# Is the everywhere advice of 1" water per week right?



## Delmarva Keith (May 12, 2018)

I keep seeing everywhere when folks are talking about irrigation, the 1" per week advice. Also admonitions to water as infrequently as possible, maybe twice a week. Always has me scratching my head - where the heck does that actually work?

Where I am, soil water capacity runs around 0.9" in our sandy clay using (perhaps optimistic) 10" root zone depth. ETo averages around 0.23" per sunny day. Rain is pretty much negligible mid-June through right around the end of August. On a predicted basis, starting at 100% , to maintain minimum 40% soil water content, allowable depletion would be 0.54" which would be reached basically every other day. Actual depletion would run about 0.46" to be applied every other day to keep things healthy. That's 1.61" total per week watering every other day as a minimum. Actual field experience bears out the predictions.

Other than the Northeast maybe North of New Jersey, are there any places where watering the "standard" 1" per week split into two waterings per week is right and actually works? Just looking at a map, a wild guess tells me no. Should we think about changing the "standard" advice?


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## Kmartel (Feb 12, 2019)

1" is a baseline. Adjust as needed based on soil and weather conditions of the site. As with everything there is no one rule for all situations.


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## Delmarva Keith (May 12, 2018)

Kmartel said:


> 1" is a baseline. Adjust as needed based on soil and weather conditions of the site. As with everything there is no one rule for all situations.


Ok, but if it's a baseline, where is the baseline valid? Seems (just thinking out loud) the baseline is arbitrary and mostly wrong.


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## MMoore (Aug 8, 2018)

the baseline would be so your grass does not get burnt off in dormancy... not to keep it thriving all summer long.


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## Delmarva Keith (May 12, 2018)

MMoore said:


> the baseline would be so your grass does not get burnt off in dormancy... not to keep it thriving all summer long.


I've never seen that attached to the advice when given. Should it be?


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## MMoore (Aug 8, 2018)

the advice would be given to basic questions.

"how can I keep my grass from dying?"
"give it an inch of water a week"

the level of detail you went into within the OP would confuse most people outside of this forum.


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## NewLawnJon (Aug 3, 2018)

The 1" guideline is just that, a guide. For the average cool season lawn, in the average cool season climate you need 1" per week based on the average eto. If you track your weather in an eto modeler there you might use more or less depending on the weather that week. An example is during the 90+ weather my models showed the grass using 1.7" of water, and after the heat when we cooled off back to 1".

Short of each person running their own eto based on a localized weather station I would say 1" of water per week on cool season grass when the average temp is below the mid 80s, and 1.5" per week when the average temp is above (i.e. add another day to your watering program is rain doesn't supplement).

If you really want to geek out you can use the spreadsheet from gman and pull in your local weather to model the eto. I do that, and only water when I have a rolling 7 or 4 day deficiency of 80% of eto of 1/2". By doing the 7/4 day (whichever gives me the deficiency first) it takes out the large rainfalls quicker, but also keeps the grass under a slight water deficiency as much as possible. By doing this I rode out the 90+ heat without watering, and didn't see much, if any heat stress.


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## NewLawnJon (Aug 3, 2018)

MMoore said:


> the advice would be given to basic questions.
> 
> "how can I keep my grass from dying?"
> "give it an inch of water a week"
> ...


I completely agree. On the forum we dig a bit too deep into the science (which I enjoy), but for the average person the 1" per week over 2 waterings, if it is really hot add 1 more day is a good basic rule.


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## Delmarva Keith (May 12, 2018)

MMoore said:


> the advice would be given to basic questions.
> 
> "how can I keep my grass from dying?"
> "give it an inch of water a week"
> ...


I guess I'm not making my point. If someone did that where I am (and I suspect many places) the grass would actually be completely brown, dormant with much of it dying. It is really bad advice for here.

Where does this magic 1" come from? Does this emperor have any clothes? Where are these places where 1" is enough because it seems like it wouldn't be many places.


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## Delmarva Keith (May 12, 2018)

NewLawnJon said:


> The 1" guideline is just that, a guide. For the average cool season lawn, in the average cool season climate you need 1" per week based on the average eto. If you track your weather in an eto modeler there you might use more or less depending on the weather that week. An example is during the 90+ weather my models showed the grass using 1.7" of water, and after the heat when we cooled off back to 1".
> 
> Short of each person running their own eto based on a localized weather station I would say 1" of water per week on cool season grass when the average temp is below the mid 80s, and 1.5" per week when the average temp is above (i.e. add another day to your watering program is rain doesn't supplement).
> 
> If you really want to geek out you can use the spreadsheet from gman and pull in your local weather to model the eto. I do that, and only water when I have a rolling 7 or 4 day deficiency of 80% of eto of 1/2". By doing the 7/4 day (whichever gives me the deficiency first) it takes out the large rainfalls quicker, but also keeps the grass under a slight water deficiency as much as possible. By doing this I rode out the 90+ heat without watering, and didn't see much, if any heat stress.


I'm not looking to geek out on anyone who doesn't want to know how to build a watch when they ask the time. Just something that's been bothering me because I see it everywhere - 1" a week. Maybe it's right but doesn't seem right to me for most places. My question is, does it work in most places where constantly repeated and if not, should it be changed?


