Surface area is one of the most important pieces of information that a manager needs to appropriately manage a pond. Yet it is surprising how many people estimate or guess a pond's surface area rather than measure it. Many pond management decisions depend upon accurate surface area measurement. These include fish species selection, fish stocking rates, fish harvest rates, aquatic plant herbicide treatment rates, rotenone application rates, clay flocculation chemical treatment rates, alkalinity adjustment lime application rates and phytoplankton fertilization rates (lime, gypsum, alum, rotenone and some herbicide applications also require relatively accurate average depth measurement). When pond surface area is estimated, rather than measured, subsequent management is generally no better than the initial guess.
Pond surface area typically changes little over the years. Once measured, the information can be filed and reused many times for future management decisions. Pond surface areas can be obtained from multiple sources. The Noble Research Institute digital imaging specialist usually calculates pond surface areas for Agricultural Division cooperators using USDA aerial photographs. The USDA Natural Resources Conservation Service and the USDA Farm Services Administration also can measure surface areas of ponds using their aerial photographs.
Some ponds have been constructed or modified after development of the most recent USDA aerial photographs, and some managers do not want to ask for help. In these situations, pond managers can measure dimensions of ponds and calculate surface areas using geometric formulas that fit pond shapes. When a single geometric shape does not fit a pond, the pond can be divided into segments with each segment measured separately and the individual areas added together to determine a pond's overall surface area.
The following geometric formulas commonly apply to pond or pond segment surface area measurements:
- Rectangle = length x width
- Triangle = 0.5 x width of base x height or length
- Circle = 0.7853982 x (diameter)2 or 0.0795775 x (circumference)2
- Ellipse = 0.7853982 x length of short axis x length of long axis
When dimensions are measured in feet, the products of these formulas are expressed in square feet. Divide the square footage by 43,560 square feet/acre to convert to acres. For most management purposes, surface areas should be measured to the nearest tenth of an acre.