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Question: Could you tell me how to determine the volume of water in a local lake like Afton Lake?
Water is a key resource so it is important to know the volume of water in our lakes and reservoirs. While you can't know the exact amount of water in a lake, you can estimate the volume if you know the average depth and surface area. If a lake had the shape of a rectangular swimming pool that had a uniform depth, the volume equals the length times the width times the depth. Since the length times the width is equal to the surface area, you can also multiply the surface area times the depth to get the volume.
To estimate the average depth, take readings at regular intervals and find the sum of all your readings. Divide this by the number of readings to find the average depth. In a Lake like Afton you could do this in a row boat with a sinker tied to a piece of fishing line. Don't forget a life jacket. Since Afton Lake is somewhat circular in shape you could approximate its average diameter from a map or by making measurements from the shore. The area would then be equal to the diameter times 3.14, which is the mathematical constant known as pie. If your estimates are in feet, the volume would be in cubic feet.
Lake volume is not constant throughout the year. In Afton, June tends to be the month when the volume peaks. February is usually the month when it is at its minimum. The volume depends on how much evaporates due to heat from the sun, how much rain falls, and how much flows in and out via streams that feed and drain the lake.
Scientists who estimate lake volumes have special equipment that makes their work easier and more accurate. They use sonar to measure the depth. Sonar, which is short for sound navigation and ranging, measures the time it takes for sound to bouce off the bottom. The deeper the lake the longer it takes. The speed of sound in water times one half the time it takes for a round trip equals the depth. Scientists also use motion sensors so they can take readings at regular intervals as their boat moves.
If you would like to see the type of equimpent scientists use you can visit the National Oceanic and Atmospheric Administration's web site for its research vessel the Thomas Jefferson. http://www.moc.noaa.gov/tj/index.html
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