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## g-man (Jun 15, 2017)

I think it works in spring but not in the summer. I think it is bad advise. 0.5in every 3 days might be a more realistic advise or just let it go dormant.


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## KoopHawk (May 28, 2019)

Delmarva Keith said:


> NewLawnJon said:
> 
> 
> > The 1" guideline is just that, a guide. For the average cool season lawn, in the average cool season climate you need 1" per week based on the average eto. If you track your weather in an eto modeler there you might use more or less depending on the weather that week. An example is during the 90+ weather my models showed the grass using 1.7" of water, and after the heat when we cooled off back to 1".
> ...


Many folks probably have their systems set to run twice a week and they set it and forget it until its time to winterize their system. Those are the people that have their irrigation system running during or shortly after a heavy soaking rain. It is not the best way to keep your lawn healthy but it is a good way to rack up an unnecessarily high water bill each month. Each lawn is unique and there is no miracle solution. If you were to drill down on weather, ETs, etc. you would probably find the average water needed to be right around an inch per week. The best advice if you're not looking to 'geek out' is to water deeply when your lawn looks like it needs it. When your lawn is starting to look stressed, water it. Pretty simple and effective.


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## Delmarva Keith (May 12, 2018)

KoopHawk said:


> Delmarva Keith said:
> 
> 
> > NewLawnJon said:
> ...


Ok, I think I surrender. I have drilled down and have found this standard advice to water total 1" split for twice a week I see repeated often and everywhere like sacred scripture makes absolutely no sense to me. If it makes sense to you, carry on.


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## Delmarva Keith (May 12, 2018)

g-man said:


> I think it works in spring but not in the summer. I think it is bad advise. 0.5in every 3 days might be a more realistic advise or just let it go dormant.


That makes a lot more sense! The debate here was for a long time 2x or 3x per week. Then after actually calculating things, basically every other day is actually needed (for here). I suspect similar in many places.


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## Harts (May 14, 2018)

Not to beat a head horse....I get where you're coming from @Delmarva Keith. I have followed 1" per week - usually all at once. However, I think I need to experiment more with what works here, taking all factors into account. Whether that's 0.5" every 3 days or 0.75" twice a week or 1" once per week etc.

You're OP basically asks how did someone come up with 1" per week. What tests were done to determine this is the baseline. I'll give you the worst answer to that question: That's just the way it's always been done! :lol: :lol: :lol:

You make a valid point. Who decided 1" was best and how did they determine that?

I think if you were to do a field study across many regions, you would find some find 1.5" works spread across 3 days; some would find 1" all at once work and other might find 0.75" twice a week works.

Point being, we all need to find what works based on the needs of our grass, which is dependent on a number of variables.

It's like my 4 year old asked me the other night, "Daddy, who made the sky?"


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## Ylli (Sep 24, 2018)

I don't have irrigation, so have to drag a hose around. The house is fed with a 1.25" plastic pipe from the mains. Best I can tell, maximum flow rate will be around 24 GPM.

I have about 15,000 ft² of lawn. 1" of water over 15,000 ft² equates to 9,375 gallons. At a flow rate of 24 GPM, I need to water for 6.5 hours.

If I take the hose and sprinkler into account, the flow rate is probably closer to 6-8 GPM. At that rate to get 1" of water over the 15,000 ft², I would need to water for around 22 hours. With 2 hoses, I might get that down to 15 hours.

I think I'll wait 'till it rains......


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## NewLawnJon (Aug 3, 2018)

Ylli said:


> I don't have irrigation, so have to drag a hose around. The house is fed with a 1.25" plastic pipe from the mains. Best I can tell, maximum flow rate will be around 24 GPM.
> 
> I have about 15,000 ft² of lawn. 1" of water over 15,000 ft² equates to 9,375 gallons. At a flow rate of 24 GPM, I need to water for 6.5 hours.
> 
> ...


That was my life last year with new sod. I had 8 hoses and sprinklers on timers on my 2 spigots, and could only put down 5gpm on each spigot. I ran the sprinklers every other day for 10 hours a time, running 2 at a time, and even then it wasn't the best coverage.

After doing that for a year I decided to go full irrigation, and if I built again I wouldn't hesitate to do full irrigation again.


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## Delmarva Keith (May 12, 2018)

Ylli said:


> I don't have irrigation, so have to drag a hose around. The house is fed with a 1.25" plastic pipe from the mains. Best I can tell, maximum flow rate will be around 24 GPM.
> 
> I have about 15,000 ft² of lawn. 1" of water over 15,000 ft² equates to 9,375 gallons. At a flow rate of 24 GPM, I need to water for 6.5 hours.
> 
> ...


A lot of work. My hat off to you. Rain is definitely the best when it comes. We just can't count on it though  . All Summer the rain blows inland and we get basically none. The rest of the year we mostly get a little too much. Without irrigation, cool season is absolute toast by mid-July. The choices are irrigate or go for a warm season/weed salad mix. Unirrigated, everything, even the weeds, decline all Summer and then when rain returns a cornucopia of random crap erupts like a second Springtime. The smart thing would be let nature take its course and just maintain whatever grows. Nobody does the smart thing though. :mrgreen:


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## Delmarva Keith (May 12, 2018)

Harts said:


> Not to beat a head horse....I get where you're coming from @Delmarva Keith. I have followed 1" per week - usually all at once. However, I think I need to experiment more with what works here, taking all factors into account. Whether that's 0.5" every 3 days or 0.75" twice a week or 1" once per week etc.
> 
> You're OP basically asks how did someone come up with 1" per week. What tests were done to determine this is the baseline. I'll give you the worst answer to that question: That's just the way it's always been done! :lol: :lol: :lol:
> 
> ...


