In any year a tax may be levied for the maintenance of the water system in the county at a rate calculated to raise a sum of money equal to the amount of the excess, if any, of the amount of the estimated cost of such maintenance over the estimated amount of revenues derived that are or will become available to defray such cost. The rate of such a tax shall not in any year exceed fifteen cents ($0.15) per one hundred dollars ($100) of the assessed valuation of all taxable property within the county, unless a proposal for a specified higher rate for a particular year is submitted to the qualified electors of the county at a general or special election and is approved by a majority of the electors voting thereon.