Ala. Code § 41-10-655

There is hereby irrevocably pledged and appropriated such amount of pledged revenues deposited in the special fund, and earnings thereon, as may be necessary to pay and to redeem prior to their respective maturities the principal, interest and premium, if any, on the bonds; provided, however that the bonds may not be issued in principal amounts and maturities such that the debt service payable on such bonds may ever be greater than the amounts permitted by Division 1.