Should the Crop Coefficient be changed during the year?

Schedules & Zones
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Our weather in Spain will change from mid 90’s in the height of summer down to mid 60’s in winter. We have warm season grass and I think it’s either St. Augustine or Centipede grass. I’m using Flex Daily and we have our own PWS.

I feel the grass is currently being overwatered. I have the coefficient set to 65%. The roots are short at only about 5cm. The nozzles are still at the default of 3.81cm since I have no idea of the actual amount. However, I think it could be up to double the actual rate.

@rraisley I saw you commented on an old post of a similar situation and I think you implied that the default nozzle size if overstated ?? Based on my temperatures, what co-efficient would you suggest ? I am unable to find any details of Kc values for Europe for garden lawns

Should coefficient values be changed as the seasons change ? If so, what would the suggestions be for the 4 seasons for the temperature range I indicated above ?

Any suggestions gratefully received fromm anyone

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A Crop Coefficient of 0.65 is recommended for Warm Season Grass, which includes St. Augustine. I have found that for Centipede grass 0.85 is the published value; it requires more water than other warm season grasses, so I use that.

As to whether Crop Coefficient should be changed throughout the year, there are some crops, mostly flowers I think, that the CC does change on. I guess they require more water when flowering than when not, that kind of thing. So you can find some published info on that. I’ve found almost nothing on grass, though, and feel that either it doesn’t vary as much, or most turn off sprinklers in winter, so don’t really need to change it.

Your root depth of 8 cm (3.1") is definitely low, resulting in only 0.6 cm of water at one time (0.23"). For St. Augustine, I think they recommend 8+ inches. For my Centipede, 6" is a good value. But increasing the root depth will give your grass more water at one time (not more over time, though), so make sure it’s not just running off. At least your soil holds a good bit at 0.15. And your roots MAY only be 8 cm deep, but if so I’m sure you want them to get deeper, so you would normally want to increase that over time.

All that sounds like you may be UNDER-watering. But your statement says you think you may be OVER-watering. The nozzle default you mention of 3.81 cm (1.5") is on the high side, from my experience, but certainly not impossible. Mine run from 0.35 to 1"/hr. IF that value is higher than actual, then you would actually be under-watering the lawn. Which again, you say you think it’s the opposite. But as I recall, we worked through a lot of this before, and you do have a pretty high flow, commercial system. Didn’t we calculate the values based on flow and area at that time? I can’t find the thread right now that has it.

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Thanks @rraisley, the local Administrator and gardener is driving me nuts. Our system finally went “Live” about 3 weeks ago. However, I am still awaiting the map of all zones to calculate the area of each zone and I’m also still waiting for a one minute test to get the quantity used. I suppose it’s pointless in me talking like this until I have actual figures to hand.

Just one question regarding the root depth setting. The roots are only 5cm, so I was setting it to 8cm to start the process of driving the roots lower. Should I have set this to 5cm for now ?

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I wouldn’t lower it. First, you really want the roots to grow much deeper than they are, and this will entice them. Second, it’s a bit hard to tell actual root depth, because there are many hair-like roots that extend deeper, but are harder to recognize as being roots.

But yeah, having those figures or areas and flows would be a big help.

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Right your Precip rate is high. I mentioned this 3 months ago when you guys started this to; check your nozzles and you just wanted to base criteria on flow. Reduce your Pr and get some appropriate water conservation nozzles in 1 GPM or Rotary Nozzles @ .6 GPM

Once, again a 1 minute test is not an accurate test!
Do you put a Car (automobile) on a Dyno for 1 minute to get an accurate reading? No. There’s discrepancies in Static system and then live action of an irrigation system when you open Dynamically the Meter, Pipes, Valves, Gear Drives, Springs, Heads, Nozzles etc…

No you don’t change KC over the year. That’s where Et and seasonal adjust etc… come into play.

It also looks like, there’s a data error in ET reporting prior to 3/4 so your overwatering. No fault of yours. Though after that it looks like the correct ET for your Temp. If you just came online that makes sense as well.

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Clarification, I did mean for example 1 GPM Precision nozzles, or Rotary Nozzles but PR @ .6 inches per hour about. That’s mostly what we are working with for soils.

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So for now, should I set the Nozzles Per Hour to One Inch / 2.5cm @ProWater ?

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Yes, and I was pretty sure I did some approximate calculations at the time, and it turned out reasonable considering the size of pipes, number of nozzles, etc. And this is a very large, established commercial/residential operation, so don’t think he’d want or need to replace all the nozzles.

