Cal. Water Code § 71638.1

For property within a district, a portion thereof, or an improvement district, which property was previously charged or assessed for such purposes, the district by resolution may fix on or before the third Monday of August, in each fiscal year, a water standby assessment or availability charge in the district, in any portion thereof, or in any improvement district, to which water is made available by the district, whether the water is actually used or not.