In case the volume of water in any stream or river shall not be sufficient to supply the continual wants of the entire country through which it passes, and susceptible of irrigation therefrom, then it shall be the duty of the water commissioners, constituted as hereinafter provided, to apportion in a just and equitable proportion, a certain amount of such water upon certain or alternate weekly days to different localities as they may, in their judgment, think best for the interest of all parties concerned, and with due regard to the legal and equitable rights of all. The water commissioners shall consist of the chairman of the board of directors of each of the districts affected.