I need about 20 GPM for irrigation from a river. Most of the landscape is about 10' higher than the river, but my house and lawn is about 50' above the river. I'm using a 4" submersible mounted horizontally in the river and can get 20 GPM from 1/2 HP, 3/4 HP, or 1 HP. Of course, the 1 HP costs more. Why not use the lowest HP that will deliver 20 GPM? What does the extra HP (and dollars) give me since all the pumps produce 20 GPM with 50' head.