depletion roots will start to die. I can say moisture depletion certainly isn't the only factor in root depth.
> 
> I would also point out that total water needed is only dependent on evapotranspiration. Deeper roots help to extend the time before you need to irrigate which can decrease water use if it rains before irrigation is needed, but the total amount of water needed has nothing to do with how deep the roots are or how much water your soil holds.
Click to expand...

Isn't the total amount of water needed going to be more if your roots are deeper? Assuming you are watering to the bottom of the root zone. Then it would also take longer to evaporate, just because your 'bucket' of water is bigger.


----------



## davegravy

TSGarp007 said:


> bernstem said:
> 
> 
> 
> 
> 
> Ohio Lawn said:
> 
> 
> 
> Ok thank you for your help. I appreciate it. Last thing, is there any correlation between the moisture depletion percentage and encouraging roots to drive deeper?
> If I constantly maintain a 50% depletion, will the roots just stay at their current depth? Does it need to go to 0% to encourage roots to search deeper?
> 
> 
> 
> Good question. I don't have a definitive answer, though someone on this site probably does. Traditional thinking is that the less often you water, the deeper the roots will go since that is where the moisture is. That seems very reasonable, but... that doesn't mean it is true. Assuming it is true, I have no idea what percentage of depletion is ideal. At some point of depletion roots will start to die. I can say moisture depletion certainly isn't the only factor in root depth.
> 
> I would also point out that total water needed is only dependent on evapotranspiration. Deeper roots help to extend the time before you need to irrigate which can decrease water use if it rains before irrigation is needed, but the total amount of water needed has nothing to do with how deep the roots are or how much water your soil holds.
> 
> Click to expand...
> 
> Isn't the total amount of water needed going to be more if your roots are deeper? Assuming you are watering to the bottom of the root zone. Then it would also take longer to evaporate, just because your 'bucket' of water is bigger.
Click to expand...

Using the bucket analogy, it'll take longer /more water to fill a big bucket initially, but after that you have to put water into the bucket at the same rate as its being removed (ET) else it'll eventually be empty. Having a bigger bucket (deeper roots) doesn't change this, it just means it'll take longer before you run dry if you're running a deficit.

... I think?


----------



## TSGarp007

davegravy said:


> TSGarp007 said:
> 
> 
> 
> 
> 
> bernstem said:
> 
> 
> 
> Good question. I don't have a definitive answer, though someone on this site probably does. Traditional thinking is that the less often you water, the deeper the roots will go since that is where the moisture is. That seems very reasonable, but... that doesn't mean it is true. Assuming it is true, I have no idea what percentage of depletion is ideal. At some point of depletion roots will start to die. I can say moisture depletion certainly isn't the only factor in root depth.
> 
> I would also point out that total water needed is only dependent on evapotranspiration. Deeper roots help to extend the time before you need to irrigate which can decrease water use if it rains before irrigation is needed, but the total amount of water needed has nothing to do with how deep the roots are or how much water your soil holds.
> 
> 
> 
> Isn't the total amount of water needed going to be more if your roots are deeper? Assuming you are watering to the bottom of the root zone. Then it would also take longer to evaporate, just because your 'bucket' of water is bigger.
> 
> Click to expand...
> 
> Using the bucket analogy, it'll take longer /more water to fill a big bucket initially, but after that you have to put water into the bucket at the same rate as its being removed (ET) else it'll eventually be empty. Having a bigger bucket (deeper roots) doesn't change this, it just means it'll take longer before you run dry if you're running a deficit.
> 
> ... I think?
Click to expand...

Well, if you let 50% of a 10 gallon bucket evaporate, that's 5 gallons to refill. If you let 50% of a 5 gallon bucket evaporate then that's 2.5 gallons to refill. Takes longer to evaporate, but takes more water to refill...


----------



## davegravy

TSGarp007 said:


> davegravy said:
> 
> 
> 
> 
> 
> TSGarp007 said:
> 
> 
> 
> Isn't the total amount of water needed going to be more if your roots are deeper? Assuming you are watering to the bottom of the root zone. Then it would also take longer to evaporate, just because your 'bucket' of water is bigger.
> 
> 
> 
> Using the bucket analogy, it'll take longer /more water to fill a big bucket initially, but after that you have to put water into the bucket at the same rate as its being removed (ET) else it'll eventually be empty. Having a bigger bucket (deeper roots) doesn't change this, it just means it'll take longer before you run dry if you're running a deficit.
> 
> ... I think?
> 
> Click to expand...
> 
> Well, if you let 50% of a 10 gallon bucket evaporate, that's 5 gallons to refill. If you let 50% of a 5 gallon bucket evaporate then that's 2.5 gallons to refill. Takes longer to evaporate, but takes more water to refill...
Click to expand...

