The reason the pump must be below the tank is that this is the way these pumps are designed to work.

The water tank being above the pump gives it a positive suction pressure, which is measured in 'feet of head', on in this case inches of head pressure.

In any case there is always pressure on the system ready to be pumped to the nozzles. The pump cannot run dry since it below the level of the fluid in the tank and the fluid is always trying to reach its' own level. The pump just provides the lift (which could also be described as the pressure) to bring the fluid up out of the tank and beyond, like onto you windshield.

Or, in simple terms... Keep the pump and the pump inlet below the level of the tank and it will always be ready to pump when the pump is activated through the switch.