Along with "flow rates" you also have "head pressure" this term is often used in pump literature to describe how powerful a pump is. "Flow rate" is simply the volume of water a pump can move during a given amount of time.. To Determined your water flow for your watercourse or waterfall, use a hose and bucket.
(For example, lets say it has taken you 2 mins to fill a 20 liter bucket at a particular flow that looked good for you, all you do then divided this to liters per min. 1min = 10 liters ) now you know your flow rate.

"Head pressure" on the other hand is more complicated. "Head" may be simply defined as any resistance to the flow of a pump. When pump manufacturers list the head pressure, they are referring to the vertical discharge pressure head. Described in very simple terms, a pump's vertical discharge "pressure-head" is the vertical lift in height (usually measured in feet of water) at which a pump can no longer exert enough pressure to move water. At this point, the pump may be said to have reached its "shut-off" head pressure. When you look at a flow curve chart for a pump, the "shut-off head" is the point on the graph where the curved line becomes horizontal as the flow rate at that point is zero. The higher a pump's head pressure, the more powerful the pump.

To use flow rate and head pressure to help you select a water pump, you first need to know:
1) how much water you need to move through your system, in other words the volume.  (volume of your pond or water feature and filters combined),
2) how many times per hour you need to turn over the volume (flow rate - typically three to five times per hour, this can vary depending on fish quantities etc.)
3) how much resistance (head) the pump will encounter as it moves water from point A to point B.

With all this information in hand you will be able to make an informed choice about which pump would suit you.
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