The pay for the services of the watermaster and the watermaster’s assistants shall be assessed against the land of the water users to which said water was so delivered. The amount assessed to each user shall be a pro rata share based on the volume of water delivered to each water user in proportion to the whole amount delivered to all water users. When any portion of the allotted waters is distributed by said watermaster to the canal of any water delivery organization, the amount of the expense chargeable for such services shall be assessed against such canal.