Who made the sky indeed.  I have to just be conscious that from now on whenever I see the 1" and once or twice a week, to just ignore it. It had become like nails on a chalkboard, hence my starting this thread.

Four year olds know best - like Frozen, I have to "let it go, let it go . . . " :lol:


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## SpiveyJr (Jun 7, 2018)

I'm sure the origin of the 1" of water guideline came from the result of a controlled environment indoors under perfect conditions with perfect soil.

At the end of the day as long as the grass is getting enough water without over watering then everything else is moot.


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## Harts (May 14, 2018)

@Delmarva Keith my daughter sings that line over and over. ALL DAY LONG.

And now that is stuck in my head. So, thanks for that.


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## Green (Dec 24, 2017)

@Delmarva Keith, simply, no.

If you track ET, you see (Northeast or mid Atlantic) that in much of July when temps are high 80s, 90s, and full sun, ET can be about 0.25 inch per day. 7÷4=1.75 inch of use per week. As such, I often prefer to water as close to 1.5 inch per week as possible during these times if the rain leaves a deficit (which it usually does). I try to schedule the watering events around mowing, and space them to mitigate heat stress as much as possible. Some of it also depends on grass type, and visual stress, stand age, amount of abiotic and biotic stress, etc.

I'm usually watering 2.5 times per week in July on average.

I try to keep the growth rate low, but not so low that rust or something will damage the heck out of the grass. Unfortunately, most KBG cultivars tend not to modulate water use well...they're either green and growing vigorously, or wilting and on the verge of dormancy. I try to avoid the latter. Tall and fine fescue and even ryegrass are better in this regard, as are certain KBG cultivars...they slow down without wilting immediately. Of course too much direct heat causes fine fescue to go dormant readily. I'm always on the lookout for cultivars of any species that handle water use well.

That and hand watering dry spots as needed.


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## ken-n-nancy (Jul 25, 2017)

Delmarva Keith said:


> I keep seeing everywhere when folks are talking about irrigation, the 1" per week advice.


No, I don't think it's right, either.

Plus, as you mention, it is everywhere (not just on this forum) -- the "1 inch per week" advice is basically ubiquitous. A google search for "lawn 'water per week'" will turn up hundreds of thousands of hits, with what appears to be 1-inch per week by far the most common, but with other recommendations going as high as 1.5 inches per week, or down to 0.5 inches per week, and everything in between.

However, for at least a couple weeks each year, in the same way that a broken clock tells the correct time twice a day, 1" per week is likely to be good advice for almost every cool season lawn location for at least a couple weeks each year.


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## tgreen (Oct 20, 2018)

Delmarva Keith said:


> I keep seeing everywhere when folks are talking about irrigation, the 1" per week advice. Also admonitions to water as infrequently as possible, maybe twice a week. Always has me scratching my head - where the heck does that actually work?
> 
> Where I am, soil water capacity runs around 0.9" in our sandy clay using (perhaps optimistic) 10" root zone depth. ETo averages around 0.23" per sunny day. Rain is pretty much negligible mid-June through right around the end of August. On a predicted basis, starting at 100% , to maintain minimum 40% soil water content, allowable depletion would be 0.54" which would be reached basically every other day. Actual depletion would run about 0.46" to be applied every other day to keep things healthy. That's 1.61" total per week watering every other day as a minimum. Actual field experience bears out the predictions.
> 
> Other than the Northeast maybe North of New Jersey, are there any places where watering the "standard" 1" per week split into two waterings per week is right and actually works? Just looking at a map, a wild guess tells me no. Should we think about changing the "standard" advice?


It's a great question and I don't know where it comes from but what would you tell the average homeowner if they asked the question 'how much should I water the lawn'?

Also, curious about your calculations and measurements. Do you have a way to measure the current soil moisture? Also, how do you get the evapotranspiration rate? Presumably the rate would vary not just on temp but also on whether there are trees, for example? Are you basically starting with an assumption of actual soil moisture and then assuming potential evapotranspiration is the same as actual and where the PE is a function of temperature?

Not being a wise guy. I'm genuinely interested in these calcs and whether I could get more precise on watering. Not sure where to even start or if there are way too many variables to make this anything other than theoretical. Thanks


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## M32075 (May 9, 2019)

I put my two impact sprinklers that get head to head coverage on every morning for 30 mins every day from end of June to September. I have a all PRG it fries if I don't. One inch a week definitely doesn't work for PRG short roots.


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## g-man (Jun 15, 2017)

PRG roots can go deep enough for every other day. No issues with fungus with this approach?


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## Green (Dec 24, 2017)

tgreen said:


> Delmarva Keith said:
> 
> 
> > I keep seeing everywhere when folks are talking about irrigation, the 1" per week advice. Also admonitions to water as infrequently as possible, maybe twice a week. Always has me scratching my head - where the heck does that actually work?
> ...


