Any entryman or applicant shall at the time of making water-right application or entry, as the case may be, pay into the reclamation fund 5 per centum of the construction charge fixed for his land as an initial installment, and shall pay the balance of said charge in annual installments. The first of the annual installments shall become due and payable on December 1 of the fifth calendar year after the initial installment: Provided, That any water-right applicant or entryman may, if he so elects, pay the whole or any part of the construction charges owing by him within any shorter period: Provided further, That entry may be made whenever water is available, as announced by the Secretary of the Interior, and the initial payment be made when the charge per acre is established.