((a)) The Secretary shall determine for each lease the grazing fee to be paid. Such fee shall—
((1)) Be fixed on the basis of the area leased or on the basis of the number and kind of stock permitted to be grazed;
((2)) Be fixed, for the period of the lease, as a seasonal or annual fee, payable annually or semi-annually on the date specified in the lease;
((3)) Be fixed with due regard to the general economic value of the grazing privileges, and in no case shall exceed such value; and
((4)) Be moderate.
((b)) If the Secretary determines such action to be for the public interest by reason of (1) depletion or destruction of the range by any cause beyond the control of the lessee, or (2) calamity or disease causing wholesale destruction of or injury to livestock, he may grant an extension of time for making payment of any grazing fee undue any lease, reduce the amount of any such payment, or release or discharge the lessee from making such payment.