Calculating evaporation rates
Phyllis Hurley
and a handy formula for the intrepid calculator with lots of resources. I
like best the parallel pan method proposed by one answerer. I will try
that: A pan of water alongide the pond to see what its rate of evaporation
will be. I note, however, that it needs to be a shallow one so the walls
don't protect from wind. It also might be set into the earth. Will report
on results eventually. Keep help coming!
Note rate for arid, cold, windy Colorado near Denver. The peak is about
6"/mo or 1.5 per week. My winter rate is 4"/mo. ...although one response
for Alabama suggested as much as 1/5" per day in the summer.
Table 5. Average Monthly Evaporation Rates (ft./mth) for the City of Aurora,
CO.
Jan Feb Mar Apr May Jun July Aug Sept Oct Nov Dec
.0083 .025 .05 .15 .2667 .4414 .5 .4917 .35 .2 .0917 .0417
http://www.pr.erau.edu/~meteo/cwx/penpan.html
A disadvantage of the the Penman formula for Eo is that it requires
measurements of temperature, humidity, wind and net irradiance, which are
not all everywhere available. This was discussed in a previous paper [12],
leading to the following simplified version -
Eo = (0.015 + 0.00042 T + 10-6z) (0.8 Rs - 40) + 2.5 F u [T - Td]) mm.d-1
(1)
where T is the average of the daily extreme Celsius temperatures, z the
elevation (m), F allows for the change of air density with elevation and
stands for [1.0 - 8.7 x 10-5 z], u is the wind speed (m.s-1) at 2 m, Td is
the dewpoint temperature and Rs the solar irradiance of the lake's surface
(W.m-2). The dewpoint can be estimated graphically from the daily extreme
temperatures [13]. As regards Rs, two methods of estimating it were
described earlier [12]. The more accurate method requires monthly rainfall
data, as proxy for the cloud which cuts out some of the extra-terrestrial
radiation, given in a table [14]. Often it is sufficient to estimate the
wind speed approximately from measurements nearby or at other times, since
evaporation varies much less than proportionally with changes of wind [3,
13]. Eqn 1 shows that the dependence on wind is least when radiation is
strong. Consequently, only daily extreme temperature measurements are
essential, with rainfall and wind speed data as desirable. Errors of
estimating Eo with Eqn 1 were shown to be around 0.3 mm.d-1 at Copenhagen,
for example.
Howard Cutler
"Phyllis Hurley" <james...@worldnet.att.net> wrote:
Hi Phyllis,
IMHO This is just too technical. Ideal if you we looking after a large
reservoir. Suggest two pans/saucers of water, but in different locations.
As per my earlier posting to your original thread.
Early advice - don't worry too much.
Regards,
Adge
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
********* My own Water Garden is at: **********
Howard Cutler http://www.adgecut.demon.co.uk
If you live near 'Stonehenge' Contact me: adg...@adgecut.demon.co.uk
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
DOWatDPG1
>Eo = (0.015 + 0.00042 T + 10-6z) (0.8 Rs - 40) + 2.5 F u [T - Td]) mm.d-1
OMG! LOL. If two trains left New Orleans at 4:40 am and one travelling at 55
mph stopped in Baton Rouge to pick up ten people and the other, travelling at
60 mph stopped in Jackson to let 8 people off at what time will both trains
return to New Orleans?
LOLOLOLOL. Okay -- it is not THAT funny but it sure made me giggle. My mind
just boggles at 7:00 am (woke up at 6:30 Laurie).
I think elevation and wind are good points. The friction of the air moving
across the water and the pressure of air on the water itself certainly would
effect evaporation. I'd suppose that Aurora has a higher elevation and thus
less pressure and more wind... But of course I am not a meterologist or even
particularly good at geography :) but I take care of someone who is.
I'll mention it to him when I go back to work on Thursday and see what he says.
Maybe there is an eaiser way? What about .gov publications related to
agriculture. Maybe they have a suggested constant amounts for your area in
Mississippi? Maybe your county extension office has some statistics.
For the most part, however, I would say do you best to look for overt signs of
leaking. The waterfall (if you have one), the tubing of your pump, especially
wet areas around the perimeter of your pond. I look forward to here about the
container/pan experiment.
.
Dowser-- 800/741-3867
Daydreamer Perennial And Aquatic Gardens
http://www.daydreamergardens.com
~ A Plethora of Plants make Ponds Prosper Pleasantly and with Panache ~
Ceos twpc
From: dowa...@aol.com (DOWatDPG1)
Date: Sun, Mar 14, 1999 7:10 AM
Message-id: <19990314071005...@ng-ch1.aol.com>
Phyllis wrote:
>Eo = (0.015 + 0.00042 T + 10-6z) (0.8 Rs - 40) + 2.5 F u [T - Td]) mm.d-1
OMG! LOL. If two trains left New Orleans at 4:40 am and one travelling at 55
mph stopped in Baton Rouge to pick up ten people and the other, travelling at
60 mph stopped in Jackson to let 8 people off at what time will both trains
return to New Orleans?
