Cal. Water Code § 72072.5

The taxes imposed by this article shall be collected, insofar as practicable, at the same time as, and along with, the charges made in accordance with the regular billing practice of the service supplier. If the amount paid by a service user to a service supplier is less than the full amount of the charge and the tax which has accrued for the billing period, a proportionate share of both the charge and the tax shall be deemed to have been paid.