I know that a lot of what i'm about to post might as well be in latin for most people, by I assure you it is very easy to do the calculations once you figure out all the symbols and coefficients.

The first thing you will likely have to due is convert your flow rate (Q) from gallons per hour (GPH) to cubic feet per second (CFS). 1 cubic foot is 7.48 gallons and 1 hour is 3600 seconds, so divide your gph by (7.48 x 3600); or multiply your gph by 0.00003714.

Flow (Q) is equal to Velocity multiplied by the cross sectional area of your pipe; Q=VA. Contrary to what I said in my previous post, there will not be any headloss due to pipe friction if you use PVC as for all intents and purposes it can be considered smooth.

z2-z1 is the change in elevation, z1 is pump height (taken to be zero usually), z2 is the height of the outlet.

Okay heres the textbook stuff, I left it as big as I could so you can read it easily. (Engineering Fluid Mechanics, 8th edition, Crowe/Elger/Roberson, 2005)

Questions?