Jason,
It is recommended to use at least the next size tubing up from the pump outlet diameter to maximize flow. If your pump outlet is 1/2" diameter, then you should be using 3/4" tubing if not larger.
Since fittings like couplings or elbows and valves and even the filter itself restricts flow, the larger you go with the tubing the better as you can make up some of the losses created from the fittings
and filter by reducing the losses created by the tubing.
The coefficient of drag (or friction of the walls of the tubing) is of importance as this can reduce the flow rate. You want LAMINAR flow restrictions only - which is just the restrictions based on the confines of the cross sectional area of the tubing and not turbulence or friction from the side walls. A smooth walled tube is best. Corrugated tubing, if it is corrugated internally, will create eddy currents and turbulence which will restrict the water flow.
Here is a helpful statement from the Pondmaster pump website explaining a bit of this and some other highly informative details. I obtained some of the information for my figures from here:
What Does Head Height (shutoff) Mean
All pumps produce water flow and pressure. The water flow is easy to see when you plug in your pump especially as it flows over your waterfall. On the other hand, pressure cannot be seen and is sometimes difficult to visualize. Pumps are rated in Head Height which is an easy way to visualize the pressure a pump produces. One pound per square inch of pressure will lift a column of water 2.3 feet in the air so that a pump that has a shutoff height of 34.5 feet produces 10 psi of pressure. This is important to understand because the higher a pump has to lift water, the less water it can deliver. Naturally, a pump will deliver more water to a lower waterfall than a higher one. Head Height charts or graphs tell you exactly how much volume a pump will deliver at any given Head height. You may see 2000 gph @ 5' of Head.
In designing your pond you will need to know the head height of your own system. It is not enough to know just the highest point above the pond as there are other variables to consider. The water from the pump is carried to the waterfall via tubing, and the size of the tubing and the number of fittings will come into play in determinging what pump you will need. Both tubing and fittings cause friction resulting in a reduction of water flow which in turn can cause the pump to work much harder producing less flow. In order to keep this friction loss to a minimum, always increase the tubing at least one size larger(if possible)than the outlet of the pump. If the outlet is 3/4", use 1" or larger plumbing. If it's 1-1/2", go up to 2" tubing. There are several shortcuts in computing the total head required.
For every 10' of Tubing, add 1 foot of Head
For every Check Valve, add 1' of Head
For every fitting, add ½ foot of Head
So, if your waterfall is 5 feet tall, with 20 feet of tubing and two elbows and a check valve, your system would have a total head of 5 + 2 + 1 + 1 = 9'. with this information you can check the manufacturers charts to see which pump gives the flow you need at 9' Head. Keep in mind that in this example we did not calculate the addition of a pond filter which in itself can reduce the flow as much as 20%. In most cases it is better to opt for a larger pump as you can generally reduce the flow but if your pump is not large enough there is nothing you can do to increase the flow.
Hope this helps you and others.
Catfishnut
