FRET and Water Needs

If you search for water requirements for the average lawn on the Net, chances are you’ll see the figure of 1" water/rain per week quoted. Quite often. With little consideration for temperature or location it seems.

I’ve just ordered a Rachio 3, but I currently have a standard programmable controller, and after some research, decided to use the FRET values quoted for my area weekly by the NOAA site ( then Total Weekly FRET (in) from the second dropdown). Today, for example, it shows 1.50" for my area in South Carolina. For reference, Bangor Maine is 1.15". It is my understanding that FRET (Forecast Reference crop EvapoTranspiration) is basically the amount of rain a crop (grass, hopefully, or close enough) requires in a week. Definitely different in SC than ME, as expected. In setting my system, I’ve assumed that if a “normal” lawn needs 1" rain a week, and my FRET is 1.50, then I set my Budget for 150% (I’ve calculated all my irrigation times based on actual gpm and actual square feet and number of runs per week to give 1" of water, but will refine that with the Orbit cup measurements).

When I go to Rainbird’s site for ET (, it gives 0.14 for June (x 7 = 0.94") for Bangor and 0.17 (x 7 1.19") for SC, which is about 80% lower than NOAA. I’m not sure about Bangor, but our 90+- temperatures here are pretty normal, and I’ve seen FRET values up to 1.8 or so on the NOAA site.

I just received Orbit’s cup gages, and while I haven’t used them yet, I did play around with the area of the cup top vs inches of flow, and readings on the cup sides, in both ml and inches. Turns out Orbit recommends for the whole summer here in SC a total of 0.87" per week. I think my grass would literally dry up and die at only that rate (maybe not quite, but still).

Why all this discrepancy? (You don’t know, I know, they’re all different sources). But I’m concerned what my new Rachio will recommend. Based on its recommended times and the inches per minute measured by my cups, I can calculate it. Any idea what it will end up?

It’s probably silly asking at this point, but what I do NOT want to do is blindly let Rachio (or ANY system) put a completely different amount of water on my lawn than it is used to, and see after a few weeks that it’s drying up and hurt the lawn.

(Again - sorry - retired engineer overthinking things.)

My suggestions…

Don’t worry about Underwatering especially at first. If anything it’ll overwater at first because it’ll assume it’s not been water… just let it…

And again I’d ignore those fret values. You might look at them for reference but let the rachio figure it out rather than some web site other than theirs because the other sites don’t assume the dynamic abilities of the rachio for watering.

As for big discrepancies a lot of random sites will base their suggestions on general areas and not your specific yard so using the ranges you see as general ranges is fine but those sites don’t also adjust for a dynamic controller which you will have. Some may be pushing using as little water as possible where as others may prefer better root depth for overall healthier yards. Rachio falls towards the latter…

Expect it to be different than what you expect…but that doesn’t mean it’ll be wrong. I’d love to see what it plans on for you vs what you used to do.

Start one zone on Flex daily, the others on the Fixed schedule you had on the old controller. You don’t have to wait and see what the watering amounts will be – make use of the soil moisture table to see what’s going on – e.g., how weather is impacting watering, how many inches are being put down, forecasted crop evapotranspiration etc (you’ll be looking at everything that impacts watering). Tweak the advanced zone settings using your knowledge and perhaps a bit of research. That table is everything since it’ll update on the fly as you update advanced zone settings.

Once you have one zone dialed in, then consider moving over the other zones from a fixed schedule to a Flex daily schedule.

I’ve only ever looked at the digital FRET product by NOAA, which gives daily Forecasted RET. If your state has decent ag services, they may have calculated from actual measurements RET (ET0) for previous days (e.g., California has CIMIS). Rachio won’t match exactly something like CIMIS, but will be close enough. iirc Rachio calculates it from known good NOAA stations (e.g., Penman–Monteith)

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