What if you wanted to know the flow of an existing waterfall, perhaps because you want to recreate the same effect? You can measure the thickness of the water flowing over a spillway or fall rock. Turn a ruler or tape sideways or perpendicular to the flow to get as accurate a reading as possible. (Remember, the water will be in motion.) For an irregular surface, measure every 4 — 6 inches and take the average depth of the water spilling over the top of the waterfall rock or spillway. Just like before, multiply by the width of the waterfall to get the total flow.
- ¼” — 3/16” depth of water over spillway ≈ 50 gph/inch
- ½” — 5/8” depth of water over spillway ≈ 100 gph/inch
- 1” — 1¼” depth of water over spillway ≈ 200 gph/inch
For a more accurate reading on smaller falls, it’s sometimes possible to measure the flow directly using a piece of scrap liner and some gravel. Turn off the pump and cover the falls with the liner, anchoring the upstream edge with gravel. Fold the rest of the liner into a 5-gallon pail and turn on the pump. Measure the number of seconds it takes to fill the pail. (You will get wet.) The flow in gph will be equal to the number of seconds in an hour, times 5 gallons, divided by the time it takes to fill the bucket in seconds:
GPH = 3,600 sec/hour x X gal / Y sec