How To Choose a Garden Pump for Waterfalls, Streams or Cascades

Most man-built waterfalls are unique. This means specifying the right pump is also unique. By the way these days it is generally easier and even better to look out for those streams, waterfalls and cascades made from fibreglass or polyurethane that have stunning natural rock, slate, stone or log forest type finishes.

The Importance of the Waterfall Overflow

Take a look at the picture to the left and you see that there is only a trickle of water flowing over the edge (or lip). It is the width of this overflow that determines the volume of water you need to pump to the waterfall. Of course you can create as many different effects as you like ... in this case the own chose to have a reduced flow.

There is a "rule of thumb" to help you decide on the flow rate. The formula is ...

Width of overflow in cms x 100 litres per hour = minimum flow rate.

Obviously if you want a larger flow then use a factor of 150 or 200 or whatever you think you might like. Maybe you want a mini Victoria Falls. However do not go below the 100 used in the equation.

• Example ... Your waterfall has 3 different places where it overflows (ie 3 lips). The first lip is 3 cm, the second 5 cm and the third 10 cm. What minimum water flow rate is needed for a waterfall to look and sound good?

• Flow rate required = (3 + 5 + 10) = 18 cm x 100 litres per hour

• Flow rate must be at least 1,800 litres per hour.

But what if the pond contains koi and the total volume is 10,000 litres?

If you refer to the article about pumps for ponds I suggest that the flow around a koi pond should be equal to the pond volume per hour. In this case 10m,000 litres per hour. Obviously this would be too much for the waterfall above so you would need to split the flow from a single pump capable of pumping 10,000 litres per hour or use 2 pumps ... a smaller pump (1,800 litres per hour) for the waterfall and a large pump (about 8,000 litres per hour) just circulating water through the pond filter.

Knowing the Water Flow is Only Half the Story

Once you've determined how much water you want to flow over the waterfall you need to know how high the pump must be able to deliver that volume of water to. This is normally referred to as pump head. In general for any submersible pump the higher you want it to pump then the less water volume it can push to that height. This is because the pump motor can only deliver a certain fixed amount of energy which is divided into lift and volume ...

• high lift and low volume or

• low lift and higher volume

These are the basic choices. I often use this analogy ... Carrying one bucket of water up 100 steps uses the same energy as carrying 2 buckets of water up only 50 steps.

IMPORTANT Pump Head

The head for a practical waterfall installation is the vertical distance from the POND SURFACE to the point where water comes out of the pipe onto the waterfall itself (not the lip or overflow of the waterfall). You do not include the length of pipe and you do NOT measure from the bottom of the pond.

Let's continue with the example above ... we worked out we need 1,800 litres per hour. So now we measure the straight up distance from pond surface to pipe outlet and record this measurement as 1 metre.

At this stage we have almost enough information to specify the right pump for this single waterfall ... we want a pump that will deliver 1,800 litres per hour at a height of 1 metre from the pond surface (this is the head). In practice if we just used this information to specify the pump we would find that the flow would be less than 1,800 litres per hour. Why?

Pipe Friction Losses

Remember what was said above about the fixed amount of energy from the pump motor being divided between height and volume. Well some of that energy also goes into pushing the water against the sides of the pipe connecting the pump to the waterfall and the longer the pipe and the lower the diameter the more energy is needed to overcome what are called PIPE FRICTION LOSSES. These losses are very complex to calculate so add 20% to the head as an approximation of the pressure loss due to friction.

In our case we then need a pump that will deliver at least 1,800 litres per hour NOT at 1 metre but at 1.2 metres head. This is the information you need to pick the right pump ... but only after you understand the pump running costs and how to calculate or compare different makes and models. There can be highly significant differences in running costs between pumps that do the same or similar duty.

I have created 12 different pond calculators and these are available free at my other site Perfect Pond Keeping One of the calculators allows you to accurately specify any pond pump even taking pipe friction losses into account. I would suggest you get these Excel calculators and play around with them.

Pond Keeping Units

Now we've started to talk litres you might find this table below useful. It helps to translate one set of units to another quite easily.

Unit	Imp Galls/min	cu ft/min	Litres/min	cu m/min
Imp. Gallons	1	0.16	4.55	0.005
Cu.Feet	6.23	1	28.32	0.028
Litres	0.22	.04	1	.001
Cu. Metres	220	35.32	1,000	1

Example ... 1 cu.m per minute is the same as 1,000 litres per minute or 35.32 cu.ft per min or 220 imperial gallons per minute.

Factors to be aware of with all pumps. This is a good page to read before you buy any pump.