I am doing some research for the head using in drive. Appreciated if somebody can explain how the negative presusure generate. Thanks in advance.

To me your question is unclear - so I will try to answer what I think your question is. I will assume this is a very basic question about the units used to measure pressure.

1 ATM pressure = 101.3 kpa = 14.7 psi = 10.33 m of water etc. It is common in civil engineering to describe pressure in terms of equivalent height if water pressure as "head" pressure. Therefore a civil engineer could describe one atmospheric pressure as 10.33 meters of water. If the pressure is less than one atmosphere then the pressure is said to be negative (or a vacuum = the amount the pressure is BELOW atmospheric pressure). The units used are the same as those to measure the pressure above atmosphere so it can be said that a pressure that is negative 0.1 atmosphere = negative 1.033 meters of water pressure or a vacuum of +1.033 meters. Hope this is the answer you wanted. Otherwise please repost with more details.

Allan