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Hydrocarbons

Substances containing only hydrogen and carbon. Fossil fuels are made up of hydrocarbons. Some hydrocarbon compounds are major air pollutants. See fossil fuel.

Hydrochlorofluorocarbons (HCFCs)

Compounds containing hydrogen, fluorine, chlorine, and carbon atoms. Although ozone depleting substances, they are less potent at destroying stratospheric ozone than chlorofluorocarbons (CFCs). They have been introduced as temporary replacements for CFCs and are also greenhouse gases. See ozone depleting substance.

Hydroelectric power plant

Structure in which the energy of fading or flowing water spins a turbine generator to produce electricity.

Hydroelectric power water use

Water used in the generation of electricity at plants where the turbine generators are driven by falling water, classified as an in-stream water use. Consumptive losses are traditionally considered negligible and assumed zero in accounting.

Hydrofluorocarbons (HFCs)

Compounds containing only hydrogen, fluorine, and carbon atoms. They were introduced as alternatives to ozone depleting substances in serving many industrial, commercial, and personal needs. HFCs are emitted as by-products of industrial processes and are also used in manufacturing. They do not significantly deplete the stratospheric ozone layer, but they are powerful greenhouse gases with global warming potentials ranging from 140 (HFC-152a) to 11,700 (HFC-23).

Hydrograph

A graph showing stage, flow, velocity, or other property of water with respect to time.

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, reservoir, or irrigation project.

Hydrologic cycle

The circulation of water from the sea, through the atmosphere, to the land, and then back to the sea by overland and subterranean routes.

Hydrologic drought

An extended period of below-normal precipitation that results in uncommonly low levels of streamflow, shallow groundwater, and/or reservoir storage. Hydrologic drought usually infers some adverse impact, or the threat thereof, to the availability of water for instream and/or offstream uses. For Illinois, a hydrologic drought is typically considered to represent infrequent conditions that may recur on average no more than once in roughly 8-10 years, and, like other types of drought, affects large regions within the State.

Hydrologic equation

The equation balancing the hydrologic budget.

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