[Green]

I've started manually tracking ET data, and want to see how it lines up with subjective indicators that the grass needs water. Here is my ET data from this week:

It last rained the night between June 3rd and 4th, somewhere between 0.6 to 0.8 inch of rain. Looking at the ET data, a half inch of water has been used up so far. How does this work...Do you wait until the entire 0.7 inch of water is used up before you irrigate? The grass looked fine as of yesterday.

 

[g-man]

This file from aty started me in the rabbit hole of ET. This just scratches the surface.

https://aroundtheyard.com/index.php?option=com_content&view=article&id=119:cactus-water-balance-irrigation&catid=9&Itemid=117

 

[Green]

Thanks; will read it when I get a chance.

 

[Green]

I read through it, as well some other similar articles. I was thinking of doing the math out. Right now, I'm having trouble finding numbers for available water capacity to use. This article has some numbers in it...

https://ag.umass.edu/turf/fact-sheets/turf-irrigation-water-conservation

I was thinking maybe I should use 1.3 because of sandy loam/almost loam soil type, but am not totally sure.

 

[g-man]

Water capacity starts with root depth. Those values are for 12in root zone. I use a 6in root zone (the default on the rachio system is also 6in). But the right thing to do is go take a root depth sample of you yard. If 80% of thr roots are at the 12in depth, then you use 12in. If 80% of the roots are at 6in then use 6in.

But there are exemptions to the guidance. If the roots are at 2in. I don't want to keep them at 2in, so I set to 4in in the growing time. To me this assures that there is water available for the roots to grow there. This is just my approach and not based on any formal research. This scenerio only happens after a reno.

For anyone reading this in the future, while all these are great calculations that guide to effective use of water, it will never replace walking the lawn and looking at the signs of stress with your own eyes. If you see the gray/green color, water then lawn and then adjust your calculations (ie. allowed depletion). There are assumptions made and it is not perfect. Soil probes are a better/$$ approach.

 

[ABC123]

Soil probes are a better/$$ approach.

Like a probe that replaces the rain sensor for a soil sensor?

Found this article, sounds a bit promising as the water savings is crazy. https://www.turfmagazine.com/maintenance/soil-moisture-sensors-vs-rain-sensors/

 

[g-man]

Yes soil moisture probes/sensor.

Like this one from dfw_pilot: https://thelawnforum.com/viewtopic.php?t=67

I think there was an updated list he made with links to his weather station. This takes the guesswork out of the equation. You place the probes at a depth and water when the moisture goes too low.

 

[Green]

Well, @g-man, I know from experience when digging that most of my roots reach down to 8-12 inches or so. I don't think many go beyond 12. Assuming 8" is the depth that I'm supposed to plug in. Now, how do I get a coefficient for this depth range in my soil type?

 

[g-man]

You get the 1.3in/ft, then divide by 12 to convert to inches (0.108in/in) and then multiple by 8in (your depth) = 0.86in of field capacity for the 8in of soil depth.

 

[Delmarva Keith]

. . . For anyone reading this in the future, while all these are great calculations that guide to effective use of water, it will never replace walking the lawn and looking at the signs of stress with your own eyes. If you see the gray/green color, water then lawn and then adjust your calculations (ie. allowed depletion). There are assumptions made and it is not perfect. . . .

Words of true wisdom g-man! :thumbup: :thumbup:

 

[g-man]

@ABC123 here is dfw_pilot website with his soil data. http://www.castlehillsnorth.com/index.html

 

[ABC123]

ABC123 here is dfw_pilot website with his soil data. http://www.castlehillsnorth.com/index.html

Wow that's impressive. Thanks g-man!

 

[Green]

So, my overseeded areas were clearly in need of water...even though I didn't see wilting, blue, or footprints, I noticed that it started to feel a little dry and the slightest bit crunchy yesterday, so I've started watering. According to the ET data (which is from an airport maybe 5 miles away...better than nothing), we've used up more than inch of water since the rain a week ago.

Is ET the only use of water for grass, or are there others that need to be considered as well?

