How To Calculate What Size Pond To Build

Greetings to all.

I am building a pond for irrigation and am trying to calculate how big to build it. I have Toby Hemenway's book and according to it my 30 X 28 roof will deliver 25,200 gallons of water per year. This is based on average rainfall in central North Carolina of 45 inches. 25,200 gallons for 45 inches of rain will give me 560 gallons of water per 1 inch of rain. Will someone please tell me how big to build my pond? Or please refer me to a calculator that can help me determine this pond size? Also, is 45 inches of rainfall per year the correct amount for central North Carolina?

Thanks and have a great day.

Jim,

How big is your property?  Surely you can direct more water into a pond than what lands on your roof.  I don't know if your rainfall info is right or not, but another question is how much irrigation water will you require during the year.

Kevin

Kevin Elmore wrote:Jim,

How big is your property?  Surely you can direct more water into a pond than what lands on your roof.  I don't know if your rainfall info is right or not, but another question is how much irrigation water will you require during the year.

Kevin

Hi Kevin,

Thank you for the great input. Unfortunately the pond will be on the corner of my property within about 10 feet from the borders. The 1.25 acre property is situated in a residential neighborhood that averages .5 acres per home. Our lot is at the bottom of a 600 foot hill of about a 5 degree grade. The pond will be 200' from the bottom of the hill and 400' from the top of the hill. There is a slight trench running from about 200' above to the proposed pond location so there should be some run off I am hoping.

About the required amount of irrigation water needed? Without knowing much about that either my hunch is enough to irrigate 400 sq ft of vegetable rows a day with drip irrigation via an electric pump.

Thanks,

Jim

Without speaking to your pond sizing, you might take a look at what water you can divert from above your property line to feed your pond. Look for where water might run down a road, or a contour where you could put a slightly off contour swale/ditch to divert surface flow towards your pond. I'm pretty sure that Hemenway's book looks at things like harvesting water from roads before it is lost into drains. Any hard surface can be used to collect water; not just a roof line.

Do you notice flowing surface water during heavy rains?

Michael Cox wrote:Without speaking to your pond sizing, you might take a look at what water you can divert from above your property line to feed your pond. Look for where water might run down a road, or a contour where you could put a slightly off contour swale/ditch to divert surface flow towards your pond. I'm pretty sure that Hemenway's book looks at things like harvesting water from roads before it is lost into drains. Any hard surface can be used to collect water; not just a roof line.

Do you notice flowing surface water during heavy rains?

Thank you Michael. Since the property above mine is not mine there is not much that I can do. it has been raining for about 14 - hours and there is no surface water anywhere near the pond. That said, do you think a 20' x 10' x 3.5' pond is a correct size for my roof size and average annual rainfall?
Best regards,
Jim

Rain water calculations from a roof are generally calculated based on enuff storage to last the dry months. If you use 3000 gallons a month and the longest rain lapse is 3 months, then you need 9000 gallons of storage to get you thru the dry months.

With a pond there are several other  variables. Mainly  evaporation and whether the pond leaks. I might have heard evaporation is ruffly 1" a day.  Leakage is an unknown.

Jim. Drip irrigation typically is done for finite periods in the early morning and late afternoon to maximize soakage and minimize evaporation. For example, you might run a schedule of 2 X per day for 30 minutes each. That is one hour of water use per day. How much you use during that hour depends upon the system you put in (size of tubing; number of ports) and the drip rate you set.

I was planning on building a pond on my property to run a drip irrigation setup until I considered that, for about the same cost, I could put in a couple of large water tanks. So far, it is working fine and I have not had to worry about pond leakage or evaporation.

Your situation might be different. In my case, I focused first on things that would maximize water retention in the area where i put my market garden - putting rows on contour, using compost to create raised planting beds, etc. My rows on contour are for access, not planting. This creates the effect of having mini-swales. This strategy has enabled me to run my drip irrigation setup at a relatively low drip rate and conserve water.

Caveat: I only have had this going for a year, so it is too early to pronounce it a success. However, I am optimistic .