That could be, or it could be that he changed the KC on that day to a lower number, therefore reducing the required KC. As he’s been playing with numbers, I think it’s that. I’ve seen the same thing on mine.

1 Like
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The last 24hrs have seen a huge amount of rain, with more expected too over the next 48hrs. So I’m on hold I think before I can get details of even just one zone - to get the area calculated, confirm the number of nozzles in the zone, and confirm the litres using during a test. There is a meter on the pipe in the main distribution pump house. There is a plan to install a Master valve onto the whole system, possibly in the coming week.

I’ve looked again at our older posts and although I cannot find our previous posts with calculations, reading your posts again make it 100% clear that I MUST get a better idea of the size of the zones, as I am wasting my time and everyone elses’ too until I get this data.

Oh how I wish I could fly down there. It could be June or July I reckon :frowning:

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You don’t need the number of nozzles, just the area of the zone, and the litres used during the test (preferably 10 minutes or so; definitely more than 1 minute, I’d say a minimum of 5).

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The ET is not something, a user is manipulating in Rachio Platform. Unless, of course you are the Sun, Wind, Rain, etc…

I had checked the weather conditions, and the relative ET looked like it was misreported data…I have no claim except for the temperature reporting.

The thought was since he was just receiving data on a startup. Not anything to do with him changing Crop Coefficients in his Zones. If you’ve seen something similar, either Rachio in error or an extra adult beverage could have marginal made the difference in your objective analysis in reference. Though, since I have seen similar nuances without, I would verify reference ET daily, from various sources especially during the hot summer days, and not from Rachio.

It’s just common sense that the reference ET doesn’t go from .25 to .09 or there about, with the same temperature running at a daily basis. Maybe, with a Tornado at that current ambient temp. I don’t know.

@championc You cannot just set your precipitation rate at whim. Again, when you initially asked for guidance months ago, my suggestion which is what we normally do is to check each of the nozzles, and it’s manufacturer’s specifications and use it as a guide. Start there!

@rraisley I enjoy your mapping interest which could help me greatly and I would have to go back to your posts in the future would like to use your proficiency and would be a good student, however…

One, you must get the PR from the watering orifice (Nozzles) as an element to figure out a baseline and water schedule, this is how it’s done. This is how it’s done in Rachio and in every Irrigation class, and design book will tell you this. Not sure why you @rraisley insist it’s not important? It is!

Irrigation Run Time in it’s simplest form:

Is based On Time, Interval, Plant Factor or Use, Application Rate in inches/hour, and the System Efficiency.
[http://irrigation.wsu.edu/Content/Calculators/General/Irrigation-Run-Time.php]

Since, I base a lot of my business on efficiency, the systems balance in specification is important to realize the best performance, and eventual water savings.

PR as defined and calculated.

https://www.rainbird.com/professionals/irrigation-design-tip-calculating-precipitation-rates

You need to find out GPM, yes you need the quantity of nozzles and it’s performance for the area. This is another design elements. these are all basics of what is translated in the software . This is all outline in the Rainbird link above.

To be simple, we always ask ourselves what’s the PR? We then correlate that to the Plant’s needs, and ET. Exactly, what Rachio is asking you to enter, and every other company tries to do, you just have to indicate the correct parameters for it to calculate and run. Not all Companies are the same and Rachio is far from perfect.

The fact though is, your able to make corrections and adjustments in the platform to work with your landscape needs, at a key stroke; be connected to weather data, in almost real time, you can operated it remotely with your hand held device, and it’s a far cry from a MWF 10 minutes each Zone Setting.

Lastly, should you change nozzles? Well, if new nozzles are 75% more efficient and uniformity goes up than the existing nozzles which could be at 50 % efficient with less uniformity, and your current system is 60 % efficient, than I know what I would do, how about you? Again, put the system in balance and in maximum specification and you can then determine water conservation upgrades and investment strategy. Good luck.

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You are absolutely right that normally ET does not have extreme changes, but here in the South I’ve actually seen similar changes to the above. We’ll go from 80 degrees and sunny one day to 40 degrees and rainy the next. Really! That has extreme changes in ET.

And again, yes, ET is controlled by the environment, not the user, USUALLY, we’ve been playing around with changing KC as a good method of changing the total amount of water applied to a zone. So it is done quite often. For example, I wanted to apply water when my lawn got /really/ dry this winter (in SC we normally don’t water at all in the winter), but didn’t need it to water a normal amount for dormant grass. So I halved the KC, which in turn halved the ET. This is what I suggested MIGHT have happened, because I’ve been talking to the OP (back in December) about changing KC, and thought that might explain the unusual change.

