Cal. Water Code § 35502

The district may provide that such rates or other charges may be collected with the water rates of the district and that all rates shall be billed upon the same bill and collected as one item, and that in the event of failure to pay the whole or any part thereof, the district may discontinue any and all service for which such bill is rendered, but this provision shall not be construed to prohibit the collection of rates or charges by the district in any other lawful manner.