It is unlikely that a whole year's water would be available to put into the pond at any given time; water would be used/ evaporated continuously. So you would have to calculate average inputs/ outputs month by month and see what the largest cumulative total would be, and then add some extra to account for unforeseen events.

Angela Aragon wrote:Jim. Drip irrigation typically is done for finite periods in the early morning and late afternoon to maximize soakage and minimize evaporation. For example, you might run a schedule of 2 X per day for 30 minutes each. That is one hour of water use per day. How much you use during that hour depends upon the system you put in (size of tubing; number of ports) and the drip rate you set.

I was planning on building a pond on my property to run a drip irrigation setup until I considered that, for about the same cost, I could put in a couple of large water tanks. So far, it is working fine and I have not had to worry about pond leakage or evaporation.

Your situation might be different. In my case, I focused first on things that would maximize water retention in the area where i put my market garden - putting rows on contour, using compost to create raised planting beds, etc. My rows on contour are for access, not planting. This creates the effect of having mini-swales. This strategy has enabled me to run my drip irrigation setup at a relatively low drip rate and conserve water.

Caveat: I only have had this going for a year, so it is too early to pronounce it a success. However, I am optimistic .

Hello Optimistic,

Thank you for the help. I priced a 3,000 gallon rain barrel online and I believe that it was about $1500 whereas I can rent a trachoe for $300 and dig a pond.
May I ask how big your garden is? How many gallons of rain barrels do you have for this garden?
I like your mini-swale design. Did you just build raised beds on contour and not dig swales? Do you find that raised beds conserve water?

Best regards,
J

Gilbert Fritz wrote:It is unlikely that a whole year's water would be available to put into the pond at any given time; water would be used/ evaporated continuously. So you would have to calculate average inputs/ outputs month by month and see what the largest cumulative total would be, and then add some extra to account for unforeseen events.

Thank you Gilbert.  

Do you know of any formulas or general guide lines or possible consultants that I can use to calculate my size for me? I have 1.5 acres total land with about 10,000 sq ft cleared for either chicken or pig grazing and or forest gardening. Of all of that I have about 400 sq ft set aside for annual vegetables and herbs. My plan is to design a very low maintenance edible forest garden, with the annual vegetable bed and maybe a couple of chickens and or pigs. All are designed to be very low maintenance and producing just enough vegetables, herbs, berries and animal fats and meats for my wife and I. Thanks again.

Best,

Jim

look into data that local agencies have collected- NC state research/extension, local NRCS office, etc. you may need to adapt to permaculture concepts, but the data and equations available through these public agencies should help with sizing. nc state in particular is known for being pretty progressive (in SE) in the designed-ponds field (rainwater/retention ponds, etc.)
maybe this will help you get started http://iswm.nctcog.org/training/Ponds_Class_082913/Ponds_handouts_combined.pdf

carly bee wrote:look into data that local agencies have collected- NC state research/extension, local NRCS office, etc. you may need to adapt to permaculture concepts, but the data and equations available through these public agencies should help with sizing. nc state in particular is known for being pretty progressive (in SE) in the designed-ponds field (rainwater/retention ponds, etc.)
maybe this will help you get started http://iswm.nctcog.org/training/Ponds_Class_082913/Ponds_handouts_combined.pdf

Hi Carly,

This is a helpful link.  The local Ag Extension Agent is coming over tomorrow to help me identify the plants in my permaculture paradise (lot sounds so boring ).
I will ask him to assist me in calculating my required storage amount.

Thank you and good planting.

Yesterday I had the local County Ag Ext Agent over to identify plants for me. It was a productive endeavor learning the names of various plants and pointing out a particularly non-native evasive plant called a Chinese Prevet.
I could not let him leave without also asking him the question that I have been posing to you folks on this thread which is how much water storage do I need based upon the amount of gardening area that I have. He gave me some good guidelines to follow.

QUOTE FROM AG EXTENSION AGENT

"For every 1,000 sq feet of garden area, 1 inch of rain will yield 600 gallons of water.