Yup - either way you are only replacing water at the rate of evapotranspiration. You can replace a large volume less frequently with a big bucket or a small volume more frequently with a little bucket but the average rate (total volume divided by time interval) in each case is the same, ie it's the ET rate.


----------



## TSGarp007

I thought 'total water' meant total needed to fill up the bucket, so to speak. Guess I read that wrong.


----------



## bernstem

It is a bit hard to explain on the internet, but I like the bucket analogy (thanks @davegravy) . A large bucket will take more water to fill, but will also take longer to evaporate. Regardless of bucket size, though, it evaporates at the same rate. The only thing that changes is how often you need to refill the bucket and how much water you need to add when it is empty. When I said total water, I was referring to the total amount of water evaporating. I should have been clearer.


----------



## jeffjunstrom

I think I have this down, but just to confirm, and maybe simplify for anyone else reading through (I had to hand-write out the equation to make sure I got it):

There are a couple of numbers of importance, and I'm going to stick to just the numbers...the explanations of the numbers are above. Also, this ignores the rain variable for now.

First, 0.8(x), where x = daily FRET number, which will be a true variable at any given point during the year. For me, right now, it's 0.19. So my first important number is 0.8(0.19) = 0.152 (how much water I'm losing daily to ET).

Second,  * (z) (and  * (z) * 0.5), where y = root depth and z = a standardized constant based on the soil type table in the initial article (and, generally speaking, y will end up being a constant as well, I'm assuming, since root depth, once established probably doesn't fluctuate a lot?). For me, I'm using y = 6, and z = 0.16 (silt loam). So my second important number is (6)(0.16) = 0.96 (my 100% capacity) and (6)(0.16)(0.5) = 0.48 (my 50% capacity).

Third, (0.5 * y * z) / (0.8 * x). For me, that's (0.5 * 6 * 0.16) / (0.8 *0.19) = ~3.15 (every number of day's I should be putting down my 50% capacity).

TL;DR - To keep up with losing 0.152" of water loss due to ET, I need to water my lawn every 3.15 days with 0.48" of water. Alternatively speaking, I need to water my lawn with 1.064" (0.152 * 7) of water per week, so however I get to that it's what's important (it can be 0.48" every 3.15 days, or 0.3" every other day, and so on). And, I should never water more than 0.96" (either at one time, or cumulatively based on current saturation), otherwise it's a waste.


----------



## g-man

@jeffjunstrom yes your approach is very good. I can see how this could help someone get to their numbers. I do think you have a small calculator error in the days. I get 3.15 days instead of 2.5.

One thing that I need to address in the main guide. The ET is for sunny areas. Shade areas will have a lower ET (hard to calculate), so those areas can go more days between watering. Still try to target the 0.48in whenever your irrigate.


----------



## jeffjunstrom

g-man said:


> @jeffjunstrom yes your approach is very good. I can see how this could help someone get to their numbers. I do think you have a small calculator error in the days. I get 3.15 days instead of 2.5.
> 
> One thing that I need to address in the main guide. The ET is for sunny areas. Shade areas will have a lower ET (hard to calculate), so those areas can go more days between watering. Still try to target the 0.48in whenever your irrigate.


Good catch, edited accordingly.

So, 2 waterings per week at 0.48"/watering would be ideal, while 0.3" every other day is the same amount per week. Is there an optimal amount of water per session, as long as you're in the range? I know you (and others) have said longer, less frequent sessions are better, I'm just trying to see what the range of "long" is in that context.


----------



## g-man

0.3in every other day will get you maintaining at like 30% depletion. Not ideal since you might irrigate the day before a rain.

Also remember that the ET changes with the weather. It could increase to 0.20in, so that mean the same 0.48in but more frequent (every 2 days).


----------



## jeffjunstrom

ALSO, what about time of day? I've always heard that early AM/dusk are the best times, but then I vaguely remember seeing someone around here citing a study that showed timing doesn't really matter.


