Pond Boss Forums POND CONSTRUCTION Selecting a site watershed size - theoretical question

Assume annual rainfall equal to the evaporation rate, and a well sealed pond that's not leaking. This theoretical pond does not have near enough watershed, but it does have enough to barely exceed the rate of seepage into the pond banks.

Will this pond ever fill up?

Eric,
I believe you are going to need supplemental water from another source. I don't see how it will ever work.

Example from my area (arid):
- 15 inches annual precipitation.
- 48 inches annual evaporation.
- 30 gpm perennial stream flow considered minimun for succesful pond. (or) 1500 acres of clay or silty soil pastureland for watershed runoff.

Good luck,


Ed

Ed,

What a difference the location makes...

In my area, for example, annual rainfall is 50 to 60 inches. I built a 4 acre pond with only about 3 acres of watershed and until this recent Texas drought, it stayed spillway full for 4 years. Even now it is down 2-3 feet, which is a managable number.

Where do you get the annual evaporation number from? Presumably it varies by local also.

Location, location, location.

Everywhere in the southeast annual precipitation exceeds evaporation. Otherwise we wouldn't have so many wonderful streams. If the pond is well sealed it should eventually fill from precipitation falling directly into the pond (sky pond). However, this may take years instead of days, weeks or months with a good watershed. Also, the water level may fluctuate quite a bit over the year.

Hillbilly

This is a theoretical pond, I'm trying to understand watershed needs better:) (And getting some ideas for other spots on my property where a pond might fit)

I would think that rainfall equal to evaporation, and a pond that doesn't leak would eventually fill up if it had any watershed at all. (Although it might takes years to fill and the water level might fluctuate severely at times)

My new pond is .36 acre and it only has about an acre of watershed. If I followed the "rules" it should have at least 3 acres. There is alot of ground above it, but that ground slopes the wrong way for water to run into the pond. The pond was dug at the end of February and it is filling slowly. We will have a heavy rain or two and the water level will go up alot, then the water level will slowly drop as the water seeps into the banks before the next rain. The good thing is it seems to be "stabilizing" at higher water levels after every heavy rain, so I guess the banks are gradually getting saturated. It had the most water in it yet about 3 weeks ago, but is down about a foot from that level because there have not been any heavy rains since then.

I've seen two figures for our annual rainfall here ... 51 & 52 inches. The two figures I've seen for our evaporation rate are much different ... 50 & 37 inches. I tend to believe the former is more accurate. If it is 51" of annual rain and 37" of annual evaporation then theoretically a pond on top of a hill with no watershed would eventually fill, right? (Assuming seepage/saturation isn't too high)

TN Hillbilly, we must have just posted at the same time. Do you know what the numbers are for our annual rainfall & evaporation? One place that has rainfall numbers is bestplaces.net. I found evaporation maps on some school project websites, but can't find anything "official".

Thanks,
Eric

ML,

Exactly, location does make a huge difference! I rarely make recommendations on this forum anymore regarding watershed hydrology, tree and grass species, etc.etc. The differences across this Country are way too diverse for �cookie cutter� recommendations.

You may want to check with the National Weather Service for evaporation rates. I located my evaporation from some old hard copy files.

Ed

OK, dug around and wound up at the NOAA website. Strap on your thinkin' cap. You're gonna need a program called Djvu to read this stuff Djvu link
but dang...it's pretty damn interesting.
NOAA Main Pg for Precip/Evap
NOAA Evap Map

Good find Brettski, I've been digging around the site some too.

Here's a link to some nice precipitation maps:
http://www.wrcc.dri.edu/precip.html

Based on what I'm seeing my county in Tennessee has around 52" average annual precipitation, and 36" of average annual evaporation. Most of that evaporation occurs during May to October. I guess a non leaking sky pond might eventually fill here.

If you just packed the pond with a dozer, most likely the water is seeping into the soil. You may get lucky and it will seal itself as fines fill the pores in the soil. Maybe.

Eric_in_Tn,
How deep is your theoretical pond. Say, if it's 12 feet deep and you get 50 inches of rain directly into it, theoretically you will have 4 feet 2 inches per year. But, you have 37 inches of evaporation, which means you have a net of 13 inches per year staying in the pond. Theoretically to fill a 12 deep pond would take you around 12 years, not including soil saturation. How old are you?

Eric,

Soooooo.... what you are saying is; if you place a 5 gallon bucket in your yard it will eventually fill with rain water and remain near full indefinately?

Geesh....I knew I lived where it was arid but I had no idea how dry it really is here.

