The board of supervisors of each county in which any part of the district is situated, at the time of making the levy of taxes for county purposes, shall levy a tax for that year upon the taxable property in the district, at the equalized assessed value thereof for that year, for the interest and redemption of the bonds. The tax shall not be less than sufficient to pay the interest of the bonds for that year, and such portion of the principal as is to become due during the year. In any event the tax shall be high enough to raise, annually, for the first half of the term the bonds are to run, a sufficient sum to pay the interest thereon, and during the balance of the term, high enough to pay the annual interest and to pay, annually, a proportion of the principal of the bonds equal to a sum produced by taking the whole amount of the bonds outstanding and dividing it by the number of years the bonds then have to run.