The Secretary is directed to proceed as nearly as practicable with the construction of the Animas-La Plata, Dolores, Dallas Creek, West Divide, and San Miguel participating Federal reclamation projects concurrently with the construction of the Central Arizona Project, to the end that such projects shall be completed not later than the date of the first delivery of water from said Central Arizona Project: Provided, That an appropriate repayment contract for each of said participating projects shall have been executed as provided in section 620c of this title before construction shall start on that particular project.