----------



## Spammage

jeffjunstrom said:


> ALSO, what about time of day? I've always heard that early AM/dusk are the best times, but then I vaguely remember seeing someone around here citing a study that showed timing doesn't really matter.


You will have less evaporation loss if you water at night or early in the morning. You should try your best to minimize the time period that the grass blades remain wet though, so this is kind of a balancing act. I try to have my irrigation stop about the time the sun comes up.


----------



## jeffjunstrom

Spammage said:


> jeffjunstrom said:
> 
> 
> 
> ALSO, what about time of day? I've always heard that early AM/dusk are the best times, but then I vaguely remember seeing someone around here citing a study that showed timing doesn't really matter.
> 
> 
> 
> You will have less evaporation loss if you water at night or early in the morning. You should try your best to minimize the time period that the grass blades remain wet though, so this is kind of a balancing act. I try to have my irrigation stop about the time the sun comes up.
Click to expand...

I'm currently doing the same, running between 530-6 depending on length, time of year, etc. But if I wanted to add an extra session in a given week, or I want to water in some product and give some extra water, do I really lose a significant amount if I water at 3p and not 6a?


----------



## Spammage

jeffjunstrom said:


> Spammage said:
> 
> 
> 
> 
> 
> jeffjunstrom said:
> 
> 
> 
> ALSO, what about time of day? I've always heard that early AM/dusk are the best times, but then I vaguely remember seeing someone around here citing a study that showed timing doesn't really matter.
> 
> 
> 
> You will have less evaporation loss if you water at night or early in the morning. You should try your best to minimize the time period that the grass blades remain wet though, so this is kind of a balancing act. I try to have my irrigation stop about the time the sun comes up.
> 
> Click to expand...
> 
> I'm currently doing the same, running between 530-6 depending on length, time of year, etc. But if I wanted to add an extra session in a given week, or I want to water in some product and give some extra water, do I really lose a significant amount if I water at 3p and not 6a?
Click to expand...

Doing that occasionally with purpose is fine as long as your municipality doesn't have rules against it - I can't ever water from 10:00am through 6:00pm here.


----------



## g-man

@jeffjunstrom The idea of early am is that it is the type of day the grass might be wet already (from dew) and it is cooler. This limits fungus development. And no, you dont loose a significant amount of water at 3pm vs 6am.


----------



## JDM83

awesome article @g-man


----------



## kay7711226

Wow...I actually read through all of that and the link below actually starts making sense to me. I use the Orbit B-hyve smart timers for my DIY above ground irrigation and it references a lot of the calculations mentioned in this thread. Good stuff guys!
https://help.orbitbhyve.com/advanced-settings/


----------



## BH Green

@kay7711226 I didn't realize B-hyve had such advanced settings. I might have to look into that for next year.

In that link you shared, I found this definition of the root zone helpful. And the idea of having a "target" depth if trying to promote deeper roots was something I wondered before:

_"Root Zone (RZ): Effective root depth which is generally considered the top 50% of the maximum root depth. Typical Root Zone is 4-6 inches for annual flowers and ground covers, 4-8 inches for cool season turf, 6-12 inches for shrubs and warm season turf, and 12-24 inches for trees. In the case of new plants with undeveloped root zones, it often makes sense to set the Root Zone at a "target" depth which encourages roots to expand downward."_


----------



## dsc123

This is some really interesting stuff. I'm trying to apply it to myself and would appreciate any feedback, as it seems so far off from conventional wisdom of 1" per week.

Depletion
- Daily FRET per NOAA - 0.07 [I understand this changes; last week it was 0.11]
- Most of my lawn gets only partial sun. That Orbit link above suggests a 25% reduction.
- Crop factor .08
0.066*.75*0.8 = ~ 0.04

Soil Capacity
- Silt loam 0.16
- Root depth--I've never measured this, and just renovated 1/2 my lawn [I'm about a month after germination]. But that orbit link suggests picking a "target depth" for new lawns, which makes sense to me. So I went with 6".
0.16*6 = .96"

Today's a good starting point because I got 1.5" of rain the past 2 days and I can safely assume that put me at 100%.

Assuming my FRET remains steady and my depletion around 0.04, I should water a 1/2 inch about 12 days from now?


----------



## g-man

I would not use 6in until you get roots there. No point in stressing your young lawn. Keep monitoring the Daily FRET as it changes with the weather.


