(a) If the use of the appropriated water is authorized for multiple purposes, the water right shall contain a special condition limiting the total amount of water that may be actually diverted for all the purposes to the amount of the water appropriated.(b) If a water right has appropriations with different priority dates, the oldest priority water shall be credited against the water first used unless the water right expressly provides otherwise or the water right holder requested the watermaster to count the water use against the junior portion of the right.