Whenever lands situated in an irrigation district are acquired by the department of transportation, and the lands, at the time of their acquisition by the department of transportation, were irrigable and were being served or were capable of being served by facilities of the district to the same extent and in the same manner as lands of like character held under private ownership were served, the department of transportation, as part of the cost and expense of the acquisition of rights-of-way and with funds available for the acquisition and at the time of the acquisition, shall make a lump sum payment to the irrigation district in an amount that is:

(1) Sufficient to pay the pro rata share of the district’s bonded indebtedness, if any, and the pro rata share of the district’s contract indebtedness to the United States or to the state of Washington, if any, allocable to the lands, plus interest on the pro rata share if the indebtedness is not callable in advance of maturity; and

(2) Further, sufficient to pay any deferred installments of local improvement district assessments against the lands, if any; and

(3) Further, sufficient to produce, if invested at an annual rate of interest equivalent to that set forth in current tables issued by the state insurance commissioner, a sum of money equal to the annual increase in operation and maintenance costs against remaining lands in the district resulting from the severance from the district of the lands thus acquired by the department of transportation. For the purposes of determining the amount of the lump sum payment, the annual maintenance and operation assessment of the district shall be considered to be the average for the ten years, or so many years as the district has assessment experience if less than ten years, preceding the date of acquisition.