Durring todays NASCAR race they ran a info-segment on how the teams run both

[Qikbbstang]

Water Temp and Water Pressure guages.  They stated the Water Temp gives you a good idea how hot the motor is, but that as the water temp increases the Water Pressure decreases (sounded odd to me). That when the water pressure dropped toward zero the motor was just about to become toast.
    I was thinking where would they source the Water Pressure from? ...... I thought the discharge of the pump may be very pressure sensitive?............but do not know.

[Woods]

Simple BB, it does not matter how much water is in the cooling system, it will read a temperature.  When the pressure drops it just means that they have run out of water to pump!!  The temperature gauge does not know the difference between steam and water.