To Size a Pump for your Water Feature Follow These Basic GuidelinesTo size a pump you should have some understanding of “Total Dynamic Head” or TDH. TDH is how much resistance to the flow of water the pond system produces. TDH is the total sum of three components: Static Head + Friction Head + Pressure Head.
A. Static head is the highest point in feet above the surface water level.
B. Pressure head is the resistance to flow as the water travels through filters, UV lights, and other pond appliances.
U.V. Light - 1 to 2 ft.
Gravity style filters - 1 to 2 ft.
Pressurized filters with foam pads - 5 ft.
Pressurized filters with plastic media - 3 ft.
C. Friction Head is the resistance to flow that is produced when trying to push water through a pipe (hose).
Add: 1 ft. of head to every 10 ft. of pipe used in the system.
Resistance increases by the number of fittings, elbows, valves, and unions used. Flexible PVC pipe is preferred because it reduces the use of fittings. GPH should be matched to the appropriate pipe for your pond system. See chart below.
Now determine TDH. If your waterfall is 5 ft. above the top of the water system your static head is 5 ft. If you are using a gravity style filter add 2 ft. and using a U.V. light add another 2 ft. then your pressure head would be 4 ft. If you use 17 ft. of pipe between your pump and filter then your friction head would be 1.7 ft.
Add static head, pressure head, and friction head to come up with TDH. The total TDH for this example pond system would be 5 + 4 + 1.7 = 10.7 feet.
At this point you should have a pretty good idea how wide your waterfall should be. ¼ inch deep-water flow over flat slate rock requires 169 GPH for every inch in width of waterfall. Note: if a stream is filled with loose stones it will require more GPH to fill up voids between stones before water will be on top of stream stones.
If your waterfall is 22 inches wide you would need 169 GPH x 22 inches = 3718 gallons delivered into your waterfall.
To select a pump refer to either a performance table or curves to find a pump that will deliver 3718 gallons at 10.7 feet of head.
Selecting the correct pipe is critical to pump performance.