Cal. Wat. Code § 51892.5
If no application for reapportionment is received under Section 51890, the district secretary may, upon learning that a parcel upon which an assessment has been levied has been divided or subdivided, reapportion the assessment against that parcel among each of the smaller parcels in proportion to the relative acreage contained in each parcel. The secretary shall give to each owner of the smaller parcels written notice of the reapportionment so determined, advising that the reapportionment will become final unless the owners, or any of them, within 10 days of the mailing of the notice, file with the secretary a written request for a hearing before the board to determine the manner in which the assessment shall be reapportioned. If no request is received, the reapportionment shall become final upon adoption by the board. If a request is received, the secretary shall give to the owner of each of the smaller parcels written notice not less than 10 days prior thereto that a hearing will be held by the board to determine the reapportionment. The determination of the board at the hearing shall be final unless, within 10 days thereafter, an objection is filed in writing with the secretary, in which event the board shall file its order of reapportionment with the clerk of the board of supervisors, and the reapportionment shall be approved by the board of supervisors in the manner provided in Article 1 (commencing with Section 51230) of Chapter 2.
Ca. Water Code § 51892.5