Ed

Ed :

For reference our ponds stay full and lose water, in addition to evaporation, through the outflow pipe and do so for most of the year. By July through Oct. they don't lose water and at worst only drop 12-18in. below full pool in dry years. I have never seen them more than 2 ft. down. These water levels include the normal drawdowns of 3-4 ft which we purposefully do about every other year. Water , water everywhere -- to bad we can't send some to you.

The question is not how long will it take for the theoretical pond to fill nor if it will stay spillway full, but rather will it eventually fill? I think the answer is yes assuming seepage doesn't make up the difference between rainfall & evaporation.

Ed, in a word "No". The bucket will not stay full indefinitely, because rainfall is not predictable in terms of amount and frequency to perfectly offset evaporation. However there is no seepage in a bucket, and if the annual averages are met, there should be just over 1' of water in the bucket by year's end. Our rainfall is spread out pretty well, with a little more in the Spring than other times of year and less in August/September. But most of our evaporation is from May to October. In Winter and Spring, after filling, the bucket may be full and running over after a rain. But the water level may drop considerably in August or September, our driest months, only to fill up again that Winter. Temperature & wind will play a part as well. Is the bucket in sun or shade, what is the air temperature, and is the bucket exposed to wind? Rainfall in inches will be the same no matter how it's measured. But there are several factors that can influence the evaporation rate.

My experience with ponds in this region is the same as Ewest's. In the worst droughts I can remember, water levels didn't drop more than a couple feet in established ponds.

Now I have one more question ...

how many of you have already placed buckets outside to test these theories?

Hay Eric, a close look at precipatation & drought indicates rainfall can be very erratic, double the average some years, and only about 25% of normal 2 yrs. straight! So, when 'ya get a wet 100+ in. of rain in a year, you might have a full pond(or 5-8 ft fuller before runoff). My concern is will our ponds stay full enough to sustain the fish when the drought years come? 50inches evaporation and seepage concerns me.

7/25 Due to the extreme fluctuations of rain we get, I assumed the South did the same, yet gotta watch those assumptions. Looks as if a "wet" year 'round TN may only be around 75" so that pond may take an extra year or two to fill.

Yea, I put out a bucket

7/25 My bucket indicates 16" of evaporation over 30 days here in Sunny Oildale California.

Brettski and Eric, thanks so much for the (evap. links) research. Looks like our area (NorCal foothills) gets 35-45 in. loss/yr. Since my pond is currently down about 2 feet, and we have to make it to ~Nov., I must still have some (underground) seepage.......the creek below is not flowing at all like in years past, so we did "fix" the major leak.

Looks like if I want to keep full to brim, I'll need that well sooner rather than later.

I think I am having trouble understanding the amount of acres needed for a watershed. I have been reading Bob Lusk's "Raising Trophy Bass" and one of the examples it had was a 12 acre lake with an average depth of 6 ft or 72 acre feet. The lake had 50 acres of watershed with 36 inches of yearly rain and 24 inches available for runoff. The book said that this was more than enough. But according to UDSA Pub.590 even the best areas require 1 acre of watershed per acre foot. In the example it is about 5/7 of an acre per acre foot. Thanks for any help.

To Eric in Tn and others,
I may have found a page that explains the difference between the two evaporation rates that Eric in Tn reported to have found. Take a look at:

http://www.grow.arizona.edu/Grow--GrowResources.php?ResourceId=208

It has a US Class A Pan Annual Evaporation Map that you can click on for a little better view. In Tennessee, the 50 inch line runs right through the state. However, if you read further down the page it states that you need to multiply the Pan Evaporation by .7 to get the rate of evaporation for larger impoundments like ponds and lakes. That might explain the variation in the two numbers. I'm not sure of the science behind it, but 50 inches would be the Pan Evaporation, and about 35 inches would be the actual loss from a larger body of water. Hope this link might help others view the evaporation rates for their location.

Good find, thanks for the link!

Perhaps large enough bodies of water keep the air over them partially saturated with water vapor - this should lower the evaporation rate somewhat (to 70%?).

Yes, Theo, I believe your answer is plausible. Your idea led me to do a bit more research and I found this somewhat related link...

http://van.physics.uiuc.edu/qa/listing.php?id=1440

It states that evaporation rates are dependent upon: temperature of water at air/water surface, humidity of air, area of air/water surface, temperature of air, water currents convecting heat, and air flow past the air water surface. The site above stated that there is a tendency to raise humidity at the air/water surface and thus reduce evaporation.

That made me wonder if bottom diffusers could actually slow down evaporation in a pond by reducing surface temperature? Or, would the additional currents created in the water counteract any reduced temperature benefit? Could active efforts to lower water temperature reduce necessary watershed size, or would the reduction be negligible? Actually, this is mostly wishful (some might say desperate) thinking from someone who just needs a good rain .