----------



## Captquin

Calculated mine at 4" and 50% to be safe. Fine sandy loam. Had a question on the impact of rainfall. Some will migrate through the soil and be unavailable to the turf. That is captured in the soil type factor, correct?

So if I need to water .25" every 3 days and we get 3" of rain on day 1, has the remaining 2.75" migrated through the soil and I still need to water my .25" on day 3?


----------



## g-man

Yes the extra rain after saturation is not available. It either runs off (even on a lite rain, there is runoff) or it moves to the subsoil.


----------



## Captquin

That's incredibly impactful information. Thanks again.


----------



## burntfire

This has to be one of the best discussions on watering I've ever seen. Bravo!


----------



## burntfire

So if I'm doing this correctly I would need to water my lawn .5" every 20 days? I feel like I'm doing something wrong here.

This is all theoretical so bear with me as I'm basing everything off of 6" root depth of bermuda.

I have clay soil at 0.17 * 6 = 1.02 * .5 = .51

Hawaii is only giving me evap on a monthly basis so for December it was 0.7596 / 30 = .02532 daily FRET (Is this where i'm going wrong?)

.51 / .02532 = 20.1

This seems.. not right.

Info pulled from here... http://evapotranspiration.geography.hawaii.edu/interactivemap.html


----------



## g-man

0.75in for the entire month seems low. It depends on where you click in that map. In most areas I get more than 1in.

December is a colder month with less sunlight. It will be an infrequent irrigation if it doesn't rain.


----------



## burntfire

I would agree. Even in August it's only .7463.

I live in Kihei, HI and I used the above website. It seems low as it rarely rains here and is typically hot year round. I would think at least watering every 5 days.


----------



## Green

Just an example, but where I live (a typical Northern climate) the ET for December totals just under 1 inch of water. Temps are generally about 40F for highs, and 20s at night.

At the same time, it's exceedingly rare that we wouldn't get an inch of rain during the whole month.

For the exception to the norm, take a look at Ryan Knorr's recent video. He had a drought, and had to water last week.

The only time I ever watered in December was one year to water in fertilizer, and I hope that was the last time. It was a weird situation because I had applied it, then we got like three inches of rain, so it probably leached. So I applied more but then had to water it in. Never again.

Every climate is different, though. It's not uncommon to have an 80F day in Hawaii right now, as I understand it.

Point was, even in places like mine where the grass is "dormant" right now, a full inch of water is still needed for the month.


----------



## burntfire

Especially where I live as it's considered the desert of the Island and we constantly have temps around 90 for most of the year. It's hard to argue with the actual numbers but I will say we go months without rain but when it does rain it RAINS.

I might skew my numbers a bit to assume 1" per month and go from there unless that's going the wrong way with it.


----------



## g-man

When I look at the map, I see 1.5in per month.


----------



## JerseyGreens

This is an awesome thread that of course gets quieter in the winter months. Hoping this post kickstarts it!

Bear with me, here are my calculations:


*Went with 3 inch root depth as that is what I witnessed when pulling soil samples on my reno area.

Here is the precip amounts for the past 72 hours - basically no rain:


Does that mean I should be turning on my Irrigation system and get to watering? The AM's are still mid 30s temp wise but the daytime temps are hitting the high 60s most days.


----------



## Wiley

Looking at the greencast soil moisture map I was wondering how the percentages are derived? How does this fit-in with ET? What's the practical application of the percentages?


----------



## johnklein25

This thread is making my head hurt. I did figure out how to make it rain tho. Step 1) wait until the lawn looks like it needs a soak. Step 2) don't believe the local forecast. Step 3) set your sprinklers, put down an inch of water. Step 4) Watch it rain 1 or 2 inches a day or two later.

Seriously, this is good information. I'll have to come back and hit it again to make sure I get it. Thanks G-Man.


----------



## smsrmdlol

If I'm trying to "catch up" on drought spots, can I be aggressive and water everyday or every other day until the spots perk up? Or should I do deep & infrequent watering on those spots?


----------



## g-man

Water more frequently to catch up. Soil becomes hydrophobic when it gets really dry and it takes a lot of water to fix it.


----------



## dleonard11122

This thread was really helpful in understanding the Flex Daily schedule of my Rachio irrigation controller. I'm happy to let it do the continous math to figure out when I need to water, but it's nice to have an understanding of the science behind it so that I can tweak things if needed. Thanks!