LOLOLOLOL. Okay -- it is not THAT funny>>
Well.... ;-)
<< but it sure made me giggle. My mind
just boggles at 7:00 am (woke up at 6:30 Laurie).>>
Stuff like this can boggle minds WAY beyond the wee hour's! Or is it just me??
:-)
Preparing for the Privet, Dow??
<<I think elevation and wind are good points. The friction of the air moving
across the water and the pressure of air on the water itself certainly would
effect evaporation. I'd suppose that Aurora has a higher elevation and thus
less pressure and more wind... But of course I am not a meterologist or even
particularly good at geography :) but I take care of someone who is.
I'll mention it to him when I go back to work on Thursday and see what he
says.>>
This ought to be cool!
<< Maybe there is an eaiser way? What about .gov publications related to
agriculture. Maybe they have a suggested constant amounts for your area in
Mississippi? Maybe your county extension office has some statistics.
For the most part, however, I would say do you best to look for overt signs of
leaking. The waterfall (if you have one), the tubing of your pump, especially
wet areas around the perimeter of your pond. I look forward to here about the
container/pan experiment.>>
Me too!!!!
.
Dowser-- 800/741-3867
Daydreamer Perennial And Aquatic Gardens
http://www.daydreamergardens.com
~ A Plethora of Plants make Ponds Prosper Pleasantly and with Panache ~
Laurie
Phyllis Hurley
The experiment comparative evaporation is now under way! Lacking the
information or interest in the complex formulas, I have started the
empirical test which several people recommended. My sophisticated devices
for comparative evaporation studies are two milk jugs, cut so that the
sloped top is gone (my pond has straight sides and a flat bottom, as do the
jugs). The jugs are at different locations to test for shade/sun and
sheltered/open differences. I measure the pond and the two jug depths at
the same time (One jug is deeper than the other, so one yard stick can
measure all three in sequence(shallow jug, deeper jug, pond: the depth is
easy to see on the wooden yardstick...darker wood). I will report on the
ongoing outcome after a few days. I will also try measuring with the
circulation on and the circulation off. 3,000 gallons per hour through two
55 gal. barrels with returns via 4" pipe should, in theory increase
evaporation...although it did not seem to when I previously measured
evaporation rates.
This two-jug method allows me to handle the rain which is messing up any
effort to see if the pond stabilizes at some level of evaporation. I just
take a measurement of the jugs and the pond after the rain and see how much
lower it is the next day. Comparative levels are the issue, rather than the
absolute level or predicting the rate of evaporation. My 1/8" per day is
about 20 gallons the 4,500 gallons in the pond.
As the rains stop, I can see if the rate levels out at some point and try
the milk test which some of you recommended.
It is mazing how a pond can entertain/harass you.
Thanks for the ideas...y'all. Keep sending any new ones. I'll post the
results soon.
Ceos twpc
<< The experiment comparative evaporation is now under way!>>
Yes!
<< Lacking the
information or interest in the complex formulas, I have started the
empirical test which several people recommended. My sophisticated devices
for comparative evaporation studies are two milk jugs,>>
hehehe :-)
<< cut so that the
sloped top is gone (my pond has straight sides and a flat bottom, as do the
jugs). The jugs are at different locations to test for shade/sun and
sheltered/open differences. I measure the pond and the two jug depths at
the same time (One jug is deeper than the other, so one yard stick can
measure all three in sequence(shallow jug, deeper jug, pond: the depth is
easy to see on the wooden yardstick...darker wood). I will report on the
ongoing outcome after a few days.>>
Cool! We'll be waiting ;-)
<< I will also try measuring with the
circulation on and the circulation off. 3,000 gallons per hour through two
55 gal. barrels with returns via 4" pipe should, in theory increase
evaporation...although it did not seem to when I previously measured
evaporation rates.
This two-jug method allows me to handle the rain which is messing up any
effort to see if the pond stabilizes at some level of evaporation. I just
take a measurement of the jugs and the pond after the rain and see how much
lower it is the next day. Comparative levels are the issue, rather than the
absolute level or predicting the rate of evaporation. My 1/8" per day is
about 20 gallons the 4,500 gallons in the pond.
As the rains stop, I can see if the rate levels out at some point and try
the milk test which some of you recommended.
It is mazing how a pond can entertain/harass you.>>
You said it best ;-). But the entertain will win out!
<<Thanks for the ideas...y'all. Keep sending any new ones. I'll post the
results soon.>>
We're glad to have you! Anything you want to share about your garden? ;-)
Laurie
>>