The total amount of bonded indebtedness shall not at any time exceed 5 percent of the taxable property of the county as shown by the last equalized assessment roll. If water conservation, flood control, irrigation, reclamation, or drainage works, improvements, or purposes, or the construction of select county roads is included in any proposition submitted, the total amount of bonded indebtedness may exceed 5 percent but shall not exceed 15 percent of the taxable property of the county as shown by the last equalized assessment roll.