(a) The district may impose an ad valorem tax as provided by Subchapter G, Chapter 36, Water Code, only if:
(1) the user fee authorized by this subchapter is held unconstitutional by the Texas Supreme Court; and
(2) a majority of the voters approve the tax at an election held in the district.
(b) The district may impose the ad valorem tax at a rate not to exceed three cents on each $100 of appraised value.