Selecting a pond pump for a waterfall. If you would like to test how much water looks best for your waterfall do the following .....

i) Place a hosepipe into the waterfall inlet
ii) Adjust the flow until you are happy with the effect
iii) Without closing the hose fill a known volume container from the hose
iv) Calculate the flow rate by dividing the volume of the container by the time taken
to fill it. This week's free calculator could come in useful here.
v) This calculator would also allow you to weigh the container as an alternative to
knowing its volume. For water use the following:

1 US gallon weighs 8.83 lbs
1 Imperial gallon weighs 10 lbs
1 litre weighs 2.2 lbs

vi) If one hose does not deliver sufficient water then use 2 hose pipes.

You can also estimate a good approximation to the flow over the lip of the waterfall by multiplying the width of the overflow lip by a factor. Here are the factors

For every 1 inch of waterfall lip width multiply by 50 gallons per hour to get the right flow rate

For every cm of width waterfall lip multiply by 100 litres per hour