I'm using local weather station ET as a proxy for exact measurements. It's close enough, and then I adjust based on the results. Too dry? Water longer. Not working over a longer period? Add an extra watering session instead. That's the art aspect. But local weather station ET data gets me very close.

My answer for the average person in CT or RI: Water as deeply and infrequently as practical. And measure rain so you know how much to add. If no rain, and assuming near full sun, that usually means 3x per week at a half inch per session in July, a bit less frequently and maybe deeper in August, in September no more than twice a week (but a bit deeper, such as 0.75 inch per session), and in October an inch a week all at once. June is similar to September. May is as needed, the full inch all at once if possible (pre stress conditioning). I would also suggest they water shaded areas less to prevent water from building up, and/or adjust their in-ground heads if applicable to reduce out put accordingly in those areas. And hand water the driest spots as needed to supplement (this may be due to amount of sun, soil type, etc.). And these are specifically for CT/RI and a small part of New York near NYC.

Of course sun angles, tree cover, etc. varies by season and even per year (e.g. insect defoliation...been there, dealt with that), so you adjust accordingly and try to time your watering to when it wilts at the latest, but not more than a day later.

My back lawn areas get somewhat more shade and/or less direct sun, and therefore are on a different schedule. I've nailed it down pretty well the last few years.

You also don't try to induce dormancy and then water, but that could be a misconception that some people think is the same as infrequent. My first couple of years, I waited for induction of dormancy before watering because I misunderstood "infrequent".

Not too complex, right?

Disclaimer: Great rough guide for CT/RI. Overkill in, say Maine, and likely not enough depth and/or watering frequency in, say Virginia.


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## jrubb42 (Jun 20, 2019)

This is how I felt reading this thread. Lol.


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## LIgrass (May 26, 2017)

The 1" per week, deep and infrequent mantra is a YMMV thing in my opinion. In sandy soils like we have on Long Island you have no chance getting away with that if you want to keep your lawn green in the summer. Grass type, shade, humidity levels...I take it week by week on my watering. IMO, WHEN you water is more important than how frequent.


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## Bigfeather1 (Jun 11, 2019)

Thanks for the thread Kieth, it gave me something else to think about today, beings I spent last weekend laying on the ground checking the nozzle size in seven zones. Now..... perhaps.... I'll program in a couple more days , maybe just the zones that turned brown. Living the good life in the LAND OF PLEASANT LIVING, I remember Bailey Goss and Diamond Jim. Joe


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## Delmarva Keith (May 12, 2018)

tgreen said:


> Delmarva Keith said:
> 
> 
> > I keep seeing everywhere when folks are talking about irrigation, the 1" per week advice. Also admonitions to water as infrequently as possible, maybe twice a week. Always has me scratching my head - where the heck does that actually work?
> ...


For what to tell a homeowner, in Summer heat in places that are hot and humid and generally sunny in the Summer (majority of places where cool season grows, far as I know) would be a minimum of 1.5" per week, less actual rainfall. In hot and low humidity, more. 1" per week corresponds to average daily ETo of 0.14". Not realistic. 1.5" per week would correspond to 0.21" per day average which seems to me to be closer to reality (assuming irrigation DU of 1.0, which is also not realistic - would need to add a roughly 10 or 20% bump for that on the assumption that the average homeowner gets optimistic about coverage uniformity when checking the catch cups if a water bill is involved). That would apply to full sun. A safe assumption is most properties include full sun areas. If the added complexity of explaining about less for shade is wanted, it should be explicit when blanket advice is given.

For the data source, good question - I was hoping someone would ask.  The University of Delaware oversees the DEOS environmental observing system which is among the most sophisticated in the Country. One outgrowth of that is their DIMS irrigation management system which automatically integrates hyperlocal weather, ET, soil type, root zone depth, plant hydration status (plants use more when there is more), etc., and also manually input irrigation amounts. The system updates calculated soil moisture content at midnight every night, and allows real time "what if" forecasting based on forecast weather and planned irrigation for the next several days. Wasted water and runoff are both undesired and the system is intended (among other things) to finely tune irrigation to actual water need.

While University of Delaware might be ahead of the curve on the level of sophistication available, I think just about every extension office can provide daily ETo and help with soil water capacity calcs. You can pretty much dive as deep as you want. :thumbup:


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## pennstater2005 (Jul 17, 2017)

I feel that I see deep watering more than any actual amount listed. Although 1" does seem to be the baseline then +/- from there. I don't irrigate so it's pretty easy actually :lol:


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## Pete1313 (May 3, 2017)

To @Delmarva Keith or everyone else, do you calculate watering needs based on 100% of ETo or 80% of ETo which is the general recommendation for cool season grasses. For that if ETo is .20" each day for a week(1.40" ETo for 7 days), actual watering needs would be .16" daily or 1.12" weekly. Closer to the 1" per week when using the 80% Kc. I've been keeping to the 80% figure for most of this year with good results, but agree and dont water 1" at a time due to my soil. Instead I try to water .60"-.75" at a time.


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## Bigfeather1 (Jun 11, 2019)

Thanks again Keith, more to study.

http://dims.deos.udel.edu/


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## Delmarva Keith (May 12, 2018)

Pete1313 said:


> To @Delmarva Keith or everyone else, do you calculate watering needs based on 100% of ETo or 80% of ETo which is the general recommendation for cool season grasses. For that if ETo is .20" each day for a week(1.40" ETo for 7 days), actual watering needs would be .16" daily or 1.12" weekly. Closer to the 1" per week when using the 80%. I've been keeping to the 80% figure for most of this year with good results, but agree and dont water 1" at a time due to my soil. Instead I try to water .60"-.75" at a time.


