Correct me if I am doing this wrong. Since most pumps are rated
in head measured in feet of lift one cubic foot of water weighs 62.43 pounds
so that would be 62.43 pounds/144 square inches of area  or .433 psi.
50 foot of water would then be about  22 psi.
Looking at the page in MaxMaster Carr the 3/4 HP pump is
pumping 35 gallons per minute at 10 feet or 4.3 psi.
It pumps only 20 gpm at 13 psi. 
It cavitats at a back pressure of only 14 psi.
What Macmaster Carr calls "shutoff".

The one HP pump in the group looks more typical of an automotive pump. 

These pump are probably more efficient than automotive pumps as they are larger
and rotating at about half the RPM so seal friction HP would be less.

Paul Lamar
 
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