Evaporation and heat loss of your pond

[pecan]

@MitchM Thanks! Our temps do drop at night we can have a 40 degree swing between highs and lows. I think lately it's been about a 30 degree swing, getting down into the 60's at night.

 

[pecan]

I just tried pecan's numbers with a 7.5 mph wind speed and calculated a loss of 105 gallons per day. A 26% increase in water loss.
Wind speed seems to make a big difference.
Also, pecan's pond is protected by a fence, so the 5 mph wind may not be actually reaching the pond surface.
Interesting.

I have noticed when we have hot dry strong winds, my water levels drop very fast.

 

[Jimmy Gibson]

Ok Mitch..... I tend to agree with your formula, let's throw in a few more variables.... 18' long, 16" wide...... 3000 gallons, almost zero wind due to fences and trees, direct sunlight from 10:00 am to about 2:00 pm. Now throw in a waterfall that is flowing at 90 gallons a minute, dropping approximately 18" to the water.... A 40 cfm aerator....Median temperature during the day about 82 deg. 70 at night .... In these conditions I am losing about an inch a day

 

[MitchM]

Ok Mitch..... I tend to agree with your formula, let's throw in a few more variables.... 18' long, 16" wide...... 3000 gallons, almost zero wind due to fences and trees, direct sunlight from 10:00 am to about 2:00 pm. Now throw in a waterfall that is flowing at 90 gallons a minute, dropping approximately 18" to the water.... A 40 cfm aerator....Median temperature during the day about 82 deg. 70 at night .... In these conditions I am losing about an inch a day

The formula relies mostly on the vapour pressure immediately about the water surface.
Wind will constantly reduce the vapour pressure, and I would imagine an aerator will also reduce vapour pressure, which is why aeration is so effective at cooling off a pond.
Can you specify the temperatures of your pond water and ambient air?

Here's an explanation of vapour pressure vs. relative humidity.
https://laulima.hawaii.edu/access/c...80e8-ed89faa22c33/book/chapter_5/humidity.htm

 

[Meyer Jordan]

All we are looking at is a snapshot in time

This is an extremely important point to remember. Given that the prevailing weather is always in constant flux any computations that are done are already out-dated as the factors involved in these computations have changed. As such, any result obtained may lead to wrong assumptions concerning the overall evaporation rate over day, weeks or months.
The use of this and other formula have falling out of practice in agricultural circles in favor of Pan evaporation rates used in conjunction with the pan coefficient for that particular geographical area.
Maps and charts should be available for each state.
As pondkeepers, it is more important to know what the expected weekly rate may be for each of our locations..
In addition, water loss due to transpiration of any aquatic plants must be taken into account over and above the evaporation rate.
In this information age, it is quicker to consult an existing chart.

 

[MitchM]

This is an extremely important point to remember. Given that the prevailing weather is always in constant flux any computations that are done are already out-dated as the factors involved in these computations have changed. As such, any result obtained may lead to wrong assumptions concerning the overall evaporation rate over day, weeks or months.
The use of this and other formula have falling out of practice in agricultural circles in favor of Pan evaporation rates used in conjunction with the pan coefficient for that particular geographical area.
Maps and charts should be available for each state.
As pondkeepers, it is more important to know what the expected weekly rate may be for each of our locations..
In addition, water loss due to transpiration of any aquatic plants must be taken into account over and above the evaporation rate.
In this information age, it is quicker to consult an existing chart.

I agree.
I was hoping that by running through some computations, we could appreciate the forces at play that contribute towards evaporation and heat loss, instead of wondering if they have a leak or buying pond heaters thinking the heater is accomplishing something that it's not.

 

[Meyer Jordan]

I agree.
I was hoping that by running through some computations, we could appreciate the forces at play that contribute towards evaporation and heat loss, instead of wondering if they have a leak or buying pond heaters thinking the heater is accomplishing something that it's not.

I would venture to guess that no more than 0.1% of the ongoing processes in a pond are visible to the naked eye, but control 99.9% of the visible results.

 

[Jimmy Gibson]

The formula relies mostly on the vapour pressure immediately about the water surface.
Wind will constantly reduce the vapour pressure, and I would imagine an aerator will also reduce vapour pressure, which is why aeration is so effective at cooling off a pond.
Can you specify the temperatures of your pond water and ambient air?

Here's an explanation of vapour pressure vs. relative humidity.
https://laulima.hawaii.edu/access/c...80e8-ed89faa22c33/book/chapter_5/humidity.htm

I have seen the water temperature as high as 84 on a really hot day ( we have already hit 90 some days) and on those days early morning about 79 degrees, but on normal springtime days maybe 80 late afternoon and 77 to 78 in the morning......

