Any irrigation district, city, town, or other water user or users whose lands are irrigated by water carried in works transferred by the United States to a board of control are hereby authorized to enter into contract with the board of control for the operation and maintenance of the irrigation works within the district by the board of control and to pay such district in a lump sum the cost of maintenance and operation of such works within the district: PROVIDED, That the amount of the cost of operation of the works in the district may be estimated and the estimated amount paid to the board. At the end of each year the board shall, after determining the true amount of such costs of operation, require such district to pay the balance, if any, of such true amount.