 

[Delmarva Keith]

So, my overseeded areas were clearly in need of water...even though I didn't see wilting, blue, or footprints, I noticed that it started to feel a little dry and the slightest bit crunchy yesterday, so I've started watering. According to the ET data (which is from an airport maybe 5 miles away...better than nothing), we've used up more than inch of water since the rain a week ago.

Is ET the only use of water for grass, or are there others that need to be considered as well?

If memory serves, the reference ET is short cut cool season grass. You do have to make adjustments for shade or full sun, grass height, nutritional status, hydration status, local wind, etc., etc. On the inputs side of things, you have to adjust for soil type, rootzone depth, slope, etc., etc.

In other words, as g-man says, in practical terms the reported reference ET (and of necessity related determination of soil water content) is a guide for making a good guess at irrigation needs. Your observations at the site and judgement based on what you see are going to be way more accurate.

 

[Rain Bird Corp]

If memory serves, the reference ET is short cut cool season grass.

Reference ET is the amount of water necessary to keep a 6 inch stand of bluegrass well watered.

 

[Delmarva Keith]

If memory serves, the reference ET is short cut cool season grass.

Reference ET is the amount of water necessary to keep a 6 inch stand of bluegrass well watered.

That may be what RB uses, but respectfully, that's not a universal reference ET. E.g.:

http://edis.ifas.ufl.edu/ae256
https://www.cocorahs.org/Content.aspx?page=et
https://extension.arizona.edu/sites/extension.arizona.edu/files/pubs/az1195.pdf
Etc., etc.

(Usually for our purposes) cool season grass of unspecified (but common local) species cut short at 12cm (4.7" - short compared to just letting it grow). An attempt was made to completely standardize short crop height at 12cm but that does not seem to have been universally successful. There has been no successful short crop species standardization that I've ever seen.

There are about as many definitions as there are locations and uses. Doesn't matter; short crop reference ET (ETo) is just a reference against which more crop specific Kco and ETc are determined. Just sort of an intermediate value that encompasses a lot (most) of the variables to try to simplify ETc determination.

When ET is reported for landscaping purposes, it is short crop reference ET (ETo).

 

[Rain Bird Corp]

If memory serves, the reference ET is short cut cool season grass.

Reference ET is the amount of water necessary to keep a 6 inch stand of bluegrass well watered.

That may be what RB uses, but respectfully, that's not a universal reference ET. E.g.:

http://edis.ifas.ufl.edu/ae256
https://www.cocorahs.org/Content.aspx?page=et
https://extension.arizona.edu/sites/extension.arizona.edu/files/pubs/az1195.pdf
Etc., etc.

(Usually for our purposes) cool season grass of unspecified (but common local) species cut short at 12cm (4.7" - short compared to just letting it grow). An attempt was made to completely standardize short crop height at 12cm but that does not seem to have been universally successful. There has been no successful short crop species standardization that I've ever seen.

There are about as many definitions as there are locations and uses. Doesn't matter; short crop reference ET (ETo) is just a reference against which more crop specific Kco and ETc are determined. Just sort of an intermediate value that encompasses a lot (most) of the variables to try to simplify ETc determination.

When ET is reported for landscaping purposes, it is short crop reference ET (ETo).

This is the first paragraph from the arizona.edu doc you linked. (I added the underline.)
"Accurate estimates of turf water use are required to effectively manage a turf irrigation system. In Volume I of this series entitled "Basics of Evaporation and Evapotranspiration (ET)," we indicated that actual turf water use (ETt) is rarely measured in the real world. Instead, we use meteorological data and a mathematical model known as the Penman-Monteith Equation to estimate reference evapotranspiration (ETos) - the ET from a tall, cool-season grass that is supplied with adequate water."
So, I can see where there are a lot of opinions. I suppose I should have been more specific and said ETo for turfgrass. And I have seen the word 'tall' above replaced with '6 inches' and 'cool-season grass' replaced with 'bluegrass'. In any case, this is just another example of needing to know exactly what you are dealing with when trying to be as precise as this conversation seems to want to be. In reality, irrigation in the standard home is not nearly precise enough to warrant splitting these hairs. Thanks for the links!