In the Piedmont region of North Carolina
For every 1,000 sq feet of garden area that you have you need about 600 gallons of water per week, except July and August when you may need 1.5 to 2.0 times that amount.

However, this is based upon the area that actually has a plant growing on it so you can subtract the rows and other spaces in that area."

For my purposes I am assuming that I may have as much as 10,000 sq feet of garden area some day. I subtract the spacing between the plants and the rows which gives me approximately 5,000 sq feet.
5,000 sq feet * 600 gallons = 3,000 gallons per week of water needed
3,000 gallons * 4 weeks = 12,000 gallons per month of water needed.
In order to account for July and August we better double that so the 12,000 gallons now becomes 24,000 gallons per month of water needed.
My roof will put 600 gallons of water into my proposed tank or pond per inch of rain.
I supposedly get at least 3 inches of rain per month which will give me 1800 gallons per month
The rain will naturally water my 5,000 sq feet garden with 3 - inches of rain per month which will give me  5 * 1,800 = 9,000 gallons per month.
If I build swales whether inbetween the rows of the annual beds and /or in the edible forest garden I should be able to calculate a net savings of at least 25% of that 9,000 gallons per month falling directly on the garden so 9,000 * 0.25 = 2,250 gallons of swale water storage. Along this same thread If I figure out a way to rotate chickens on about 6,000 sq feet of this garden space on land that has swales built on it then I can still use this 2,250 gallons however if I cannot rotate chickens on the area because the swales will leave gaps below the chicken tractor which will let in predators then I pretty much lose a large chunk of that 2,250 gallons of swale storage.

Lets do the numbers.
How much do we need per month = 24,000 gallons

How much do we have = 1,800 per roof + 9,000 gallons per rain on garden + 2,250 gallons per rain on swales = 13,050 gallons

With the 13,050 gallons available
What happens if the 9,000 gallons does not come at the proper time? Lets subtract 25% = 6,750 gallons so 13,050 is now 10,800 gallons available.
It looks like we can get close to our needs every month with the exception of the months of July and August when we will need double the amount of water available.
One option is to collect the water off of our house roof in addition to the water off of our storage building roof. This will give us an extra 3,600 gallons of storage per month.

Since we have enough water available lets calculate a tank size. In July and August we are going to need a total of 48,000 gallons of water.
We can not subtract from this by calculating in rainfall on the garden and the swale savings because August will be already at a deficit. So we are stuck with needing a 48,000 gallon water storage tank which is an impossibility dUe to cost.
I wonder if the Ag extension agent calculated the amount of water needed if we used drip irrigation. Maybe the figure of 600 gallons per 1,000sq feet per week was based upon inefficient sprinkler irrigation?
If anyone has some input here please chime in? Thanks

Okay back.

I asked the local Ag Agent and he said.

"The 1" of water per week is the general recommendation for vegetable gardens no matter the source of the water. That could be rainfall, sprinkler or drip. The drip calculation would be made on the actual planted area vs. the entire garden if using a sprinkler. A garden bed that is 30 feet long and 3 feet wide would require about 62 gallons of water per week.

I don't usually recommend investing in water storage because it is very expensive and there will be many times during the summer that you can not capture enough water to meet the garden requirements. It is fine to capture what water you can, but the rainwater collection will not be able to supply all the water the garden needs. A drip system connected to the house water supply is the least expensive and most dependable way to water the garden.  

Berryhill Irrigation is the vendor most of the farmers in this area use for their drip systems.

http://www.berryhilldrip.com/    "

This puts me in a difficult position because I do not have an adequate well for irrigation, too shallow.

I now plan to design my site based on the exact water needs of each plant I wish to grow.

Here is a site that will help me https://www.growwater.org/plantneeds.html   along with the book "Edible Forest Gardens" and the weekly ET evapotranspiration rates of my area which I hope the ag agent can give me.



*Sorry but I have to go to work now but will continue this at a later date*

Thanks,