(a) If bonds are secured as provided in Section 51.450(b)(3) of this code, at the time that net revenue together with money derived from taxes accumulates a surplus in the sinking fund equal to the amount required in the succeeding year to liquidate the interest and principal on the district's bonds maturing in that year, the district's annual tax levies may be lowered to produce not less than 25 percent of the bond maturities for the succeeding year.
(b) If three successive years demonstrate that this net revenue is adequate to protect the district's bonds as they mature, the district's tax may be discontinued until further experience demonstrates the necessity to continue the tax to avoid default in the payment of the district's bonds as they mature.