Pond Boss Forums GENERAL QUESTIONS Water chemistry Evap rates

I guess I could look around the internet and find this info but thought you guys could help speed up my search. Is there a chart somewher on evparation rates for certian temps? You know rate is 50 gals/day/acre at 70 and 300 gals/day/acre at 90, etc. Anything like that woudl really help, we are in pretty bad drought at this point. thanks in advnce. This water quality section does not get many hits hope someone finds this.

Greg
That info would be interesting.

By the way, it has been 41 days and counting without rain for me in middle Georgia. It is getting really bad...I am 10+ inches below normal.

Frank

Frank, with all the floods going around us in Texas, our N.E. Texas ponds are about 2 ft low, and Lake Cooper is 10 ft low.
The rains keep passing us by...

wow

I cannot imagine 10 feet low!

Frank

its pouring cats and dogs in illinois we just picked up 1.75 inches and it is still pouring. i wish i could send it your way. i need it to dry out so i can get the backhoe in, excavate the remaining area and seal the dam

I was thinking, Greg. Any answer must include wind velocity, too.

Theo

Will relative humidity be a factor?

Frank

Greg, I just read this and figured that it would be vital to your quest for information concerning evaporation - "the sun evaporates one
trillion tons of water each day". There, now you can wrap up your
search and get some sleep. :rolleyes:

this gives me a great idea for a senior year science fair project considering im going into meteorology

Frank:

You're darn tootin' it will.

bump, anyone else want to help? Frank this would be good info but can not find it. Seems another year that you might have been able to fertilize after all. I know some data is there, b/c I have heard it over the years but can not seem to put my finger on the info. eric, Cody you out there?

Frank, my main pond, close to the house, was 7 ft. low before we finally got some drought breaking rains.

Greg, somebody once posted a map here from the National Weather Service showing evaporation rates by area of the country but I haven't been able to find it.

I have this link on file in my fav's.
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this one too

Brettski,

Great! We are in a drought here as well and that table says we are losing almost an inch per week which is pretty close to actual. It has been extremely hot and dry here for this time of year.

You should be able to get similar data for your area of the country to the information posted below. This is from NOAA historical information on rainfall and pan evaporation. I have used this to predict and verify runoff rates vs. various type and amount of rain events ( such as a 1" hard rain vs. a 2" slow rain) and to track pond level change against expected drop due to evaporation.
This might help,
http://hyper20.twdb.state.tx.us/Evaporation/evap2.html

depts of Ag keep historic pan evapo rates that will give you a historical range.

I have tried to tackle quantifying evap rates on several occasions without much success. It seems to me that there are so many variables, assumptions, and special equipment that the average person doesn't really stand a chance at achieving a value that is statistically or scientifically significant. Any automated tool is going to assume so many inputs that its value will be diminished to a hasty generalization. Precip charts and tools of that sort are very interesting and have their uses but microclimates between a ridge top and valley floor only a couple hundred feet away can be quite different and this is not reflected in these types of tools. I hate to be a negative Ned (my apologies to any Neds out there) but I don't think it's possible without isolating all the variables in some sort of scientific atmosphere. I guess were left with a best guess that may or may not be anywhere close to reality. How would we ever know for sure?

Yeah, that's the basis/problem with all statistical sampling. You have to first define a population and parameters. Statasticians and Economists always end their analysis by using the Latin term, ceterus parebus(sp?) for "all other factors considered equal".

Like the auto industry says, "Your mileage may vary.".

I still think that using the 30 year (some places 50 year) NOAA pan evaporation data provides a good approximation to answer your question Greg. Pan evaporation rates are measured at all large airports and in multiple locations in some counties. They also measure precipitation data. Lakes and ponds are said to experience approximately 70% of the pan evaporation. The data is provided as annual median and mean, as well as monthly median, mean, lowest recorded, maximum recorded and raw data is provided. Based on this and your estimation of the type of year you are having, you location vs, the measurement point, local humidity and temperature, etc, might help you estimate your particular situation.

If you want to set up your own pan evaporation tool, they provide a detailed description of the pan. It is a 4' diameter pan that is 1' deep I recall. Measurements are taken daily at a set time and water replaced as evaporated. In my case I am equidistance from 2 sites with very similar data. My assumption is that I am in line with he average of those two sites. It is good enough for my needs.

Mike:

Is there a standard color for the test pan? I'm thinking black vs. white would make a difference.

Theo,
I believe you can google pan evaporation and find the official pan design. Old data did not use the new standard. There is a color specified, but I do not remember what it was. I do re-call it being a 4' wide 1' deep straight sided pan.
Here is some information I had saved with the coeficients used in the calculations. it has some information listed on data sources you might want to look into. Hope this helps.
About Evaporation/Precipitation Data for Texas

Data Sources: Monthly pan evaporation and precipitation data obtain from the following sources: (1) TWDB and NWS Evaporation Stations; (2) Hydrosphere NCDC (National Climatic Data Center), Summary of the Day Compact Disc; (3) NCDC Climatological Data Monthly or Annual for surrounding States: Louisiana, Arkansas, Oklahoma, and New Mexico; and (4) Other internet data sources.

Monthly gross evaporation rates are computed with a GIS based program called ThEvap. This program replaced an older program, WD0300, previously run by the Texas Department of Water Resources. ThEvap runs on a Unix workstation and applies updated evaporation pan coefficients to observed daily pan evaporation measurements to calculate monthly reservoir evaporation rates for Texas.

ThEvap has been used to recalculate gross evaporation rates between 1954 and 2004. Evaporation computed from data prior to 1954 is still only available as output from WD0300. Prior to 1954, WD0300 used pan evaporation data from non-standard pans, which allowed for a much larger dataset. However, updated pan coefficients were not calculated for non-standard pans, so the decision was made to keep the WD0300 data from 1940 through 1953, but to separate it from the ThEvap dataset from 1954 to 2004 (for 1940-1953 Evaporation). TWDB also ran ThEvap to produce average monthly rainfall by quadrangle from 1941 to 2004.

The pan coefficients used in ThEvap were developed in response to the National Weather Service's NOAA Technical Report NWS 33 in 1982. They reflect both a seasonal and spatial distribution, instead of the previous annual coefficient (TWDB, Report 64, 1967). The new coefficients resulted in reduced annual evaporation rates (the new pan coefficients table) .

For the monthly gross evaporation rates, there are some missing data had been replaced by the long-term mean values of Quadrangle.

Designation of Quadrangles: the smallest practical area for evaporation calculations was a square 1 degree latitude and 1 degree longitude in size.

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Wikipedia did not give a color for a "Class A" pan, but did state that a "Sunken Colorado" pan should be unpainted galvanized steel, and that evaporation rates between the two styles of pan can be converted in a standard manner.