How is Crop Evapotranspiration Calculated?

All the data :wink:

That is scary, I came to this same conclusion a couple months ago (no, I seriously did). Who are you? :slight_smile:

Yes, this![quote=“solid54, post:20, topic:6114”]
This seems to matter more than temperature differences in my experiments.
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Yes it does. I’ve actually double, triple checked our implementation and wind is such a heavy factor.[quote=“solid54, post:20, topic:6114”]
This results in the problem I see where day after day the actual ET value is much less than the forecast ( and much less than a true observation ). This results in less watering than is actually needed, and browning grass.
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This could be the case for some people. I actually increased my crop coefficient to compensate this difference which seemed to generally correct the problem.

I’m going to look into this and other possible solutions. Thanks so much for passing this along.

I need to reiterate how strange this post is, these are the exact things I’ve been re-evaluating all summer, and came to the exact conclusions you did.

How do you know so much about the Penman-Monteith equation? Do you know of any variations that don’t weigh wind so heavily? I’ve almost thought about some dampener on just wind.

:cheers:

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I don’t think this is an illustration of problem Rachio behavior. Instead I think it illustrates the next technical frontier for the Rachio to reach for.

It makes sense to me that the Evapotranspiration, post calculation using actual yesterday data, or forecast calculation using forecasted values from some national service, is going to be highly influenced by wind speed. So if the Rachio forecasted evapotranspiration values are calculated without taking into account wind speed, then I think this is the next level of calculation sophistication to reach for.

Plus, from a marketing point of view: “The only consumer irrigation controller that includes wind speed influence on evapotranspiration calculations!”

Best regards,

Bill

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As you are aware, the Penman-Monteith equation relies on some approximations, consequently there can be some errors and deviations in calculations. Additional research developed alternatives to P-Me to address these issues.

The ASCE (American Society of Civil Engineers) created a task committee to standardize the equation. The ASCE Standardized Penman-Monteith Equation “expects the weather station wind speed data to represent that occurring at a height of 2 m over and downwind of a smooth measurement surface such as clipped grass. The Task Committee on the Standardized Equation provided guidance for adjusting wind speed measured at height other than 2 m, or, for situations when the wind speed is measured over and downwind of 0.5 m alfalfa.”

Another adjustment was developed by UC Davis and is used by the California Irrigation Management Information System (CIMIS). Here’s a link to those calculations: http://wwwcimis.water.ca.gov/Content/PDF/CIMIS%20Equation.pdf

You two get to do the math! :wink:

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I can read minds through the web :wink:

I tried this too. It worked but I didn’t like the fact I couldn’t see the true watering schedule as it was pushed out each time the actual ET values were updated.

I don’t. I skimmed the paper linked above and did the calculation in a google sheet. Looking at the equations, most temperature related ones used the mean for the day in Celsius, so 5% variance won’t change the values much. Solar is pretty much constant because it’s calculated based on the day of the year, so it had to be wind. My background is meteorology which may helped

Not familiar with any. I have some thoughts but this thread could get long, “windy”, and nerdy fast.

I switched to another station in my area a little further away that seemed to have better wind observations compared to the local National Weather Service station. The calculated values were much closer to the predicted values.

I suspect some sort of analysis grid which lumps wind and precip into grids similar to how the forecast model is output my help smooth the deviations. The RTMA I mentioned earlier covers CONUS and other US related interest. I am not sure about a similar global analysis.

@sunny

Thanks, I’ll take a look at these!

:cheers:

@solid54

Thanks for the extra information, I’m going to do some more research and see if there is anything we can smooth this out with.

:cheers:

LOL!
Can you teach me to read minds through the web, too?
:wink:

Like you, I explored stations near me and tried several, but ended up adjusting the crop coefficients to achieve desired results.

Now for a true confession: After returning from a trip and seeing a stressed out lawn with a large brown spot, the next time I left, I switched to a fixed schedule for the duration of my absence.

This is why I selected a weather station at Wiley Post airport. Plenty of wind there.

My station is http://www.pwsweather.com/obs/ALTAWOOD28.html
This is my pws I have my Rachios tune to.

I agree that Windspeeds can cause variation in the ET that’s why I mounted my anemometer at the industry standard 10 meters. But I’m still getting a discrepancy between forecast and actual reported values. The forecasted values are always high the actual report values are lower. I’ve done a fair bit of research into where to place my station and how to set it up correctly so that it reads correctly but I’m still having issues with my days getting pushed out too far between watering intervals

I also broadcast to weather underground here’s my station ID KUTSANDY61
And I also broadcast to the citizens weather observation program my ID there is EW9651
Looking at weather underground they seem to be the only one supporting the solar sensor for evapotranspiration values at least on their site. PWS and CWOP don’t show the solar radiation values I hope this helps

I was of course talking about evapotranspiration values

It has forced me as well to compensate using crop coefficient values to get the watering intervals where they should be. I’m in the process of tweaking them now hopefully I can get them to where their reasonable

Like @franz I have adjusted the crop coefficient upwards, and I am getting better and better results by constantly observing results against settings. This is why i would love to have the ability to export and save settings that I can annotate. Intuitively it makes sense that the wind is a strong influence on ET, but like scattered rain events, which wind would you go with in an urban development? I can see strong winds in the tree tops around my house, but don’t feel much in the backyard… temperature and humidity are more uniform.
Another factor that I am exploring is the irrigation efficiency % setting to converge observations with frequency and length of irrigation. I don’t think there is a complete theoretical magic wand in an environment with so many variables, and it becomes experimental.

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I appreciate all the info you guys are great!

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