No allowance of severance aid shall be made as provided in Section 831 unless the total assessed value of taxable real property within the water district is reduced by 2 percent or more during a fiscal year by reason of acquisitions giving rise to severance aid as provided in Section 831. In determining whether or not the value of such acquisitions is equal to 2 percent or more of the value of taxable real property, all such acquisitions made during each fiscal year shall be considered as one sum.