Building your own waterfall can be a rewarding experience however dealing with water can be tricky so you must arm yourself with as much information as possible in order to build something that you will be happy with.
Minimum flow rates
For every inch of width of your waterfall you should have a minimum of 100 to 150 gph (gallons per hour) of water supplied by your pump. Example waterfall: if you want the top bowl in your waterfall to have a 10 inch wide area where the water rolls over and down your waterfall, you would need a pump that can supply 1000gph. Additionally you need to understand how pumps are rated to ensure that you get the desired effect that you are looking for. There is an efficiency loss in the movement of water through pipes and you will need to account for this to make sure that your waterfall receives the water flow it needs.
Pump ratings and Head loss
The maximum rating for your pump, say 1000gph, refers to a pump with zero head pressure. Head pressure refers to the weight of the water that the pump is lifting and pushing. If the pump has to lift the water a long way it will produce less gph. Simplified, the pump will have a rating that refers to the amount of vertical climb the water will need to make. It will supply a chart in the pump specifications which states the zero head pressure gph, but also the gph to expect with one foot of vertical lift, or two feet of vertical lift, or five feet of vertical lift and so on.
Fittings cause head loss also
Another factor in this equation is fittings such as ninety degree elbows, forty five degree elbows, and bends in the pipe or valves also will increase the head pressure. A pump that has to lift the water one foot, but also through a series of four or five 90 degree elbow fittings will have significantly less gph than a pump pushing through a straight run of pipe.
The height and width of your waterfall will determine your pump requirements
For this reason you need to determine how high your pump will need to push the water to reach your top bowl and also how wide you want the spillway in your top bowl to be. This will give you an idea of what size pump you need. Assuming the same 10 inch wide spillway and a vertical climb of five feet to reach the top bowl, you would need to find a pump that can produce a minimum of 1000gph with a head of 5 feet. This means that the label on the pump would likely be around 1250 gph referring to the zero head pressure rating for the pump with a performance of about 1000gph with five feet of head.
Calculating head pressure from pipe fittings
It is recommended to take into consideration that you will have fittings, bends and restrictions in your plumbing to have it reach from the pond up to your top bowl so this will also increase the head pressure. In larger applications the number of bends in the pipe, length and fittings being used are all calculated to determine the exact head loss for the water to reach its destination. For smaller applications like your waterfall this calculation is unnecessary. When building your plumbing from your pond to the top of your waterfall try to use as few fittings and bends in the pipe as reasonably possible. Especially try to avoid having fittings back to back like two or three ninety degree bends in a short section of pipe. Fittings and bends in short succession because more head pressure than the same number of fittings spaced out further. For simplicity do not calculate the number of fittings that you will use to build your waterfall, but instead oversize your pump by a minimum of ten percent.
Now the whole plumbing equation together
If you have a 10 inch waterfall and a lift of five feet, and you account for 10% additional flow being lost in the fittings and pipe run you would conclude the following.
1000gph at a head pressure of five feet will be a pump advertised with a higher flow rate like 1250gph, again referring to the zero head pressure. To account for your fittings you would need 10% more flow, or another 125 gph more than the pump rated for 1250 gph. This means that the minimum pump rating you would consider would be 1375 gph. Round this number up to 1400, or maybe even 1500 gph rating as pumps are typically sold in increments of flow rate and you most likely will not find a 1375 gph pump.
The reason that you need to consider this at this stage is because you need to determine what size of pipe to run in your waterfall form from the pond to the top bowl. If you have estimated the flow rate that you will need to accomplish the scope of the waterfall you are planning you can find the pump that you will need to accomplish this from your local hardware or pump supply store. The type of pump best suited for most DIY waterfalls is a submersible pump. It will need to sit directly in your pond and as a result a black pump is better than a light colored one which will be highly visible. It is possible to use remote pumps like what a swimming pool might use but this will increase the difficulty of installing the plumbing system considerably as well as increasing the project cost as well.
Once you locate the pump that you will use for your waterfall you can now begin to install the plumbing system as well as consider details like creating an electrical chase to allow the cord from the pump to be installed out of sight as opposed to hanging out the side of your pond.