Notwithstanding any provision of the Irrigation District Federal Cooperation Law, or any provision of this division or other law, when any district has contracted with the United States pursuant to this article or any other law for the purposes set forth in Section 31151, if the revenues of the district from water rates are, or in the judgment of the board are likely to be, inadequate to pay all charges payable to the United States or otherwise under the contract, then any tax or assessment levied for the payment of any of those charges in any manner provided by law shall be levied only upon land, exclusive of improvements and personal property, in the portion of the district to be served with water under the contract as therein described or thereunder modified.