 

[Jimmy Gibson]

I have seen the water temperature as high as 84 on a really hot day ( we have already hit 90 some days) and on those days early morning about 79 degrees, but on normal springtime days maybe 80 late afternoon and 77 to 78 in the morning......

Also something to add to this...... My filter and pump are over 100' from my pond, I have a 2" line from the skimmer a 2" line from my bottom drain going to the suction side of my pump and a 2" line from my filter to my waterfall.... And they are buried 18" in the ground...... Why I am saying this I noticed this last winter my pond would cool down with the ambient temperature and due to the fact of a vigorous waterfall but it would warm up quickly and I thought that was due to the piping being in the warmer ground..... Not intentionally trying to muddy the water but just wondering

 

[Meyer Jordan]

I thought that was due to the piping being in the warmer ground....

Certainly a factor.

 

[sissy]

Reson I like having my hose in the pond and if it ever leaks it goes into he pond and not in the ground

 

[MitchM]

Ok Mitch..... I tend to agree with your formula, let's throw in a few more variables.... 18' long, 16" wide...... 3000 gallons, almost zero wind due to fences and trees, direct sunlight from 10:00 am to about 2:00 pm. Now throw in a waterfall that is flowing at 90 gallons a minute, dropping approximately 18" to the water.... A 40 cfm aerator....Median temperature during the day about 82 deg. 70 at night .... In these conditions I am losing about an inch a day

1 inch of water loss in your pond is about 177 gallons.
With the conditions you posted, I came up with a wide variation, between 5 and 15 gallons per day loss calculated.
Different wind speed, air and water temperatures can make a big difference.
Aeration is also a factor to take into consideration, and I found this paper on the effects of evaporation on a pond.
https://research-repository.griffith.edu.au/bitstream/handle/10072/46754/78054_1.pdf;sequence=1
I haven't gone through the paper yet.
I'm sure a waterfall and/or stream design will also increase evaporation.

Using the pan evaporation rate for your state for the month of June, you should be losing only about 45 gallons per day. I found monthly pan evaporation rates here:
http://large.stanford.edu/courses/2010/ph240/harting2/docs/NWS34EvapTables.pdf

I'll have to go through the original paper I posted for this thread and have a look at what the water body was that they used for the study. I imagine it was a still pond.

Thanks

 

[Gordo33]

Using the above formula, Wp=(.097+.038v)x(Pw-Pa)xA
Wp=(.097+(.038x7.5))x(0.256-0.29)x899
Wp=.097+(.285x-.034)x899
Wp=(.097-.00969)x899
Wp=.08731x899
Wp=78.49

Still with me?

Nope
This the line that is confusing me: Wp=.097+(.285x-.034)x899
Why is your result for (Pw-Pa) - .034 multiplied with the velocity result .285 before adding 0.97 to .285 as shown in the formula (0.97 + .038v). It's been a long time since I have had to deal with math formulas but I thought you complete the work inside each bracket before performing the other functions.

I thought it should be
(.097+.285) x (-.034) x 899
(.382)x(-.034) x 899

I was trying to figure out my ponds evaporation rate and ended with a negative number as I would using your numbers as discribed above.
Straighten me out what am I missing here?

My ponds numbers are
Air temp 92 f
Wind velocity 7 mph
Pond is 200 sf

Thanks

 

[Meyer Jordan]

Using the pan evaporation rate for your state for the month of June, you should be losing only about 45 gallons per day. I found monthly pan evaporation rates here:
http://large.stanford.edu/courses/2010/ph240/harting2/docs/NWS34EvapTables.pdf

While this may be useful for historical meteorological data. It is hardly useful for current conditions.
This data is for a period of 1964 - 1982 and as such is 35 years old. Much has happened with climate in the last 35 years.
If I were to rely on what current data is available I would refer to-
https://www.usda.gov/oce/weather/pubs/Weekly/Wwcb/
Though current, even this is may not be much help as many of the pan evaporation stations have been closed in the past 35 years thanks to modern technology.

 

[MitchM]

Nope
This the line that is confusing me: Wp=.097+(.285x-.034)x899
Why is your result for (Pw-Pa) - .034 multiplied with the velocity result .285 before adding 0.97 to .285 as shown in the formula (0.97 + .038v). It's been a long time since I have had to deal with math formulas but I thought you complete the work inside each bracket before performing the other functions.

I thought it should be
(.097+.285) x (-.034) x 899
(.382)x(-.034) x 899

I was trying to figure out my ponds evaporation rate and ended with a negative number as I would using your numbers as discribed above.
Straighten me out what am I missing here?

My ponds numbers are
Air temp 92 f
Wind velocity 7 mph
Pond is 200 sf

Thanks

To complete the equation for your pond, we also need the water temperature.
What is that number?