Pond Boss Forums INTRO Questions & Observations How much watershed do you need?

Are there rules of thumb? For a 1 acre surface pond, 10' deep, how many acres of drainage are required. Assume all water into the pond is surface runoff, no springs, etc.

Your supposed to have 10 acres of water shed for every acre of pond. If it is open land. If it is wooded then you supposed to have 20 acres to every one acre of pond.
So you would need ten acres of water shed for your particulare pond...If it is open land..

I've heard that rule of thumb before, but really question it. Doesn't the real answer depend on where you live, the amount of annual rainfall you get, and the surrounding soil composition?

I had a NRCS "expert" tell me 4 years ago that my planned 3.5 acre pond would never fill up or stay full because it has only about 2 acres or less of watershed....it filled within one month of completion and has been full ever since (for the last 4 years). Now if that same pond was in West Texas, no amount of watershed could keep it full.

Seems to me that the rule of thumb needs a little common sense applied to it.

Jack,

"Basic Pond Management" by Bob Lusk and Mark McDonald (available from this site) gives the correct answer.... Depends on the evaporation rate vs. the rainfall.

Step 1. Determine if you need watershed.

If evaporation for your area is 50 inches per year and your annual rainfall is 50 inces inches per year, you have enough rain to keep the pond full. Too bad rainfall is not consistent throughout the year. If you have dry summers, you have to create a reservoir to maintain water during drought.

Suppose your evaporation rate is 24 inches more than your rainfall in the two hottest summer months, you have to have enough watershed to make up for the missing 24 inches during those months or your pond will drop 24 inches.

2. Calculate the amount of watershed to fill your pond.

To calculate the amount of watershed to fill your pond you need to know the total amount of runoff you will be getting in the time frame you are concerned with.

1 ac. times 10 ft. times 12 inches per foot equals 120 inches of runoff needed to fill the pond.

Say in July, you want to have the pond filled and you only get 4 inches of runoff after the trees and grass soak up the water. You will need 30 acres of water shed to fill the 1 acre ten foot deep pond. 120 inches needed divided by 4 inches equals 30 acres.

The problems are numerous... for example, in northern Oklahoma we only get 2.5 inches of rain in April and then 6 inches of rain in May and 6 inches in June (interfers with the wheat harvest). Then it gets very dry again. The average for the three months is 4.8 inches per month but the &*&*(%%$ wind in March and April evaporates the heck out out the ponds during the time we don't get very much rain so we end up dropping our pond levels in March and April by 6 to 10 inches and then flooding them in May and June.

I think the best thing to do is do the calculations but also check out the other ponds in the area for how much watershed they require. The pond owners will most likely be more than happy to get long winded about their problems, which reminds me..... Good luck

Does any one know the evaporation rate for GA?? Because I might have a BIG problem with my junk...could someone help?? it is Northeast GA....

Big Pond, I once got the evaporation rate from the National Weather Service. Local TV Weather types might also be able to help. I would also bet that the Engineers on local lakes can help.

Posted this web link on the sizing a spillway thread above but I think it will answer these questions about the basin required for a pond of a given size. For those interested in the runoff calculations it will show the most accurate method. The variables are given for Michigan, so you would have to obtain the corresponding runoff constants for your area.

Meadowlark is right, runnoff is determined by local conditions, especially soil and vegetation types or the presence of paved/impermeable surfaces (clay), also the condition of the vegetation. Runoff is also dependent on the rate of precipitation and slope. Does precipitation in your area tend to fall in light steady rain or big gully washers. Hard rains falling on steep clay soils will have a high runoff factor and produce huge amounts of water. Light rain on flat sandy soils with thick vegetation will produce little or no runoff.

The 10 to 1 rule is a starting point, but to do it right, all aspects of the drainage basin are included e.g., acreage, evaporation rate, annual precipitation (in the north we have to divide precipitation based on the season), transpiration rate, surface water inflows/outflows and groundwater gains or losses. Most of this information is published and available for the design of detention basins, stormwater structures, roads and other engineered structures and can be obtained and used for pond design.

big_pond, you should be able to Google "pan evaporation Georgia" and find the information.

Calculating runoff

Big Pond when you started posting about your watershed I thought you were just getting impatient, but with our rain in north Georgia this year I understand your concern.
Has your pond level gone up at all over the last two months?

Bill,
It has gone up, it has gone up about 3 to 4 feet I'd say..It still has about 4 feet to go. My concern is that I will not EVER fill up. If it did it will not stay full, but drop a foot durin the dry summer months..

Do you know the evapration rates for our area? or the Evaporation total??

This topic could go a LOT deeper than watershed, around me, it's roughly 15 acres per surface acre. A pond constructed with a dam typically needs more watershed than a pond constructed below ground level. Clay content is a MAJOR player here...I have ponds close to mine that lose water thru the ground, not evaporation. If/when you build a pond, walk down during the night...and see illuminated clay, count your lucky stars. Also, don't sell !