It's a pretty basic question. Does it use some sort of water injection scheme?
According to Wik, it does: http://en.wikipedia.org/wiki/AV-8_Harrier_II
there are three injection ports for the engine to cool itself, it can fly without any water but it does cool the engines operating temperature to allow for a higher rpm performance which increases thrust.
See this thread https://www.physicsforums.com/showthread.php?t=95868
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