Each district shall have the power and authority to list in separate ownership the lands within the district which are susceptible of irrigation from the district sources, to enter into contracts to furnish water service to all such lands, and to levy assessments against the lands within the district to which water service is furnished on the basis of the value per acre-foot of water service furnished to the lands within the district; Provided, the board may divide the district into units and fix a different value per acre-foot of water in the respective units, and in such case shall assess the lands within each unit upon the same basis of value per acre-foot of water service furnished to lands within such unit.