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.