Cal. Wat. Code § 51890
When any parcel located in a district upon which an assessment has been levied is divided, the board, upon written application signed by any or all of the landowners within the divided parcel, shall enter a minute order directing the reapportionment of the assessment upon the divided parcel in such manner as will charge each of the smaller parcels with a just proportion of the assessment.
When any parcel in a district is subdivided for the purposes of sale in accordance with Part 2 (commencing with Section 11000) of Division 4 of the Business and Professions Code, the subdivider shall make written application to the board for the reapportionment of any assessment of the district upon the parcels and the board shall proceed to make the reapportionment in the manner provided in this article. The expense of the reapportionment shall be paid by the subdivider.
Ca. Water Code § 51890