All lands found by the classification made under the supervision of the Board of Survey and Adjustments (House Document 201, 69th Congress, 1st Session, checked and modified as outlined in General Recommendations numbered 2 and 4, Page 60 of said document), to be permanently unproductive shall be excluded from the project and no water shall be delivered to them after the date of such exclusion unless and until they are restored to the project. Except as herein otherwise provided, the water right formerly appurtenant to such permanently unproductive lands shall be disposed of by the United States under the reclamation law: Provided, That the water users on the projects shall have a preference right to the use of the water: And provided further, That any surplus water temporarily available may be furnished upon a rental basis for use on lands excluded from the project under this section, on terms and conditions to be approved by the Secretary of the Interior.