Cal. Civ. Code § 731.12

Where any part of the principal consists of property subject to depletion, such as leaseholds, patents, copyrights, and royalty rights, and the tenant in possession is not under a duty to change the form of the investment of the principal, the full amount of rents, royalties, or income from the property shall be income to the tenant; but where the tenant is under a duty to change the form of the investment, either at once or as soon as a reasonable price, not representing an undue sacrifice of value, may be obtained, then the rents, royalties or income from such property not in excess of 5 percent per annum of its inventory value as fixed by the appraiser or appraisers regularly appointed by the court, or in default thereof its market value at the time the principal was established or at its cost where purchased later, shall be deemed income and the remainder principal.