Cal. Pub. Util. Code § 29502
As a condition of annexation, the district shall require any county seeking to annex to the district to reimburse the district for the county's equitable share of the previously incurred cost of financing capital expenditures within the district theretofore imposed by taxation on territory within the district. The amount of reimbursement due from such county, unless otherwise provided for in the annexation agreement entered into pursuant to Section 29501 of this code, shall be computed on the following basis: one-half in the proportion that the population of the county bears to the total population of the district and one-half in the proportion that the assessed value of the real estate of the county bears to the total assessed value of the real estate of the district. Determinations of population shall be made by the district on the latest official information available to it. Determinations by the district as to the amount of capital expenditures shall be conclusive in each case.
Ca. Pub. Util. Code § 29502