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

Moby Thesaurus II by Grady Ward, 1.0:

75 Moby Thesaurus words for "depletion":
   ablation, absorption, assimilation, attrition, burning up,
   cleaning out, clearance, clearing, consumption, corrosion,
   curtailment, cut, cutting, decrease, decrement, defecation,
   deliquescence, depreciation, derogation, detraction, digestion,
   diminution, dip, discharging cargo, disparagement, dissipation,
   dissolution, drain, drainage, draining, eating up, egress,
   elimination, emptying, erosion, evacuation, evaporation, excretion,
   exhausting, exhaustion, expending, expenditure, extraction,
   finishing, impairment, impoverishment, ingestion, leakage,
   lessening, loss, off-loading, reduction, remission, removal,
   retraction, retrenchment, shortening, shrinkage, spending,
   squandering, truncation, unloading, using, using up, venting,
   voidance, voiding, wastage, waste, wastefulness, wasting away,
   wear and tear, wearing, wearing away, wearing down