I use 100% ETo and a crop type that models that usage. My understanding is ETo is a grass reference based on well watered, full cover relatively tall cut cool season grass (isn't that the objective?). From what I've observed doing it at 100% for the past two years, it is spot on in late Spring through Summer. A bit excessive in early Spring and in Fall.

Poking around, a couple things to consider, ETc of 0.8 of ETo is an average and won't hold true in Spring and Summer; a 20% bump in ETc to match up with ETo would likely offset pretty close to (maybe a little high but close) typical irrigation uniformity issues of a typical system.


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## Pete1313 (May 3, 2017)

@Delmarva Keith I hear ya, except on relatively tall cut cool season grass being the objective.  80% seems to be working well for me thru this year, but obviously location, soil, root depth, how much N used will dictate watering amount and frequency. It's a bit of an art and agree whether 80% or 100%, at times that number can fluctuate and either be too much or not enough.


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## Green (Dec 24, 2017)

@Delmarva Keith and others, in reference to a post you wrote above: just want to emphasize that here in Southern New England, 1.5 inches per week split into 3 or so waterings per week is about right in the heat of the Summer for full sun areas. I'd assume that traveling significantly South (maybe even once you hit your area) a slight bump would be needed. Opposite for further North, on average. I read about some of the Western MA guys only watering once per week most of the time at a full inch in Summer, and that's not even that much further North...I'm always shocked that works for some of them (or that their lawns are ready for urea in early September...ours are usually still struggling).


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## Delmarva Keith (May 12, 2018)

Green said:


> @Delmarva Keith and others, in reference to a post you wrote above: just want to emphasize that here in Southern New England, 1.5 inches per week split into 3 or so waterings per week is about right in the heat of the Summer for full sun areas. I'd assume that traveling significantly South (maybe even once you hit your area) a slight bump would be needed. Opposite for further North, on average. I read about some of the Western MA guys only watering once per week most of the time at a full inch in Summer, and that's not even that much further North...I'm always shocked that works for some of them (or that their lawns are ready for urea in early September...ours are usually still struggling).


That's really what I'm thinking. Like when I read everywhere "apply 1" total and water no more than twice a week" what I actually see is "go brown out all your full sun areas by the end of the first hot, sunny week" 🤪. Might depend on regularity of rain too - we get basically none all Summer and that's my frame of reference. Without plenty of frequent irrigation, it's all toast. By the start of July there's zero "reserve" at all in the soil so unless full ET is regularly replaced, a few hot days and bye-bye.


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## Green (Dec 24, 2017)

Delmarva Keith said:


> That's really what I'm thinking. Like when I read everywhere "apply 1" total and water no more than twice a week" what I actually see is "go brown out all your full sun areas by the end of the first hot, sunny week" 🤪.


Exactly. I think that's part of the reason I used to think that the idea was to let the lawn brown slightly before watering! And then I wondered why my results weren't too good.



Delmarva Keith said:


> Might depend on regularity of rain too - we get basically none all Summer and that's my frame of reference. Without plenty of frequent irrigation, it's all toast.


Of course. And I hear you. Here in my part of the state, our average rain for July is *supposed to be* 4.6-4.7 inches. That should be enough to grow halfway decent grass if it comes split up properly. However, in reality, when you look at data from the past 10 years only, our average has consistently been 2.7 inches in July. That's far under what's needed. It WILL brown out with that amount of rain. Zoysia might hang on, crabgrass too, but not cool season stuff. I've been tracking ET to calibrate my smart irrigation system, and noticed they're using the data from the last 10 years rather than beyond. That makes sense. I had a discussion with one of the programmers about it.



Delmarva Keith said:


> By the start of July there's zero "reserve" at all in the soil so unless full ET is regularly replaced, a few hot days and bye-bye.


Tell me...do you think the deep and infrequent/1-inch per week people recommend depleting all of the reserve before each watering? And at what depletion threshold does curling up/color change/sustained footprinting usually appear? (To me, that doesn't sound like a good idea...I would think you'd want at least 15% left in the root zone, or something around that.)


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## mooch91 (May 5, 2019)

Ylli said:


> I don't have irrigation, so have to drag a hose around. The house is fed with a 1.25" plastic pipe from the mains. Best I can tell, maximum flow rate will be around 24 GPM.
> 
> I have about 15,000 ft² of lawn. 1" of water over 15,000 ft² equates to 9,375 gallons. At a flow rate of 24 GPM, I need to water for 6.5 hours.
> 
> ...


Even with full irrigation, it's tough to do. I believe my household well pump supplies around 10-12 gpm. I'm watering about 35K sq ft broken in to 9 zones. In order to get down about 1" per week, I need to run each zone about an hour per week, twice per week. So my sprinklers run 3 hours per night, six nights per week, to achieve this. Thankfully it's all happening while I'm sleeping, but that's a fair amount of wear and tear on a well pump.

