English to Portuguese Tech/Engineering Energy / Power Generation

Dear colleagues,
Does anybody know what "head" means in this context? I am attaching a few passages from the text I am working on below:
Thanks a lot in advance....

"Such system shall be able to supply oil under pressure in sufficient volume to allow that the Bulb runner wicket gates and blades servomotors will perform a full closing course in the time interval required to meet the Guarantees and governing throughout the range of heads and openings of the distributor and the runner blades."

"The above studies shall use data from tests in turbine model of the turbine and will be carried out for different conditions of minimum, rated and maximum head. "

"The relative efficiency check in certain operation points (head - discharge) of the turbines will be checked in the prototype after the commissioning of the second Generating Unit. "