----------



## JerseyGreens

dleonard11122 said:


> This thread was really helpful in understanding the Flex Daily schedule of my Rachio irrigation controller. I'm happy to let it do the continous math to figure out when I need to water, but it's nice to have an understanding of the science behind it so that I can tweak things if needed. Thanks!


I don't trust my Rachio controller with anything "AI" related. The fact that we can't input GPMs on the Sprinkler Heads leads me to have doubts with their algorithms.


----------



## dleonard11122

You can change that, but you just have to do the calculation or measurement for nozzle inches per hour and adjust that under advanced settings.


----------



## JerseyGreens

dleonard11122 said:


> You can change that, but you just have to do the calculation or measurement for nozzle inches per hour and adjust that under advanced settings.


That definitely makes sense and is very useful in the long run but I've found success using the ET calcs to water this growing season.

When all is said and done I'm sure the Flex Schedule would get pretty close to a similar outcome. Do you sense that its deep watering while on "Flex"? Might have to dig into the advanced settings as I haven't done that for probably 18 months - they may have made updates.


----------



## dleonard11122

A catch cup test will tell you your nozzle inches per minute and be more accurate than a generic calculation, but this is a good starting point.

Precipitation Rate = Inches/hr = (96.25 * gpm) / area

For instance, I have a zone that has Rotors that put out 4, 6, 6, and 4.5 GPM. Over that zone I have a total of 21 GPM. The zone covers 4k sqft.

(96.25 * 21) / 4000 = .505

I confirmed this by doing a catch cup test and most areas of that zone get a half inch of water in an hour.

Caveat: this calculation assumes you have an even distribution of your precipitation rates across a zone.

For instance, the 6 gpm rotors in my zone are half circle (180°) and the 4 gpm are quarter circle (90°). Therefore the 4gpm rotors have a higher precipitation rate for the area they are watering, but I'm okay with that slight loss of efficiency, and just run this zone until such a time that the 6 gpm rotors have reached the desired precipitation rate.


----------



## JerseyGreens

dleonard11122 said:


> A catch cup test will tell you your nozzle inches per minute and be more accurate than a generic calculation, but this is a good starting point.
> 
> Precipitation Rate = Inches/hr = (96.25 * gpm) / area
> 
> For instance, I have a zone that has Rotors that put out 4, 6, 6, and 4.5 GPM. Over that zone I have a total of 21 GPM. The zone covers 4k sqft.
> 
> (96.25 * 21) / 4000 = .505
> 
> I confirmed this by doing a catch cup test and most areas of that zone get a half inch of water in an hour.
> 
> Caveat: this calculation assumes you have an even distribution of your precipitation rates across a zone.
> 
> For instance, the 6 gpm rotors in my zone are half circle (180°) and the 4 gpm are quarter circle (90°). Therefore the 4gpm rotors have a higher precipitation rate for the area they are watering, but I'm okay with that slight loss of efficiency, and just run this zone until such a time that the 6 gpm rotors have reached the desired precipitation rate.


Thanks for the detailed response and I agree these calculations would get a user very close to a positive outcome. My only concern is that I have differing zones that overlap Head-to-Head - which made sq footage calculations difficult for me to wrap my head around.


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## dleonard11122

Oh, yeah I have to fight that battle too. That's my biggest complaint about Rachio. A good irrigation system install has head to head coverage between zones, but Rachio doesn't have an easy way of dealing with that.

My workaround has been to tell Rachio that the Nozzle inches per hour for a given zone are double what they actually are. So, for instance that last zone that we calculated a 0.5" per hour rating, I have set to 1" per hour. That way, the zone will run for a half hour and stop (thinking it's put down 0.5", when it really has only put down 0.25".) Then, the adjacent zone will turn on and fill in the remaining missing 0.25" to get an actual 0.5" of water on that first zone.

The only issue with this hack is that Rachio thinks it's putting out water faster than it actually is, so it wants to smart cycle more than it probably needs to. I don't mind that though as I have enough zones to cycle through.


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## behemyth

Out of sheer curiosity how long do your irrigation systems run per zone? I know its going to depend on head coverage, I'm just curious the times people are running for. I run mine for about 30mins a zone and it keeps my lawn looking good all summer, I just feel like I'm wasting water sometimes.


