1.1 Flow Equation

In an irrigation system, the major goal is to deliver a quantity of water from the source to the sprinkler head or trickle emitter in an efficient manner. Flow of water in a closed pipe can be described as a flow rate, i.e. volume per unit time. Imagine cutting through a pipeline and catching the water coming out in one minute. If one catches 10 gallons in one minute, then the flow rate is 10 gallons per minute. The cut exposes the cross-sectional area of the pipeline. The water passing through that cross-sectional area in one minute was 10 gallons.

Flow is therefore a function of water velocity and the pipe cross-sectional area.

The flow equation helps to illustrate the relationship of velocity and area to the flow:

Q = 7.48 x V x A


Q = quantity of water flow given in gallons per minute (gpm)

V = velocity of water given in feet per minute (fpm). Charts may use feet per second (fps).

A = cross-sectional area of the pipe given in square feet (sq ft)

Note: 7.48 = conversion factor to change cubic feet to gallons. There are 7.48 gallons of water in a cubic foot.


In our example above, the water velocity through the pipe can be found by dividing the flow rate (10 gallons per minute) by the cross-sectional area of the pipe in square feet. Water velocities are shown in a commonly used Friction Loss chart that will be described soon.

So, the area and velocity determine the volume per unit time (flow rate) of water that passes through a pipe.

In the equation above, one can increase the velocity and decrease the area, or vice versa, without changing the flow.

In the practical world, it says we might use a smaller diameter pipe and save money by requiring the water to move faster. However, later discussion of friction will illustrate additional power (energy) is needed to put water through a pipe at a higher velocity while maintaining a given end point usable water pressure. This subject confuses people and needs careful reading and understanding.