 The right combination of plumbing and pumps is essential.
A pump circulates water against the resistance caused by gravity and friction. The pump has to 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 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 (this may seem strange since it has nothing to do with height or gravity, but it's convenient to use the same units for both forms of resistance so that they can be combined and compared easily.) Dynamic head is also 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.
The total resistance to flow caused by gravity and friction combined is called the total dynamic head and, since it includes the dynamic head as a component, it increases as the flow rate increases. The total dynamic head (TDH) is the total resistance a pump will face when delivering a given rate of flow. In general, a given pump's flow rate will decrease as the TDH rises. This relationship between TDH and flow rate defines the "power curve" of a pump.
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