There are a wide assortment of pond pumps available and most are rated in much the same way; by the capacity, or volume, of water they can move in a minute or in an hour. The abbreviations GPH or LPH, correspond to gallons per hour or liters per hour and are used to measure the rated performance and overall capacity of pond pumps.
Another common term used when rating the performance of pond pumps is the maximum head pressure or head height or lift. There are a few terms that describe basically the same thing which is a measure, to put it simply, as to how high the pump could push water straight up. Head height is a relative measure of the resistance to flow caused by the friction of water through a hose and gravity as water is pushed upwards.
All pond pumps have a maximum head pressure rating, which is the approximate measure of the distance in feet or meters that a pump can push water. There is more than just distance to consider; as water is pushed through tubing or hose there is a friction caused by the wall of the pipe and there is a drag created every fitting, corner, elbow, tee and joint in the fitting as your plumbing turns which adds effort to your pump and impacts the total flow that is available as the water exits though your plumbing to the waterfall or filter or outlet. As this head pressure increases the pump volume performance is reduced and this ratio can be shown on a flow chart graph which many pump manufacturers use to quickly help you determine how to choose the best pond pump. Knowing the total dynamic head pressure of your application is absolutely critical when sourcing the pump. Every pump has a "shut off" point where it can no longer push the water. If a pump is rated to a "lift" of 15 feet and you put a tube on the end of the pump that was 16 feet high the pump would not be able to push the water up and out the end of the pipe. Of course if the 16 feet of pipe was horizontal the pump would work fine but stand the same pipe vertically and the water won't exit the pipe and the pump will either shut off or start to burn out.
The effect of the Earths gravity on the "lift" or head pressure is fairly simple; for every vertical foot of distance the pump moves the water you are adding one foot of head pressure so the ratio is a 1:1 ratio. The effects of the friction, caused by water as it travels through your hose or pipes, on the total head pressure is a little more difficult to calculate especially as there are slight variations in pipe friction in different hose materials and the smoothness of the inner bore. Basically. for every ten feet of pipe through which the water has to travel travel horizontally will contribute 1 foot of head height; the ratio of the pipe friction loss is a 10:1 ratio.
Plumbing fixtures and bends and corners in your hose also increase the total head you must calculate to ensure the proper final volume from your pump. Every corner with a 90 degree elbow in your plumbing will add 1 foot of head pressure with a 1:1 ratio. 45 degree elbows, tees and even insert couplers can all have an impact on the final flow.
If you install a pump 40 feet away from the top of your waterfall which is 6 feet above the pump and the tubing is a single run of 40 feet horizontally then you add 4 feet of head for the tubing length (the 10:1 ratio) to the 6 foot differnetial between the pump location and the final height of the waterfall so your final total dynamic head calculation would be 10 feet. This means your final volume of water flow in this water feature or application would be the volume of flow on the performance curve that equaled the gallons per hour at 16 feet. This volume will certainly be much less than the initial volume the pump can move at an open flow or a zero head.
If in the above example your 40 feet of horizontal tubing run also required 3 elbows of 90 degrees then an additional 3 feet of theoretical head would be added and your final flow result would be at 19 feet on the performance curve of the pump. In this example you would want to choose a pump that has the desired GPH rating at 9 feet of head pressure. Tubing size is also an important factor in accounting for head pressure loss, in general you should never reduce the diameter of the tubing below what the output size of the pump is, this will drastically increase head pressure, and reduce pump performance. For maximum pump performance, using the largest tubing that is practical is the best choice. A best practice is to use a hose with an inner diameter that is the same as your pumps outlet fitting.
Pumps By Flow
A good rule of thumb for small ponds is to turn over the full volume of the pond water twice every hour to ensure proper circulation and help buffer water temperature fluctuations. Your filtration system, waterfall height, hose lengths and diameters and plumbing components that are to make everything operate properly will consume a portion of the pump's overall flow. The rated flow of a pump is often with no restriction and shows the gallons per hour or liters per hour of the water as it comes right out of the pump. Any given tubing system, plumbing fitting elbows or tees, filters or opening that water moves through or around creates a drag. Frictional loss measures the amount of water flow rate lost to drag. Static head refers to the distance between the pond water surface and the highest point to which the water is lifted. These two factors added together create the total dynamic head and the final tally of total dynamic head that you measure directs you to select a pump often with with a flow rate greater than you might presume. You can restrict an excess flow, if desired, but you cannot make a pump deliver a higher flow rate than its capacity.