My whole property is about an acre and a half. The reason I never expanded the irrigation is because I never thought I'd be able to run it enough to make a difference. I've focused on planting more drought tolerant grasses in the areas that are not irrigated.


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## Green (Dec 24, 2017)

g-man said:


> I think it works in spring but not in the summer. I think it is bad advise. 0.5in every 3 days might be a more realistic advise or just let it go dormant.


Your climate is very similar to mine. That's right on the money with the 0.5 inch every 2.5 days that seems optimal for full sun areas in July here. But I prefer to round down to every 2 days (for insurance) rather than up to every 3. I feel like the higher frequency helps push the water from the previous watering deeper faster before the sun can get to it. I get more browning doing 0.5 inch every 3 days in July (which is closer to an inch a week) than every 2.5 days (e.g. Monday, Wed., Fri. - 1.5 inch per week).

When I used to do about an inch (sometimes 2 inches due to overlaps when moving a small impact head around) every 10 to 14 days, the lawn was in a perpetual state of drought stress even more than it is now...getting in-ground irrigation on a large portion helped a huge amount, but it had its own kinks that needed to be sorted out (still in progress, 4 years later).


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## Green (Dec 24, 2017)

Also, we've gotten over 5.5"+ of rain this month (raining now)...but I've still had to water, because it wasn't distributed right. There were gaps of more than a week without any rain. Still had to water the front like 7 times this past month.


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## tgreen (Oct 20, 2018)

Delmarva Keith said:


> For what to tell a homeowner, in Summer heat in places that are hot and humid and generally sunny in the Summer (majority of places where cool season grows, far as I know) would be a minimum of 1.5" per week, less actual rainfall. In hot and low humidity, more. 1" per week corresponds to average daily ETo of 0.14". Not realistic. 1.5" per week would correspond to 0.21" per day average which seems to me to be closer to reality (assuming irrigation DU of 1.0, which is also not realistic - would need to add a roughly 10 or 20% bump for that on the assumption that the average homeowner gets optimistic about coverage uniformity when checking the catch cups if a water bill is involved). That would apply to full sun. A safe assumption is most properties include full sun areas. If the added complexity of explaining about less for shade is wanted, it should be explicit when blanket advice is given.
> 
> For the data source, good question - I was hoping someone would ask.  The University of Delaware oversees the DEOS environmental observing system which is among the most sophisticated in the Country. One outgrowth of that is their DIMS irrigation management system which automatically integrates hyperlocal weather, ET, soil type, root zone depth, plant hydration status (plants use more when there is more), etc., and also manually input irrigation amounts. The system updates calculated soil moisture content at midnight every night, and allows real time "what if" forecasting based on forecast weather and planned irrigation for the next several days. Wasted water and runoff are both undesired and the system is intended (among other things) to finely tune irrigation to actual water need.
> 
> While University of Delaware might be ahead of the curve on the level of sophistication available, I think just about every extension office can provide daily ETo and help with soil water capacity calcs. You can pretty much dive as deep as you want. :thumbup:


Great info, thanks! Just so I'm clear, I basically need to find a local weather station that reports evapotranspiration. Let's say it's 0.2 inches every day constantly. Let's say I got an inch of rain 5 days ago. Is the implication that I need to water on day 6 by 0.2 inch and so on until we get rain again? If on day 6 I instead water 1 inch then that buys me 5 days? I've been researching this just now and am not clear. Thanks


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## tgreen (Oct 20, 2018)

Green said:


> My back lawn areas get somewhat more shade and/or less direct sun, and therefore are on a different schedule. I've nailed it down pretty well the last few years.


Yes, this helps. I've been trying to fine tune my watering the last couple years. I have about 2k sqf that retains a lot of water, probably subsurface water moving through it. I don't really need any water ever in that section but the sprinkler system isn't set up that way and an adjacent area needs normal water. Problem is that same zone runs through both sections. My next step is to try to find a head that will shut off. I'm using hunter pgp's now but am told that rainbirds have the ability to shut off individual heads.


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## g-man (Jun 15, 2017)

reference Evapotranspiration (ET0)* crop factor = your ET.

For cool season lawns the crop factor is 80% (under typical conditions).

You can calculate the ET0 using temp or get it from your state climate office. It is used by farmers to track for their crops (corn, soy, etc).

Your ET By day means how much water your lawn "used" per day. Today I have a 0.15in ET.

Let say we got rain for 3 day straight and the soil was fully saturated. Every day I will loose 0.15in of the total water capacity for my soil At day 3, I'm at 0.45in, day 4 0.60in, day 6 it is 0.9.

But I used a key word, water capacity for my soil. Each soil will have a different capability to hold on to water. Also the water needs to be available to the roots, so root depth matters. For my soil (loam clay), it holds around 0.16in of water per in of root depth. If I have 6in roots, then that's 0.96in of water capacity. So at ~6days I would empty out the soil.

Emptying out the soil is not good. The roots will be at stress and likely compressed by the dry soil. The typical is to maintain at 50% depletion. That means that I should refill the tank (water capacity) when I am at 0.48in. Therefore at day 3, I should water to get to 100%, by adding 0.48in of irrigation. That means 0.5in every 3 days if the ET is at 0.16. When the ET goes to 0.20 (heat wave), I would need to refill every 2 days with 0.4in or every 3 with 0.6in.

If you soil is sandy, then your capacity is way less, so you will need to water more often.


