Cal. Code Regs. tit. 18 § 24349(i)
The depreciation allowance includes an allowable for normal obsolescence which should be taken into account to the extent that the expected useful life of property will be shortened by reason thereof. Obsolescence may render an asset economically useless to the taxpayer regardless of its physical condition. Obsolescence is attributable to many causes, including technological improvements and reasonable foreseeable economic changes. Among these causes are normal progress of the arts and sciences, supercession or inadequacy brought about by developments in the industry, products, methods, markets, sources of supply, and other like changes, and legislative or regulatory action. In any case in which the taxpayer shows that the estimated useful life previously used should be shortened by reason of obsolescence greater than had been assumed in computing such estimated useful life, a change to a new and shorter estimated useful life computed in accordance with such showing will be permitted. No such change will be permitted merely because in the unsupported opinion of the taxpayer the property may become obsolete. For rules governing the allowance of a loss when the usefulness of an asset is suddenly terminated, see Section 24347 and the regulations thereunder.
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* This regulation is substantially the same as Section 26 CFR 1.167(a)-9.
Cal. Code Regs. Tit. 18, § 24349(i)