The district may borrow money and incur indebtedness, and may issue bonds or other evidences of indebtedness. No indebtedness shall be incurred exceeding the ordinary annual income and revenue of the district without the approval of 60 percent of the voters voting on the proposition to incur such indebtedness, except that a further vote of the voters is not required for any indebtedness incurred within the purposes and not exceeding the available amount of any previously authorized bond issue, and as to such indebtedness the proceeds of any of the bonds unexpended in the treasury of the district, or the par value of any of the bonds which are unsold shall be deemed a
part of the ordinary annual income and revenue of the district.