A common question people often ask is how one goes about sizing a pump to use in a pond or fountain. What constitutes TOO MUCH water flow, or TOO LITTLE?
If you buy a fountain kit with a container, the math has been done for you and you simply have to install it and make a minor adjustment to the pump. If, on the other hand, you replace a pump that has stopped working, or you create a new water feature and need a new pump, you have to do the math yourself.
Begin with a rule-of-thumb – Circulate all the water in the pond EVERY two hours for maximum aeration and filtration (either mechanical or biological, a topic for another time).
Start by measuring pond size
Before you do anything else, you need to know two things about your setup:
1] the size of the pond (cubic feet and gallons); and,
2] the height you want the water to rise, or discharge height.
To calculate the first unknown, pond size, cubic feet is estimated
by multiplying the average length by the average width by the average depth, or L x W x D = C.F. If you measure in inches, remember to convert the measurements to feet first! [For example: an 8 foot by 10 foot pond that is 2 feet deep would be 8 x 10 x 2 = 160 CF].
Then compute the gallons
Once you know the cubic feet in your water feature, calculating the pond volume in gallons is easier. Multiply the constant 7.48 (from 7.48 gallons of liquid in a cubic foot) by the cubic feet you just calculated. In our example, 160 CF x 7.48 = 1196.8 Gal. You will need at least a 600 Gallon per hour (GPH) pump to move the amount of water you have, if you turn it over every two hours.
Now figure in discharge height
The packaging the pump comes in will provide two important pieces of information for this calculation: 1] the GPH for each head height; and, 2] the maximum height the unit can pump. At either of those maximums, most pumps will only deliver a small proportion of what the pump is capable of producing. Obviously, the lower the height the water has to travel, the more output the pump will produce. To get a lot of height of travel, expect to put a LARGER pump on the system.
Other considerations
– Most pumps are adjustable. They will have something like a slot or flap to determine how much water gets into the pump in a certain period of time. A small pump “wide open” may produce the same stream as a larger pump “closed down.” You may have to experiment with this concept to get it right, so ask around and play with water features a while before you run out and buy a lot of expensive pumps (and most ARE expensive!). Serious water gardeners tend to “collect” pumps
for some reason. I’ve never figured out why, exactly, but I never
seem to have problems finding a few to experiment with.
– For waterfalls, the recommended flow over the spillway (the weir) is about 50 GPH for a thin sheet of water, up to about 150 GPH for heavy flow, a one-inch thick sheet of water. If your waterfall was a foot across, a heavy flow would require about 1,800 GPH from the pump (12” x 150 GPH).
– Never install a pump that isn’t plugged into a GFCI (Ground Fault Circuit Interrupter). This a special electrical plug that shuts off to prevent accidental electrocution in the event of mixing water and electricity in any way.
– In these times of drought (or near drought, depending on where you live), turning your pumps, waterfalls and fountains down, or even off, can cut down on water loss. Turning the water on intermittently can still aerate and filter properly, and you can add other units to the pond to aerate in place of moving water (fish tank and stock tank aerators, air bubblers, and the like).
Moving water, both the motion and sound of it, is one of the more enjoyable aspects of gardening for many people. Just look around at the expanded offerings in desktop fountains and other similar devices in every store you can imagine. There is even something healthy and relaxing about it in our hectic work schedules. Just don’t waste the water while you’re enjoying it!