For every foot of waterfall width, one will require 1500 gph for a 1/2 to 3/4" depth water fall.
Yes, head pressure, pipe lenght, elbows, tees etc. all take in effect to how the pump will run and what is required.
We have a pump calculator for waterfalls on our web site for an easy way of calculating this. See my signature for the web page.
I have built for a customer a 35 foot long, 35 foot high water fall that used a 5000 gph pump (3/4 hp) and a 2" flexible PVC Pipe. It does a great job and is on its 4th year running.
Happy ponding!
