Each spring, landowners with farm ponds begin managing their ponds for recreational fishing, livestock water and other uses. Water quality is important and having your water tested can help you decide what to do next. If you know how big your pond is that can be helpful in many farm pond management decisions.
If we know the proper size of the pond, we can know how to apply lime or fertilizer to the pond, control weeds or even how to properly stock new fish species. Avoid exceeding 1,000 pounds of fish per surface acre. Doing some mathematic calculations to determine pond size can help take some of the guesswork out of managing the farm pond.
If your pond is square or rectangular in shape, use the formula for area which is length times width. For example, if the pond is 200 feet by 350 feet, that equals 70,000 square feet. If we take 70,000 square feet divide that by 43,560, which are how many square feet in one acre, then that total would be 1.6 surface acres. If the average depth of this pond is 3.2 feet, then 1.6 multiplied by 3.2 equals 5.1 acre-feet of water.
For a triangular pond, use the formula for area, which is one-half times base times height. In a triangular pond, base is the shoreline length along the dam. Height is the distance from center point of the shoreline along the dam to the upper end of the pond. For example, if the pond averages 4.2 feet and is 200 feet along the dam and 500 feet to the upper end of the dam how many acre-feet? One-half multiplied by 200 feet by 500 feet equals 50,000 divided by 43,560 square feet in one acre for 1.1 surface acres. If we take 1.1 surface acres multiplied by 4.2 feet the pond has 4.6 acre-feet of water.
For a circular pond, the formula for area is 3.14 times the radius squared. The radius is one half the pond’s diameter. For example, if the pond averages 3.9 feet and is 150 feet across the middle or diameter how many acre-feet of water are in this pond? In this equation, 3.14 is multiplied by 75 squared equaling 17,662.5. If we take 17,662.5 divided by 43,560 square feet per acre the results are 0.41 surface acres. Now take 0.41 surface acres multiplied by 3.9 feet and the pond has 1.6 acre-feet of water.
It may be necessary to calculate the average depth of the pond as well to take the guesswork out on how deep the pond may be. The pond may have been built years ago and the depth could have changed over time due to silt and other particles settling to the bottom. To calculate the average depth of the pond, measure the depth in feet using a calibrated rope and anchor or pole marked in feet. Begin each transect at the bank with a zero and end on the far bank with zero. The more transects or the more soundings taken the more accurate your depth estimate will be.
An example of figuring the depth in feet of a circular pond having two transects if the measurement are as follows 0,3,3,6,7,4,2,0 for transect 1 and 0,3,6,6,4,4,1,0 for transect 2. The formula to figure this would be the sum of all soundings divided by the number of soundings. Add all the depths of transect 1 and transect 2 getting a sum of 49 divided by 16 which is the numbers of soundings. This pond has an average depth of 3.1 feet.
For more information on this or to receive a copy of “Pond Measurement Determining Area in Acres, Volume in Acre-Feet, and Average Depth” by Dr. Billy Higginbotham, professor and Extension Wildlife and Fisheries Specialist with the Texas A&M AgriLife Extension Service at Overton, contact our office at 903-590-2980.
Educational programs of the Texas A&M AgriLife Extension Service are open to all people without regard to race, color, religion, sex, national origin, age, disability, genetic information or veteran status.