Author Topic: New Evapotranspiration Observer program from CoCoRaHS  (Read 3220 times)

Steve

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New Evapotranspiration Observer program from CoCoRaHS
« on: May 30, 2012, 03:58:07 AM »
(Cross-posted from MacWeather.net)

I know that a few of us are CoCoRaHS observers and a few record ET on their weather stations. Recently, CoCoRaHS announced that they are starting an evapotranspiration observer program. Unfortunately, the equipment required is quite a bit more expensive than the $27 that the rain gauge costs!

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I have some interesting news to share.  For 14 years we have  focused our efforts on measuring rain (plus frozen forms of  precipitation).  Today we are launching the measurement of the "up side"  of the water cycle -- evaporation and transpiration (known together as  "Evapotranspiration"  -- ET for short.  That is the water changing from  liquid to gas and going back up into the atmosphere.  ET is largely  invisible and often overlooked, but anyone involved in meteorology,  hydrology, agriculture and related fields knows just how important ET  is.  Here in Fort Collins for every 10" of rain that falls, about 90% of  that moisture returns back to the atmosphere as evapotranspiration.  But  that ratio varies a lot from place to place across out country.  Here  our rivers and streams are small and far apart -- since not much of our  precipitation becomes "runoff".  But in other parts of the country  rivers are much larger and streams are numerous.  That's a sure sign  that a lower fraction of the precipitation is evaporating and more is  running off into rivers and streams.

We know that ET varies from place to place across the country -- not as  much as precipitation, but still a lot.  ET rates tend to be low in  areas that are cool and humid, but very high in places that are hot and  dry (assuming there is any water there to evaporate or transpire).  For  example, a yard of turf grass growing in west Texas will require much  more water to stay green and growing than a similar yard of turf grass  in upstate New York.  ET also varies from day to day as weather changes  -- with more ET on dry, hot, sunny and windy days and much less ET on  cool, cloudy, humid and calm days.  In other words, sunshine,  temperature, humidity and wind speed all influence the rate of  evapotranspiration.


Opportunity to help measure and report reference Evapotranspiration (ETr)

There is a small company down the road from us near Loveland that has  been making an attractive instrument that looks a bit like a rain gauge.   It's called an atmometer -- or ET gauge.  It works like a rain gauge  in reverse.  You fill it with distilled water and you prime the tube and  ceramic head.  Then, each day, the level in the gauge goes down as it  draws water through a porous cloth (like a leaf).  It approximates the  use of water by a crop of alfalfa that has adequate soil moisture  -- so  technically we are measuring "alfalfa reference evapotranspiration".

We've been testing this instrument at our weather station for three  years now.  We have found it to perform quite reliably.  It's extremely  interesting to watch the water cycle in action using the combination of  a rain gauge and an ETgage.  It is also satisfying to have something to  measure even on dry sunny days (actually, especially on dry sunny days).   So this adds some excitement to the many days when there is nothing in  our rain gauge.

Last year we found over a dozen people in various states from New  England to Arizona to help test the gauge.  We developed a data entry  system on the CoCoRaHS website to enter the ETgage levels.  Then we  developed data reports to show the data and summarize the results.  A  few of you noticed these new reports that showed up under your "View  Data" tab and have been asking about them.

Almost all the bugs have been worked out of the system.  Starting today,  registered members of CoCoRaHS anywhere in the county can now be a part  of the ET measurement team.

Some cautions!

Before you get too excited about this I must warn you about a few things.

1)  The instrument is expensive -- about $212 plus shipping.  And even  at that price, it still requires manual measurement. The ceramic head,  wafer and fabric cover (which simulate water use by plants) cost more  than $75 to replace if damaged.  In other words, most of us won't be  willing or able to afford this instrument -- and that's perfectly OK.

2)  The instrument isn't complicated, but it does require care and  commitment.   Also, the ET readings are affected by surrounding  vegetation so it's important that each instrument be placed in a nice,  open area -- installed 39" above the grass for optimum performance and  comparability.

3)  Reference Evapotranspiration does not vary as much from place to  place and day to day as precipitation.  Therefore, we don't need nearly  as many stations to get a realistic view of ET variations across the  country.  That's good because of #1 and #2 above  :-)

4)  You have to check in with us before you can start reporting.  Contact Zach at info@cocorahs.org  if you intend to become an ETr reporter.

Despite these obstacles, we're still hoping that we can find a few  observers in each state across the entire country so we can begin  measuring, mapping and comparing ET -- and tracking the "up side" of the  water cycle as well as the "down side" -- precipitation.

One way this could work is by finding motivated sponsors -- perhaps  local Conservation Districts, Water and Sanitation Districts, City  Parks, Storm Water utilities, agricultural businesses, etc -- who may  need local ET information and be willing to purchase instruments for a  few volunteers in their area.


Are you interested in taking measurements or sponsoring some instruments?

If you would like to help measure reference evapotranspiration in your  area please consider the qualifications and costs.  Then, if you think  it might be possible, please contact us at   info@cocorahs.org


Are you interested in learning about ET?

here is our new resource page about ET
http://www.cocorahs.org/Content.aspx?page=et  there is a link there to  the excellent instructional guide on how to measure ET(reference).

You certainly don't need to take measurements to benefit from CoCoRaHS  ET.  All our data are free and public.  Just click on "View Data" on the  top menu bar of the CoCoRaHS website    www.cocorahs.org and you'll see "ET Reports" on the left hand menu 6 items down.  As of  this evening (May 9) I see that 77 daily ET reports have been submitted  so far this month.  That means that some of our testers from last year  are still actively collecting data.  Hurray!!!   You'll also see "Water  Balance summary" where you can compare the amount of precipitation to  the amount of reference ET for any station that is measuring both elements.