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## ken-n-nancy (Jul 25, 2017)

tgreen said:


> My next step is to try to find a head that will shut off. I'm using hunter pgp's now but am told that rainbirds have the ability to shut off individual heads.


I have been upgrading my Hunter PGP heads to Hunter I-20SS heads whenever I need to dig up a PGP head for some reason. The I-20 has an individual rotor shut-off feature that I like to be able to use when doing a partial renovation of our lawn (seems that the boundaries of areas to be renovated never match the sprinkler zones).

https://www.hunterindustries.com/irrigation-product/rotors/i-20

I upgrade the regular I-20 to the I-20SS, because the price difference is small compared to my time to dig up and replace the head.

https://www.sprinklerwarehouse.com/hunter-irrigation-lawn-rotor-i20-04-ss

I would note that when the I-20 (or I-20SS) head is "shut off" it will still pop up, but just won't deliver any water.

By the way, if you are doing a renovation or some other project for which you want to deactivate a few PGP heads for a while, you can change the nozzle to a "blank" nozzle (no hole) so that it doesn't water. I have done that, too, but changing the nozzle is a fair bit more hassle then just turning the on/off dial on the top of the I-20.


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## Delmarva Keith (May 12, 2018)

Green said:


> Tell me...do you think the deep and infrequent/1-inch per week people recommend depleting all of the reserve before each watering? And at what depletion threshold does curling up/color change/sustained footprinting usually appear? (To me, that doesn't sound like a good idea...I would think you'd want at least 15% left in the root zone, or something around that.)


At about 60% depletion, I can definitely see it. Permanent wilting point (plant death or dormancy for plants that go dormant) can start to occur at around 70%+ depletion so running the tank down 60% on a regular basis is not for the inattentive.

As a practical matter I try to keep things between 50% and 100% soil water capacity. For here with my soil (typical sandy clay and not at all particularly bad for holding water) the difference between 100% soil water capacity and right around between 40% to 50% is generally two Summer days. A third day would drop it below the "well watered" threshold into the blue/gray stage (I call it the burnt stage because it looks burnt to me at some sun angles) and a fourth day and it starts entering dormancy.

Once or even twice a week would be wasting water on ultimately a brown landscape. Waiting until it shows signs of stress tends to thin the stand and also requires a lot of water all at once to recover from. Honestly whoever came up with some of the crap that seems to get mindlessly repeated over and over as gospel to be preached for all climates and locations was playing a nasty joke on a lot of gullible people. What struck me to start this whole ball of wax was where does the always repeated gospel actually work? A few states in the far Northeast? A small area of Pacific Northwest? Maybe a rainy zone in North Central US? How about all the rest of us. Seems crazy.


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## tgreen (Oct 20, 2018)

This has been a very informative thread! Thanks keith, green, gman and kennancy! Taking in all this info, as a practical matter, I will add the evapotransporation rate to the data I follow to give me a better idea on when and how much to water. I finally understand it. Also, great info from kennancy on the ability to turn off the i-20 heads. I also didn't know they made a 'blank' nozzle and am going to look for that now. It's not on the standard nozzle rack but I wish it were.


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## M32075 (May 9, 2019)

g-man said:


> PRG roots can go deep enough for every other day. No issues with fungus with this approach?


Yes I'm battling fungus but last summer I went with a inch of water once a week and my lawn was wiped out by heat stress and fungus. This year hopefully just a over seed. But to be honest I think I'm over PRG lawn . Total renovation with either tall fescue or KBG possible if I can squeeze it in early September. If not just throw down some PRG seed new lawn in 10 days.


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## Delmarva Keith (May 12, 2018)

g-man said:


> reference Evapotranspiration (ET0)* crop factor = your ET.
> 
> For cool season lawns the crop factor is 80% (under typical conditions).


Kc is different for different months in different places. For example:
https://ucanr.edu/sites/UrbanHort/Water_Use_of_Turfgrass_and_Landscape_Plant_Materials/Turfgrass_Crop_Coefficients_Kc/

Kc of 0.8 is an average that like a broken clock is wrong most of the time.


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## g-man (Jun 15, 2017)

@Delmarva Keith yes. I was avoiding going down this rabbit hole. :lol: crop factor is a function of turf type, density, fertilizer qty, shade vs full sun, soil temps, amount of sun light.

And also, while all this science stuff is interesting and guides you to better irrigation practices, if the lawn looks gray/water stressed, put the calculations away and water the lawn.


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## ken-n-nancy (Jul 25, 2017)

tgreen said:


> Also, great info from kennancy on the ability to turn off the i-20 heads. I also didn't know they made a 'blank' nozzle and am going to look for that now. It's not on the standard nozzle rack but I wish it were.


The "blank" nozzle is on a gray rack for the PGP heads for their low angle nozzle set. (Link to Hunter site with part number below.) It seems wasteful ordering an entire rack for just one nozzle, but the racks are only like $0.38 cents a piece from Sprinkler Warehouse. Make sure you order a few extra ones; I always have trouble getting the nozzles in/out and end up damaging them over time.

https://www.hunterindustries.com/en-metric/support/there-pgp-nozzle-i-can-use-completely-shut-watering

Oh, and as mentioned in the article, you'll need to put regular nozzles in to winterize the system.


