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.