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