Pond Measurements: Area, Volume and Residence Time

An explanation of basic pond measurements that are vital to basic pond management practices.

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The importance of getting an accurate estimation of your pond surface area cannot be overestimated. The majority of pond owners visually estimate their pond area, which usually results in an overestimate of the true pond surface area. Pond area and water volume should be calculated based on some simple measurements. The effort necessary to estimate pond surface area is directly related to your pond's shape and uniformity. The simplest method--using basic equations for common shapes--can be applied if your pond closely resembles a circle, square, rectangle, or trapezoid in shape.

Pond Shapes

Circular

pond shape can be estimated by measuring the distance around the pond shoreline in feet. Square the shoreline distance and divide by 547,390 to get the pond area in acres. For example, a pond that is 450 feet around the shoreline would have an area = (450 feet)2 / 547,390 or 0.37 acres.

Rectangular or square

shape area is estimated by simply measuring the length and width of the pond sides in feet. Multiply the length times the width to get the square feet of surface area. This value can be converted to acres by dividing by 43,560 ft2/acre. So, a pond that measures 150 feet long and 100 feet wide would have an area = 150 feet X 100 feet = 15,000 ft2 or 0.34 acres.

Trapezoid

Many ponds may be roughly rectangular in shape, but one side may be significantly shorter than the other. The area of this shape is best estimated using a formula for a trapezoid by taking the average length of the two unequal sides and multiplying by the width of the pond. For example, a pond that is 200 feet long on one side, 300 feet long on the opposite side, and 100 feet wide would have an area = 250 feet X 100 feet = 25,000 ft2 or 0.57 acres.

Irregular Shaped

Many ponds have an irregular shape where the surface area cannot be adequately estimated using the formulas for common geometric shapes. Three methods can be used in this case depending on the degree of accuracy you desire. Keep in mind that the accuracy of your pond surface area estimate may be very important, especially for the safe use of aquatic herbicides. The three methods are described in order from least to most accurate. You should strive to use the most accurate method that you can reasonably accomplish.

1. Average Length and Width Method: Take numerous measurements to determine the average length and average width. Make certain you get both the longest and shortest distances in calculating the average length, and the widest and narrowest distances for determining the average width. The more measurements that you make, the more accurate your result will be. The area is then calculated by multiplying the average width times the average length. If you do your measurements in feet, your result will be in square feet. You can convert square feet into acres by dividing it by 43,560 ft2 per acre. Depending on the number of width and length measurements made, the final area will probably be within about ±20 percent of the actual pond surface area.
2. Multiple Trapezoids Method: A more accurate method to determine the area of an odd-shaped pond is to divide the pond into multiple trapezoid shapes. A new trapezoid is defined anywhere the shoreline makes a rapid change in direction. Note that instead of horizontal transects, this method requires measurement of the distance between each vertical transect. This would be most easily done during winter when the pond is frozen and the transects could be easily laid out and measured. This method requires more measurement and effort, but the final area estimate will probably be within ±5 to 10 percent of the actual pond area.
3. Handheld Global Positioning Systems (GPS): Handheld GPS systems have become quite common over the past five years as they have become more affordable. They are now routinely used for outdoor recreation (hunting, hiking, camping, etc.) and navigation. GPS units allow you to determine your exact location on earth using multiple satellites in space. Various locations, or "waypoints," can be stored in the GPS unit for use with mapping software that either accompanies the unit or can be purchased separately. The software can connect the waypoints and calculate the area inside the resulting shape.
4. Geographical Information Websites: There are also geographical information programs on the internet, like Google Earth or Bing Maps, which use satellite imagery to display a map of your pond or lake. These website tools can make it very easy to determine the surface area of your water resource.

A pond surface area could be estimated by walking the perimeter of the pond and stopping at various waypoint locations along the pond shoreline. If waypoints are stored at each location where the pond shape changes, the resulting area will be extremely accurate, probably within 1 percent of the actual pond area. Even if you do not own a GPS system, friends or family members that enjoy outdoor recreation may own a unit that could be used to estimate your pond surface area.

Pond Depth and Volume Measurement

The volume of water in ponds is often expressed in units called "acre-feet." An acre-foot represents one surface acre that is one foot deep. To calculate the acre-feet of water in a pond, you'll need the surface area in acres as calculated above and an average depth of water in the pond. For a typical bowl-shaped pond the average depth can be estimated as 0.4 times the maximum depth. So, a pond with a maximum depth of 12 feet would have an average depth of about 4.8 feet.

A more accurate method for calculating average depth is to make many measurements and calculate an average. This is most often done by measuring the pond depth along two transects--one along the width and one along the length. Make sure to pick transects that represent the shallow and deep portions of the pond. Depths can be measured easily from a canoe or boat using a weight and a string marked in feet. The more depth measures that you make, the more accurate your final average will be. In the example shown on the next page, pond depths were taken at six locations across the pond length and five locations across the pond width. The average pond depth can be calculated as the average of all of these measurements.

The volume of water in the pond (in acre-feet) is calculated by simply multiplying the pond area (in acres) by the average pond depth in feet. Keep in mind that one acre-foot of water is equal to 325,851 gallons.

An even better way to calculate an average pond depth is to divide the pond into numerous (at least four) sub-areas (much like we did in the trapezoid method). Take at least one depth within each of the sub-areas and use these to calculate the overall average pond depth. This method is especially good if the pond bottom is irregular rather than bowl shaped.

Residence Time

Another important measurement for a pond or lake manager to know is the "residence time" of your water resource. Residence time is how long water resides in the pond, from the time it enters the pond to the time it leaves through the spillway or overflow pipe. This calculation is important for making herbicide or liming applications to the pond. If water flows too quickly through your pond, it may not be effective to make these types of applications.

We calculate residence time by using the formula: 226 times the pond volume in acre-feet divided by the overflow rate in gallons per minute. For example, if we have an acre pond with an average depth of 5 feet, and an overflow rate of 10 gallons per minute, our residence time would be: 226 (5) / (10) = 113 days. On average, the water entering this pond stays in the pond for 113 days before it reaches the overflow. In some ponds it may be too difficult to obtain an overflow rate to make this calculation.

A Final Word

Using the methods described in this fact sheet will allow you to calculate the surface area and volume of water in your pond with reasonable accuracy. These numbers are critical for the safe and proper use of various pond management activities such as using aquatic herbicides, liming, fish stocking, and using aeration devices.