(a) Except as provided in this subchapter, an appropriation available for obligation for a definite period shall be apportioned to prevent obligation or expenditure at a rate that would indicate a necessity for a deficiency or supplemental appropriation for the period. An appropriation for an indefinite period and authority to make obligations by contract before appropriations shall be apportioned to achieve the most effective and economical use. An apportionment may be reapportioned under this section.

(b)(1) An appropriation subject to apportionment is apportioned by—

(A) months, calendar quarters, operating seasons, or other time periods;

(B) activities, functions, projects, or objects; or

(C) a combination of the ways referred to in clauses (A) and (B) of this paragraph.

(2) The official designated in section 1513 of this title to make apportionments shall apportion an appropriation under paragraph (1) of this subsection as the official considers appropriate. Except as specified by the official, an amount apportioned is available for obligation under the terms of the appropriation on a cumulative basis unless reapportioned.

(c)(1) In apportioning or reapportioning an appropriation, a reserve may be established only—

(A) to provide for contingencies;

(B) to achieve savings made possible by or through changes in requirements or greater efficiency of operations; or

(C) as specifically provided by law.

(2) A reserve established under this subsection may be changed as necessary to carry out the scope and objectives of the appropriation concerned. When an official designated in section 1513 of this title to make apportionments decides that an amount reserved will not be required to carry out the objectives and scope of the appropriation concerned, the official shall recommend the rescission of the amount in the way provided in chapter 11 of this title for appropriation requests. Reserves established under this section shall be reported to Congress as provided in the Impoundment Control Act of 1974 (2 U.S.C. 681 et seq.).[1]

(d) An apportionment or a reapportionment shall be reviewed at least 4 times a year by the official designated in section 1513 of this title to make apportionments.

[1] Section 2, United States Code, et seq. is a reference to the Impoundment Control Act, found in Title X of the Congressional Budget and Impoundment Control Act of 1974.