Notwithstanding Section 37207, the board of supervisors annually at the time of levying county taxes shall levy an ad valorem assessment upon all lands within that portion of the district within the county sufficient to raise the amount stated in the estimate of the district board; provided, however, that until such time as a district which has not theretofore delivered water for any purpose puts into operation facilities for the delivery of water, the board may levy a special assessment of an equal amount upon each acre of land within the district sufficient to defray all expenses as estimated by the board to be incurred for the general benefit of the district during the period in which facilities for the delivery of water primarily for agricultural purposes are being planned and constructed. In no event shall such equal amount assessment exceed three dollars ($3) per acre per year.