Question
I just had a 6" irrigation well put down.  This well is 140' deep and has 80' of perforation.  The ground water rising up in the well comes to about 11' of the surface.  Is there any way to accurately determine how many gpm this well will produce with a 6" centrifugal pump at ground level.  There will be almost no head because I plan to dump the water directly on to the ground as flood irrigation.  The well paper work states 120+- gpm but, this does not seem to be allot?

Answer
Thanks for the question.
The quick answer is, trust the paperwork. The well driller probably flow tested the well to arrive at this amount. It does require a large investment to be able to test the flow of a well so, your probably not going to be able to do too much without the pump.
This is not to say the well can't produce 150-180GPM, but your well driller probably know best. A word of caution. The pump needs to be set at a point below the "draw-down" of the well. When a sub pump is pulling water from the ground it tends to draw the well down from the surface. Another way of saying this, the screen will only allow so much water, if you ask for more, the pump pulls the water from above it. You need to find the mid point of flow vs draw-down of the well to locate you pump.
hope this answers your question.

Matt