Increase the head on the suction side of the pump. Lower the temperature of the incoming liquid. The following stages are not considered because they should have plenty of head available to them. Where dissolved air or other non condensable gases are present in the liquid, pump performance may be adversely affected even though you have the NPSH required as shown on your pump curve. You are going to have to increase the NPSH available to prevent the release of these gases.
Absolute pressure can vary with weather conditions.
If you are playing it close, and we often do, this could lead to a cavitation problem. Low flow usually means a lower NPSH required, but low flow can also mean a temperature build up inside the pump. Some operators have been known to throttle the suction of a pump to reduce capacity. This may be valid for some unique dangerous products that would become a hazard if the discharge were throttled and the heat increased inside the pump, but except for these special cases, suction throttling is a bad idea. The Mc Nally Institute 1986 S. Belcher Rd.