Imagine that you have a basic water pump with the discharge connected to a short piece of pipe with a pressure gage and a gate valve in it. Suppose you shut the valve and turn the pump on. The pressure gage will reach some maximum and the flow will be zero since nothing can get past the valve. If you now open the valve a little bit there will be some flow and the pressure gage reading will drop a bit. As you open the valve more and more the flow increases and the pressure drops. When the valve is fully open you will get the maximum flow and the pressure will be zero. If you plot these readings on a graph with flow in gallons per minute on the bottom ( x axis) and pressure measured in feet of water on the left ( y axis) you will have a pump curve. This curve is determined at the foundry where the impeller is cast and nothing, within reason, will change it. It is used to tell what flow the pump will produce at any given head pressure. Note that pressure is a measure of resistance to flow. It is the total resistance in a system that determines the flow output of the pump. Without any resistance, the pump delivers its maximum flow. Pumps are quite stupid and totally unaware of what you intend them to do. They deal only in what is real to them and they don't know anything except what is at their suction and what is at their discharge. Pump curves are customarily marked in feet of head because any liquid pumped will be lifted to the same height. This is true whether it is oil, water or molten lead. ( Pressure and energy required are another thing altogether). For water applications, (pressure in PSI) = feet x .433 or, ( head in feet) = (pressure in PSI) x 2.31 for example: 60 PSI = 138.6 ft.