If, in the opinion of the board of a municipal water district, the corporate area of which has been included in or annexed to a metropolitan water district as a unit, the revenues of the municipal water district will be inadequate for any cause to pay the operating expense of such district, to provide for repairs and depreciation of works owned or operated by it, and to meet all of its obligations the board shall provide for the levy and collection of a tax, in the manner provided in this part, sufficient to raise the amount of money determined by the board to be necessary for the purpose of paying the operating expenses of such district, providing for repairs and depreciation of works owned or operated by it, and meeting all of its obligations.