Idaho Code § 42-1304

On or before the fifteenth day of April in each year the secretary of the association shall notify each water user of the amount assessed against him for that year as herein authorized, and the same shall be due and payable on the first day of May of each year and if not so paid on or before the fifteenth day of June of such year, a penalty of ten per cent (10%) shall be added thereto, and the total amount due shall then draw interest at the rate of ten per cent (10%) per annum from said fifteenth day of June of such year until paid.