(a) A district which enters into a contract with the United States shall levy annually sufficient taxes to provide payment of all installments required by the contract.
(b) The board may apportion benefits and levy and collect taxes on the benefit basis instead of the ad valorem basis with the approval of the district electors.
(c) The board may pay construction charges when provided by contract on the basis of the average gross annual acre income of the land of the district or designated divisions or subdivisions of the district. The secretary of the interior shall determine the annual gross acre income.