How to Calculate Catchment for a Pond
Catchment refers to the amount of area that feeds a particular pond. It is very important that you are aware of the size of the catchment area for a couple of reasons. For one, if you are building a pond that will be fed with surface runoff and rainwater, it is a good idea to know if you have enough catchment to keep a pond of the size you are designing filled. You also need to know how big of a spillway will be needed to accommodate a 100 year storm in your area. The amount of catchment is needed to figure this out.
The easy way to figure this out is with a contour map. You can find these on google maps, but you can also make your own with a laser level and some map making skills. Basically, you need to look at the contour lines, and draw arrows at a right angle to the contour line, to show where the water will flow. Any water that flows in to your pond at right angle to the contour line is part of your catchment area. Then simply measure or estimate the total area and that is your area of catchment.
My area of catchment for the pond I am installing ended up being about 2 acres. As you can see from my map there is part of my catchment that is coming from the property above me, which is not on the map, but I included where it would be if it were. By installing a drain along the back of my house to divert more water to the pond, I am adding another ½ acre to my catchment and getting that water away from the back of my house.
Now that we have calculated our catchment, we should calculate the gallons of water our pond will hold and approximately how much runoff water our pond is going to receive in a given year. How many gallons of water your pond will hold is a pretty simple calculation.
Length X Width X Average Depth x 7.48 gallon converter= Volume in Gallons
For my pond: 60 feet x 30 feet x 3 feet deep x 7.48= 40,392 Gallons
So my little pond is going to hold about 40,000 gallons of water, if there is enough catchment to fill it. So now you can determine the catchment area runoff.
Square Footage of Catchment x Depth of average rainfall per month .3 feet x 7.48 x 58%=
(The 58% is the runoff percentage based on my soil type and existing vegetation type as it is written in USDA Ag handbook #590 entitled Ponds- Planning, Design, Construction)
90,000 x .3 feet rain in my area x 7.48 x .58%= 117,137 Gallons of runoff directed to my pond per month
I should be able to fill my pond, and keep it filled based on my catchment and runoff calculations. I used my monthly rainfall amounts as opposed to my yearly amounts because evaporation is a big factor in keeping your pond filled, so shade can really help a pond especially in dry lands. Also, all dams will leak a little.
A simpler way to get an idea of whether or not you can keep your pond filled is to consider the following. For a surface runoff pond, ideally you have at least 15 times the catchment as the size of the pond. For example, if you had 15 acres of land that would eventually drain into your pond, you could theoretically have a 1 acre pond. Now this is a very crude way of looking at it, as you have to take rainfall and shade as well. For example, if you have a shady pond site and you receive 70 inches of rain a year, you may not need as much catchment, but if you are in a dry environment without shade, your pond would dry up with that catchment. For my 1800 square foot pond with 90,000 square feet of catchment, I end up with 50 times the catchment area. Incidentally, if you changed the size of my catchment to 17 times for my 40,000 gallon pond, you would get about 40,000 gallons of runoff a month. So while the 15 times number is crude, it is actually pretty accurate in my case.