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## Green

behemyth said:


> Out of sheer curiosity how long do your irrigation systems run per zone? I know its going to depend on head coverage, I'm just curious the times people are running for. I run mine for about 30mins a zone and it keeps my lawn looking good all summer, I just feel like I'm wasting water sometimes.


A typical zone for me takes about 2 hours to put down 1 inch. Actual watering times very depending on how much I'm trying to put down and if it has rained or not. As a rule, I almost always want more than 0.3 inch, but almost never more than 1 inch. So, a typical watering time might be 60 min for about a half inch.

Measure it if you're in doubt. But remember, you might get away with a bit less water than calculations in this thread suggest (deficit irrigation) unless your lawn is being used a lot.


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## dleonard11122

Between 15 and 60 minutes, depending on the zone. The 15 minute zone is only 1k sqft and has a bunch of spray heads that provide a continous misting. The 60 minute zone covers 7k sqft with 2 rotors doing 360* and 2 doing 180*.


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## davegravy

60 min per zone gives my 1/2" except at the edges it's only like 1/8" so I've been hand watering the edges


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## behemyth

Ok, I feel a little better now. I run my zones about 40 mins each, but when they installed my irrigation system most of my heads over lap their coverage, so I get more water down than if just a single head was covering a spot.

I also have heads that spin almost 360 degrees in the middle of some of the zones to help apply even coverage.

When do you guys start your systems running for an hour+ a zone? I've been starting mine around 3am.


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## davegravy

behemyth said:


> Ok, I feel a little better now. I run my zones about 40 mins each, but when they installed my irrigation system most of my heads over lap their coverage, so I get more water down than if just a single head was covering a spot.
> 
> I also have heads that spin almost 360 degrees in the middle of some of the zones to help apply even coverage.
> 
> When do you guys start your systems running for an hour+ a zone? I've been starting mine around 3am.


3am here. I have the whole property watered by noon if I'm diligent (the front yard and part of the back I have to drag hoses around)


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## g-man

behemyth said:


> Out of sheer curiosity how long do your irrigation systems run per zone? I know its going to depend on head coverage, I'm just curious the times people are running for. I run mine for about 30mins a zone and it keeps my lawn looking good all summer, I just feel like I'm wasting water sometimes.


Avoid guessing. Do an irrigation audit to figure out how much to run your system. For our area, target 0.5in each time you irrigate. Use straight wall containers in the zones to see how much water the zone gets after X amount of time running it. My system needs 1hr and 20min for the 0.5in (Hunter MP).


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## Green

behemyth said:


> Ok, I feel a little better now. I run my zones about 40 mins each, but when they installed my irrigation system most of my heads over lap their coverage, so I get more water down than if just a single head was covering a spot.
> 
> I also have heads that spin almost 360 degrees in the middle of some of the zones to help apply even coverage.
> 
> When do you guys start your systems running for an hour+ a zone? I've been starting mine around 3am.


Heads are supposed to overlap in most areas in order to get complete coverage. The only places where they can't overlap will be where some obstacle prevents it, such as a small area that can only fit one head, an area bounded by hardscape, a building, property boundary, or some other boundary.

Having a 360 degree head in the middle of an area is very helpful in many cases. Some installers consider it optional, but I have neighbors without them, and they battle localized dry spots. I'm not saying the dry spots are caused by the head coverage or lack of, but having less water on the dry spots than the surrounding areas makes no sense and you will be battling it forever, until you install another head most likely.


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## dleonard11122

My irrigation (6 zones) ran this morning for a total of 3 hours and 47 mins. Started at 1:45 to finish before sunrise at roughly 5:30.


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## Michael58

I'm having a hard time in the heat - On the ET map, my daily ET is .33. So if I multiply that by .8 (crop factor for TTTF), my watering losses are about .26" per day? With sandy loam, I aim to water .387", but this takes 2 hours per zone for 3 of my zones.

Since every other day doesn't seem to keep up with the losses, should I water every day? I'm contemplating using something like Hydretain also...


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## g-man

For your area and current weather, yes everyday. Of course, there might be some local restrictions with the current drought.


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## creediddy2021

Great job g-man! Great info!


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## robbybobby

Thank you for the write up @g-man !

My numbers below:

Peoria,Az. - .40 ET0
Crop Factor - .6
ET per Day - .24in

Sandy Loam - .13/in
Root Depth - 8in
Soil Capacity - 1.04in
50% Soil Capacity - .52

Result - Water .25in every 2 days?