To show this, I just changed my KC on one of my zones, and it gives the following graph:

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Surprisingly, to me, the ET change started /yesterday/ rather than today. But you can see the huge difference due to changing the KC. Don’t get hung up on the other values in this zone; it’s an experimental zone that doesn’t make much sense in real life.

Hmmm. I couldn’t get that calculator to work. Regardless of values I typed in, I got “NaN” as the result. And I /think/ I put in appropriate inputs.

I love this page! It proves what I’ve said all along: the number of nozzles doesn’t matter. In each case, the calculation of PR is equal to the water flow (in this case, the estimated factory spec flow from all the nozzles at a standard pressure that water the zone) divided by the area (row and sprinkler spacing). My suggestion simply uses the /actual/ flow (certainly more accurate than estimated flow) divided by the actual area. And since, for example, none of my zones have equal spacings or even constant rectangular or triangular spacings, it works out well.

And I didn’t mean to indicate that efficiency isn’t important. But without knowing what you have to start with, it doesn’t seem that the next step would be to replace all the sprinkler heads. I know for a fact that my house’s irrigation is /very/ poorly designed, as are most non-rectangular areas, and doing it “right” would involve redoing the entire system.

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Many thanks guys for the replies. I have really no knowledge of what brand or model the sprinkler heads are - but I do know that they evenly cover all of the grass. Of course, there is likely to be some overlap of zones, but not a huge amount I think.

I have the complication now of a gardener who now says that the system is “my responsibility” - yep, me in Dublin Ireland and it’s in Spain !!! So I’m pushing back to say I cannot be responsible unless I am provided with the relevant data. Mind you, the good thing is that we are using much less water than before. The Rachio current calculation is 11,500 litres since 02/26. The gardens are in excess of 4000 m2.

It is currently pouring rain and will be too for about the next 24hrs, so it gives me plenty of time to get the zones measured. I’m trying to get the ducks lined up to meaure the zones next Monday, and to do a 5 min test on one zone.

I’ve made a grid reference map of the gardens and hope that they can test and tell me what zones are made up of what boxes - simple enought I think to get a decent aproximation. Getting the details off the underside of a sprinkler head could possibly be asking too much at this point in time. If I was over there, I would be out at 02:00 checking stuff if necessary - but some others are not as committed as I seem to be.

From memory, all zones are made up of a mixture of directional 90 deg sprays, 360’s and sweeping rotational sprays. So there’s not much of a possibility any time soon going an doing the full PR calculation exercise, and there’s certainly nobody there who will take the amount of time needed to undertake such an exercise. I have one resident lined up for next weekend to hopefully map things out with my grid.

Thanks again, your knowledge, ideas and feedback is so greatly appreciated.

Here is my plan of the gardens. Each square is about 50m2

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1 Like
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Is that a hotel or a mansion!?

How does one get an invite?! :wink: :beers:

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Agreed! I’ve been offering to measure area of zones, hoping I’d get to do it in person!

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If they would print out your plan, but OUTLINE each zone (often separated by walkways, etc.) and send you a scan of same, you can figure the areas pretty well (and I could help).

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To me, it is rather easy to do in Google Earth. In person would still be more fun, I am sure. :wink:

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There is an area calculator tool that uses Google Maps that is easy to use and very accurate:

https://www.daftlogic.com/projects-google-maps-area-calculator-tool.htm#

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We’ll be using Draftlogic for sure.

The rains have cleared, with nothing expected for the foreseeable future, so the plan is to get the zones mapped sometime around this coming weekend.

It appears they had a power surge during the thunderstorm on Monday morning because the Rachio PSU was totally dead. Thankfully for now, they have replaced it with one of the original Rainbird PSU’s (only 650mah) but will hopefully get a proper (1ah) replacement this week.

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@rraisley Oh my god - it’s like watching paint dry in a thunderstorm. The delays are sooooo frustrating. And the information being provided so far is pityful. I’m glad I’m not paying for the info or I would be looking for a refund.

Anyway, thankfully, a 5 min test was finally done today on one area which is well defined and made sense - Zone 9. The Area calculated as 346.5m2 and the quantity used was 836 litres (167 l/m). They actually did a second zone, Zone 8. The Area calculated as 145m2 and the quantity used was 505 litres (101 l/m). I think the area for Zone 9 is likely to be more accurate than that of Zone 8.