Depletion is the actual physical reduction of a natural resource. For example, an oil field is depleted as oil is extracted from it over time. Depletion is commonly associated with all types of mining, as well as petroleum drilling and timberland usage.
The concept is used in accounting to charge the costs of natural resource extraction to expense as those resources are being used. Depletion can be considered a variable cost, since it is closely linked to the rate at which resources are consumed. This varies from the fixed cost treatment that is accorded to depreciation and amortization, since these types of expensing mechanisms do not vary with activity levels.