What I can't wrap my head around is that I have flood irrigation that provides roughly 10-24 inches of water across my property. This comes every 14 days. Water is gone in <12-18 hours for the most part. From there I don't irrigate with sprinklers for at least the first 7 days as the grass looks lush and vibrant. 7 days of ET for me would be 1.68in vs my calculated capacity of 1.04 (100%) or .52 (50%).

I suppose I'm due for a soil test to confirm my type + confirm my depth. Something doesn't match.


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## g-man

Flood irrigation is not something I've researched in regards to ET. I would guess you should not start counting the ET0 until the water is gone (18hrs) or maybe even more since it could still be moving thru the soil (water table).

When do you see the wilt point in your grass? Day 7?


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## robbybobby

g-man said:


> Flood irrigation is not something I've researched in regards to ET. I would guess you should not start counting the ET0 until the water is gone (18hrs) or maybe even more since it could still be moving thru the soil (water table).
> 
> When do you see the wilt point in your grass? Day 7?


I flood irrigate again this Sunday and will track and report back. My guesstimate is day 7. You can see the saturation in the soil (different shade) for at least 3 days post irrigation. I nearly go all 14 days in the backyard which gets closer to the 24" of water. The front maxes out at 10".


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## alnp

How do you guys decide on the root depth value?
This is an important variable in the formula but I find it hard to calculate/decide on it.
When I remove a small patch of the grass and check the roots so not only in this patch they have different length but also can be different from one area of the lawn to another. Deciding on average from a few samples like that seems very inaccurate. Would it be more reasonable to use a constant value for [Type of grass, Type of soil, maybe age of the grass?] instead of trying to measure it myself?


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## Green

Quick question for the others here using ET and with cool-season grass...

Are you multiplying daily ET by the 0.8 crop factor, or not?


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## Wiley

I found a couple of interesting articles to add to the info @g-man presented. I thought the info regarding HOC and ET was insightful.

#1
#2


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## g-man

@Wiley great articles. I've read similar ones that point that higher hoc = more water need. There is a dogma of mow high during the summer being better for using less water.


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## dleonard11122

Very interesting point. I've been mowing my TTTF at 4" during the summers. Maybe next year I'll try to maintain it at 3" or 3.5" and see if there are any noticeable differences.


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## doverosx

g-man said:


> ET and Irrigation Guide​


The Canadian ET Calculators are now located here: https://farmwest.com/climate/calculators/evapotranspiration/


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## g-man

Fixed.


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## g-man

Bump for 2022 summer season. Start a new thread for specific questions around your lawn.


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## Dono1183

@PhxHeat told me to check this thread out when I brought up HOC and water consumption. I'll be honest and say I thought he was nuts, but he was right. Thanks for the info on this. Having read the whole thread, I decided to also see what the folks at A&M said as their formula uses a grass at 4" of height to give you your requirements. I have bermuda at 1" so that didn't make sense to me. But here's the response he gave me based on my question regarding their starting assumption in their formula and bermuda HOC. Again, thanks PhxHeat for pointing me in the right direction, this was very helpful and eye opening.


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## PhxHeat

@Dono1183 To be fair, I am a bit nuts :lol: But I am glad you sought out more information from more knowledgeable sources.


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## Dono1183

PhxHeat said:


> @Dono1183 To be fair, I am a bit nuts :lol: But I am glad you sought out more information from more knowledgeable sources.


Glad to have folks like you to point us to the right resources. :thumbup: Thanks again!


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## 440mag

Just wanna shout out another "Thank You!" g-man, for composition of this reference!!!!!


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## Marzbar

I just read this interesting article from MSU. https://www.canr.msu.edu/news/lawn-irrigation-tips. Does this have any correlation to the information in this thread?


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## STI_MECE

is step 1 of this process to just soak the lawn to 100%? if you dont, you are simply staying at 0% and just replacing the water you losing to ET?


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## g-man

Sure.


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## h22lude

Great write up G-man!

So when my lawn reaches the depletion point (50% of capacity), do I want to bring it back to full capacity in 1 day?


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## g-man

Yes


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## lbb091919

Marzbar said:


> I just read this interesting article from MSU. https://www.canr.msu.edu/news/lawn-irrigation-tips. Does this have any correlation to the information in this thread?


This is basically syringing without saying it. I get what they are saying, but it sort of goes against the principles of this thread.


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