Cal. Water Code § 36575

In lieu of assessing the land, as aforesaid, the board may direct the assessor to, and the assessor shall thereupon adopt as the cash value of the land, and shall insert in the assessment book as such cash value the value of the land as the same appears on the assessment roll of the county in which the land is situated, except that any value of rights to minerals, oil, gas, or other hydrocarbon substances underlying the land which may be included in the assessed value of the land on the assessment roll of such county, shall be deducted by the assessor from such assessed value and the remainder of such assessed value shall be adopted as the cash value of the land, and inserted in the assessment book as such cash value.