No contract shall be made by the Water Resources Commission which requires a greater time than five years for construction of the works. All contracts shall state that the work shall begin within six months from date of contract; that the contractor shall secure for the use and benefit of the reclamation system all necessary water rights, rights of way, reservoir sites, or other property necessary for its construction and operation; that construction shall be prosecuted diligently and continuously to completion; and that a cessation of work under the contract with the state for a period of six months, without the sanction of the commission, will forfeit to the state all rights under the contract. The commission may extend the time in which to begin the construction of works, or for the completion of work, on account of delay caused by physical or engineering difficulties beyond the power of the contractor to control.