Cal. Water Code § 23242.4

Where a district makes a contract with the United States for a supply of water or for the payment or repayment of construction costs upon a reclamation project, and the United States has theretofore made individual contracts with landowners or entrymen as to land within the district, and such landowners or entrymen have under such individual contracts paid all or a portion of the charges to be paid to the United States upon construction costs or for a water supply, the assessment required in any year to meet the payment due to the United States for all purposes under the contract may be apportioned in whole or in part in accordance with the benefits received by such landowners or entrymen under the contract between the district and the United States. In the levying of district assessments upon the lands of such landowners or entrymen, the amounts already paid by such landowners or entrymen may be taken into consideration in the levying and equalizing of the assessments to the end that such landowners or entrymen may not be required to pay by way of assessment the amounts that such landowners or entrymen may have already paid to the United States under individual contracts.