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A pump circulates water against the resistance caused by gravity and
friction. The pump must overcome gravity to raise the water above the
pond's surface. The higher the water is pumped, the more
resistance the pump will face, and the less water will be delivered per
minute. The resistance to water flow caused by gravity is measured in
feet and is called the static head of the circulation system. The static
head is measured from the surface of the pond (or other water source) up
to the height to which the water is eventually pumped. The friction of the water moving through pipes also causes resistance
to flow. A small diameter pipe causes more friction than the same length
of a larger pipe, and the resistance increases as the flow rate
increases. Because the resistance due to friction is not constant at all
flow rates, it is called dynamic head. The dynamic head is also measured
in feet and is increased by changes in direction (such as elbow joints
or curves in flexible tubing) and by any constriction of the plumbing
system. Things like valves, pressurized filters, and pipe unions can all
increase dynamic head.
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