The Collaborative International Dictionary of English v.0.48:

Depletion \De*ple"tion\, n. [Cf. F. d['e]pl['e]tion.]
   1. The act of depleting or emptying.
      [1913 Webster]

   2. (Med.) the act or process of diminishing the quantity of
      fluid in the vessels by bloodletting or otherwise; also
      excessive evacuation, as in severe diarrhea.
      [1913 Webster]

WordNet (r) 3.0 (2006):

depletion
    n 1: the act of decreasing something markedly
    2: the state of being depleted