That whenever any irrigation district in this state, organized for the purpose of cooperating with the United States under the federal reclamation laws and acts amendatory thereof and supplemental thereto, shall have paid in full all construction charges due from such district to the United States, under the construction contract, and there shall remain any lands in the district upon which the per-acre charge for construction under the contract between the United States and the district have not been paid in full, such charges shall continue as a lien against such lands until paid in full. The district is hereby authorized to continue to assess such lands for such unpaid construction charges for a period not to exceed twenty years following the date of final payment of the construction liability of the district to the United States, and to collect the same until they have been paid in full; and the board of directors of said district shall be authorized to set the period of repayment within the twenty-year period specified above. The monies derived from such assessments shall be used by the district for operation, maintenance and betterment of the district’s works.