How to Calculate Water Flow

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The calculation for determining the flow rate of water is Quantity = Velocity x Area. This formula works when determining how much water is flowing through a pipe or stream. Many farmers want to know how much water is being pushed through a stream bed at any given time so the water can be used to create water powered generators or pumps.

Things You'll Need

  • Stop watch or time measuring device
  • Yard stick or measuring tape
  • Small plastic ball
  • Rope or string

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Determine the velocity of the water. This is done by marking off a 20 foot section of the stream or pipe. Drop the ball at the beginning mark. Using the stop watch, record how many seconds it takes the ball to reach the end mark. Divide 20 by the number of seconds and multiply by 0.7. For example, if it takes 15 seconds for the ball to travel the 20 foot distance, then the calculation would be 20/15x0.7=0.933. Repeat this process at least three times to gain a more accurate measurement.

Measure the area of the stream. Using the same area of stream used in the first step, tie a rope or string from one side of the stream to the other side. Divide the rope into equal sections and mark these sections. Using the measuring stick, determine the depth of the stream at each of the marks.

Calculate the average depth of the stream by adding all the measurements together and dividing by the number of sections measured. A 6 foot stream measured 5 times with depths of 3/4 foot, 1/2 foot, 1 1/2 foot, 1 foot, and 3/4 foot would be calculated as: 3/4+1/2+1 1/2+1+3/4=4 1/2 feet divided by 5 for a total of 0.9. Multiply 0.9 by 6 (the width of the stream) for a total area measurement of 5.4 square feet.

Use the numbers gathered to process the equation. In the examples above, the calculation would look like this: 0.933 x 5.4 = 5.0382 gallons per hour. This is an estimated calculation to show how the formula works.


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