relating to storage space for water supply, to change the method of calculation of the interest charges that began accruing on February 1, 1977, on the investment costs for the 198,350 acre-feet of future use storage space and on April 1, 1979, on 125,000 acre-feet of future use storage from compounding interest annually to charging simple interest annually on the principal amount, until— (A) the State of Kansas informs the Secretary of the desire to convert the future use storage space to present use