SPECIAL PUBLICATION NO. 27


(selenite). Gypsum is used chiefly as a soils amendment, as a retarder in
portland cement and in making plaster.

harvesting The gathering of a crop or yield of one growing season.
Commonly refers to vegetable matter which can be replanted at will. In
reference to peat, this term is used as a synonym for mining.

hectare A metric unit of land area equal to 10,000 square meters or
2.471 acres.

hemic peat (U.S. Department of Agriculture classification) Peat in
which plant fibers compose between 33.33 and 66.66 percent of the
material; more decomposed than fibric peat.

humic acid Black, acidic, organic matter extracted from soils, peat,
low rank coals and other decayed plant substances by alkalis. It is insolu-
ble in acids and organic solvents.

hydraulic peat mining Peat mining methods which do not require prior
drainage of the deposit. Typically, high pressure water guns or dredges
are used to cut peat from the deposit.

hydrocracking A process in which relatively heavy hydrocarbons are
broken up by heat into lighter products (such as gasoline) in the presence
of hydrogen.

hydrologic budget An accounting of the inflow to, outflow from and
storage in a hydrologic unit such as a drainage basin, aquifer, soil zone,
lake or reservoir (Langbein and Iseri, 1960); the relationship between
evaporation, precipitation, runoff and the change in water storage,
expressed by the hydrologic equation. Syn: water balance; water
budget; hydrologic balance.

hydrology The science that deals with continental water (both liquid
and solid), its properties, circulation and distribution, on and under the
Earth's surface and in the atmosphere, from the moment of its precipita-
tion until it is returned to the atmosphere through evapotranspiration or is
discharged into the ocean.

hydroperiod (of a wetland community) A measure of the time (usu-
ally in days per year) that water is at or above the soil surface.

hydrostatic head The height of a vertical column of water, the weight
of which, if of unit cross section, is equal to the hydrostatic pressure at a
point; static head, as applied to water.

hypnum moss peat (American Society for Testing and Materials
(ASTM) classification) Peat which contains at least 33.33 percent plant
fibers with one-half of those identifiable as Hypnum moss. NOTE: ASTM