Wash. Rev. Code § 86.09.169
In case a contract has been or shall be hereafter made between the district and the United States, or any agency thereof, or with the state of Washington, as herein provided, bonds of the district may be deposited with the United States, or any agency thereof, or with the state of Washington, as payment or as security for future payment at not less than ninety percent of the par value, the interest on said bonds to be provided for by assessment and levy as in the case of bonds of the district sold to private persons and regularly paid to the United States, or any agency thereof, or to the state of Washington, to be applied as provided in such contract and if bonds of the district are not so deposited it shall be the duty of the board of directors to include as part of any levy or assessment against the lands of the district, an amount sufficient to meet each year all payments accruing under the terms of any such contract.
RCW 86.09.169