Thanks for taking the time to read about ET today.  Hopefully in a few  months we'll have a nice batch of water balance data to analyze.
Steve - Avon, Ohio, USA


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Steve

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Re: New Evapotranspiration Observer program from CoCoRaHS
« Reply #1 on: June 01, 2012, 04:11:09 PM »
I just checked with Weather Your Way, and my ET Gauge is on the UPS truck out for delivery!

Of course it is raining now, so no ET reading for the next couple of days. We just finessed up May with only two rain days over a couple of hundredths inch. That would have been a good time to take readings, especially when it was 95˚ with relatively low humidity.

I'll take photos. ;)
Steve - Avon, Ohio, USA


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elagache

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Can't wait! (Re: New Evapotranspiration Observer program)
« Reply #2 on: June 01, 2012, 04:41:14 PM »
Howdy Steve and WeatherCat fans!

I just checked with Weather Your Way, and my ET Gauge is on the UPS truck out for delivery!

. . . . . .

I'll take photos. ;)

 ;) Well I certainly hope so!!  Do take one of the smiling UPS delivery person!   You must be one of his favorite customers on the route!!   [lol2]

Cheers, Edouard  [cheers1]

Steve

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Re: New Evapotranspiration Observer program from CoCoRaHS
« Reply #3 on: June 07, 2012, 05:50:27 PM »
I installed the ETGage Tuesday evening, and then zeroed it out during my regular CoCoRaHS reading Wednesday morning. (It really is plumb. It just looks crooked.)






First reading this morning was .07" compared to the VP2 reading of .19" reported at midnight last night.

I plan to compare the reading with my VP2 ET calculations. I already have a spreadsheet with my CoCoRaHS rain gauge compared with the VP2 rain gauge. So I'll just add two more columns to the chart.

https://docs.google.com/spreadsheet/ccc?key=0AhiOaCqzSbmedGtwSTdnbEpodTV3UDUzVFViRU8wa0E

My VP2 ET readings to date are on my work-in-progress soil page http://www.avon-weather.com/soil.html

Steve
Steve - Avon, Ohio, USA


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elagache

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Re: New Evapotranspiration Observer program from CoCoRaHS
« Reply #4 on: June 07, 2012, 10:31:57 PM »
Thanks Steve for keeping everyone up-to-date,

More meaty comments on MacWeather: http://www.macweather.net/viewtopic.php?f=11&t=593&start=10#p3681

Cheers, Edouard  [cheers1]

WCDev

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Re: New Evapotranspiration Observer program from CoCoRaHS
« Reply #5 on: June 08, 2012, 07:21:31 AM »
Hi Steve,
There's quite a discrepancy between the ET readings there, however it'll never read the same as the Vantage due to the differences in location of sensors - for example I assume your anemometer is mounted higher and will see more wind than the ETGage.

Cheers,
Stu.

Steve

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Re: New Evapotranspiration Observer program from CoCoRaHS
« Reply #6 on: June 08, 2012, 03:29:56 PM »
Today's reading was equally off. .05 for the ET Gage and .20 for the VP2. There's another station in Ohio that started one day before me. His first two were similar to mine, and then the next was right in line with my VP2 reading. I'm guessing there's some "soaking in" for the first couple of days. I'm hoping to see a bit higher ET reading tomorrow.

Stu, interesting thought about the anemometer being higher. But then, readings around .20 are "normal" according to the CoCoRaHS information for this sort of area. We've had no appreciable rain for a week, very sunny, and temps in the 70˚s F with light winds. I should put my Kestrel out by the ET Gage and see how the wind compares. Hopefully I'll get the weathervane base for the Kestrel for Father's Day and can have it record over a period of time.

Steve
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elagache

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*Gulp* . . . factor of 4 ?? (Re: New Evapotranspiration program)
« Reply #7 on: June 08, 2012, 10:03:42 PM »
Howdy Steve, Stu, and WeatherCat fans,

Today's reading was equally off. .05 for the ET Gage and .20 for the VP2.

Hmm . . . . . . .

Golly differences would be expected but not a factor of 4.  Is there anyone you could check with within CoCoRaHS to see if other folks are having similar problems?  I'm wondering if perhaps there is a bad batch of the ETGages.  That would explain why your other Ohio observer is getting the same odd results.

Bummer dude!

Cheers, Edouard  [cheers1]

Steve

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Re: New Evapotranspiration Observer program from CoCoRaHS
« Reply #8 on: July 26, 2012, 06:28:06 PM »
I discovered an error in my spreadsheet that shows the comparison between the Davis VP2's precipitation and ET readings to the CoCoRaHS rain and ET gauges. For the Davis ET, I was entering what turned out to be "net" ET (daily ET minus daily precipitation.) I've gone back and corrected the spreadsheet to show the VP2's daily ET calculation.

I understand the reasoning for calculating net ET, as it is what the soil really sees; precipitation and evaporation. However, for my purposes of directly comparing the instruments, I needed to change my spreadsheet to reflect actual daily ET.

Here is a link to the corrected spreadsheet: https://docs.google.com/spreadsheet/ccc?key=0AhiOaCqzSbmedGtwSTdnbEpodTV3UDUzVFViRU8wa0E

Steve
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