 

[Delmarva Keith]

Reference ET is the amount of water necessary to keep a 6 inch stand of bluegrass well watered.

That may be what RB uses, but respectfully, that's not a universal reference ET. E.g.:

http://edis.ifas.ufl.edu/ae256
https://www.cocorahs.org/Content.aspx?page=et
https://extension.arizona.edu/sites/extension.arizona.edu/files/pubs/az1195.pdf
Etc., etc.

(Usually for our purposes) cool season grass of unspecified (but common local) species cut short at 12cm (4.7" - short compared to just letting it grow). An attempt was made to completely standardize short crop height at 12cm but that does not seem to have been universally successful. There has been no successful short crop species standardization that I've ever seen.

There are about as many definitions as there are locations and uses. Doesn't matter; short crop reference ET (ETo) is just a reference against which more crop specific Kco and ETc are determined. Just sort of an intermediate value that encompasses a lot (most) of the variables to try to simplify ETc determination.

When ET is reported for landscaping purposes, it is short crop reference ET (ETo).

This is the first paragraph from the arizona.edu doc you linked. (I added the underline.)
"Accurate estimates of turf water use are required to effectively manage a turf irrigation system. In Volume I of this series entitled "Basics of Evaporation and Evapotranspiration (ET)," we indicated that actual turf water use (ETt) is rarely measured in the real world. Instead, we use meteorological data and a mathematical model known as the Penman-Monteith Equation to estimate reference evapotranspiration (ETos) - the ET from a tall, cool-season grass that is supplied with adequate water."
So, I can see where there are a lot of opinions. I suppose I should have been more specific and said ETo for turfgrass. And I have seen the word 'tall' above replaced with '6 inches' and 'cool-season grass' replaced with 'bluegrass'. In any case, this is just another example of needing to know exactly what you are dealing with when trying to be as precise as this conversation seems to want to be. In reality, irrigation in the standard home is not nearly precise enough to warrant splitting these hairs. Thanks for the links!

No problemo my friend. For the type of stuff you do (the advocacy side) if you ever need a sleep aid, the American Society of Civil Engineers (ASCE) worked for a while on standardizing ET reference definitions and has published several papers. Exciting reading (not!) but good for argument's sake if somebody on the other side of the advocacy table really wants to get into any nitty gritty. ASCE citations let you call, er, "bullstuff" if needed.

Does that earn me a tee-shirt? :mrgreen:

I didn't post links because I haven't kept up with it.

 

[Rain Bird Corp]

No problemo my friend. For the type of stuff you do (the advocacy side) if you ever need a sleep aid, the American Society of Civil Engineers (ASCE) worked for a while on standardizing ET reference definitions and has published several papers. Exciting reading (not!) but good for argument's sake if somebody on the other side of the advocacy table really wants to get into any nitty gritty. ASCE citations let you call, er, "bullstuff" if needed.

Does that earn me a tee-shirt? :mrgreen:

I didn't post links because I haven't kept up with it.

:thumbup: (No shirt for you!) All you get is the thumb! :lol:
Curious. Why would Civil Engineers care about ET? Because they can???
Thanks for the lead. Although, I really wonder if horticulture folks would put a lot of weight in ET info from engineers. But to your point, regulators (and other NGOs like environmentalists) might be impressed.

 

[Delmarva Keith]

. . .Curious. Why would Civil Engineers care about ET? Because they can??? . . .

It grew out of something similar to what you do with advocacy which is what made me think of it. The Irrigation Association and the ASCE Environmental & Water Resources Institute asked ASCE as a whole to put its formidible standards machinery into action to determine and publish a fully engineered standard that could be used when discussing ag and home use irrigation from the "water is a civil resource that can't be needlessly wasted, but hey, plants need water" point of view. It is, of course, one piece of a large engineering standards puzzle (that ASCE was, at least at one point happy to wade into).

Engineers are good at this stuff. If something can't be measured and proven to the nth degree, based on fully supported and agreed upon standards within the relevant scientific community, it is nothing but an old wives tale. The mind-blowing complexity of it all is trivial familiar territory for them.