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## Delmarva Keith (May 12, 2018)

g-man said:


> @Delmarva Keith yes. I was avoiding going down this rabbit hole. :lol: crop factor is a function of turf type, density, fertilizer qty, shade vs full sun, soil temps, amount of sun light.
> 
> And also, while all this science stuff is interesting and guides you to better irrigation practices, if the lawn looks gray/water stressed, put the calculations away and water the lawn.


Water use overall can depend on the factors you mention but ETo already considers amount of sunlight, soil temps, and grass type (cool season). Kc for cool season grass takes into account additional things like growth stage (season), weather effect on plant water use and a level of stress deemed acceptable back when the concept was developed. Fertilization is assumed to be nondeficient and density is assumed to be full cover. Less cover would require more water in Summer due to additional evaporation. Shade is obviously a separate factor and is accounted for differently than lumping into Kc.

If the lawn looks gray/water stressed, you are doing something wrong. :lol:

If you're not interested in the science or where generalized rules of thumb seem to deviate therefrom, this might not be the thread for you. :nod:


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## Kmartel (Feb 12, 2019)

Wow, does anyone here actually look at there lawn? I come home from work and driving up my driveway I know if my lawn needs to be watered or not. I am not talking waiting till I find dormant brown turf. I mean waiting till the first signs of heat/ drought stress then water the lawn. You should know the areas that show signs first. Maybe you don't notice the leaf fold/roll that first indicates stress but that purple/black haze should slap you in the face immediately and trigger an internal thought that says water the lawn. If you don't notice this then open your eyes and start really paying attention. Then water, but you have to water that night, I like to drop .5". Then wait for the lawn to tell me to water again. It's pretty simple, though I know simple goes against many here, those that enjoy complicating turf.


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## Delmarva Keith (May 12, 2018)

Some people are pure scientists, some people are pure artists and some try to blend the two. Turf needs a blend. I'm not yet ready to just throw up my hands and say the state of the art in 2019 is water some arbitrary amount when it looks like it needs it.

The thread has drifted a bit but I like the drift it's taken. We went from is 1" enough to what is typically enough to how do you know it's enough. There's a lot of different ways to figure that out but yes, at the end of the day you have to look at it to see if the model matches reality. Reality is complicated so to understand the reality there's a good reason to complicate things (unless that's not someone's interest, which is fine too).

What I've come to conclude is the simple model of 1" a week as a baseline, as a guideline, as any line, is arbitrary garbage that by accident alone might work in some places at some times of the year.


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## dkggpeters (May 31, 2019)

Use the following site to get forecasted ETo for your area:

https://www.weather.gov/ict/Evapotranspiration


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## dkggpeters (May 31, 2019)

g-man said:


> And also, while all this science stuff is interesting and guides you to better irrigation practices, if the lawn looks gray/water stressed, put the calculations away and water the lawn.


^ this is a very important point. We are using a lot assumptions on some of the variables like root depth, how much water we actual put down, actual ETo, and crop coefficient factor that this is not an exact science. It should drive you directionally and through observations you should adjust. Over time you can dial in your irrigation.


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## KoopHawk (May 28, 2019)

Boy, for not wanting to nerd out about 1" of water per week, this thread has really started to nerd out!


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## tgreen (Oct 20, 2018)

dkggpeters said:


> Use the following site to get forecasted ETo for your area:
> 
> https://www.weather.gov/ict/Evapotranspiration


 :thumbup:



ken-n-nancy said:


> tgreen said:
> 
> 
> > Also, great info from kennancy on the ability to turn off the i-20 heads. I also didn't know they made a 'blank' nozzle and am going to look for that now. It's not on the standard nozzle rack but I wish it were.
> ...


 :thumbup:


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## Delmarva Keith (May 12, 2018)

Come on guys. If we can predict the weather fairly accurately over the next ten days (or send men to the moon and back or whatever hyperbole you like) , we can predict the hydration status of a lawn. It's not impossible; it's just a little hard.


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## g-man (Jun 15, 2017)

If you want to geek out, then install moisture sensors in each zone and run the irrigation based on your actual moisture profile. DFW_pilot had this setup.


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## Delmarva Keith (May 12, 2018)

tgreen said:


> Green said:
> 
> 
> > My back lawn areas get somewhat more shade and/or less direct sun, and therefore are on a different schedule. I've nailed it down pretty well the last few years.
> ...


If you don't mind changing the heads, another option is the Rainbird 5000 plus series. There's a little ball valve inside the head you can shut off from a center of the head screwdriver slot. I have a bunch of them and they're handy when setting up a system (or for limiting water to areas that don't need it). A reason to do it that way if you have areas that don't need supplemental water is for seeding. You can easily turn them back on for seeding and turn them off the rest of the time. Oh yeah, and for winter blowout (almost forgot  ).

https://www.sprinklerwarehouse.com/rain-bird-irrigation-lawn-rotor-5004-plpc


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## Delmarva Keith (May 12, 2018)

g-man said:


> If you want to need out, then install moisture sensors in each zone and run the irrigation based on your actual moisture profile. DFW_pilot had this setup.


Empirical methods (a la Thomas Edison) usually work best but are not readily extensible or satisfying. 

I suspect this approach is going to become a lot more common as the tech get cheaper and more accessible. Basically foolproof.


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