monthly or annual depreciation, amortization and depletion are used to reduce the book value of assets over time as they are "consumed" or used up in the process of obtaining revenue. these non-cash expenses are recorded in the accounting books "after" a trial balance is calculated to ensure that cash transactions have been recorded accurately. depreciation is used to record the declining value of buildings and equipment over time. land is not depreciated. amortization is used to record the declining value of intangible assets such as patents. depletion is used to record the consumption of natural resources.