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These definitions have been compiled from various sources, including the USGS's Water Resources Data, Weather Service Operations Manual, Chapter E-90, and Glossary of Meteorology (AMS, 1959).

ice boom

A floating structure designed to retain ice.

ice bridge

A continuous ice cover of limited size extending from shore to shore like a bridge.

ice gorge

The gorge or opening left in an ice jam after it has broken.

ice jam

A stationary accumulation which restricts or blocks streamflow.

ice push

Compression of an ice cover particularly at the front of a moving section of ice cover.

ice run

Flow of ice in a river. An ice run may be light or heavy, and may consist of frazil, anchor, slush, or sheet ice.

ice twitch

Downstream movement of a small section of an ice cover. Ice twitches occur suddenly and often appear successively.

ice shove

On-shore ice push caused by wind, and currents, changes in temperature, etc.

impermeable

Material that does not permit fluids to pass through it.

impervious

The ability to repel water, or not let water infiltrate.

import

Water piped or channeled into an area.

inactive storage capacity

The portion of capacity below which the reservoir is not normally drawn, and which is provided for sedimentation, recreation, fish and wildlife, aesthetic reasons, or for the creation of a minimum controlled operational or power head in compliance with operating agreements or restrictions.

inch-degrees

The product of rainfall depth (in inches) multiplied the temperature in degrees Fahrenheit above freezing, used as a measure of the snowmelting capacity of rainfall.

inclined staff gage

A staff gage that is placed on the slope of a stream bank and graduated so that the scale reads directly in vertical depth.

index of wetness

The ratio of precipitation for a given year over the mean annual precipitation.

indirect flood damage

indirect flood damage

Expenditures made as a result of the flood (other than repair) such as relief and rescue work, removing silt and debris, etc.

infiltration

Movement of water through the soil surface into the soil.

infiltration capacity

The maximum rate at which water can enter the soil at a particular point under a given set of conditions.

infiltration capacity curve

A graph showing the time-variation of infiltration capacity. A standard infiltration capacity curve shows the time-variation of the infiltration rate which would occur if the supply were continually in excess of infiltration capacity.

infiltration index

An average rate of infiltration, in depth of water per hour, equal to the average rate of rainfall such as that the volume of rainfall at greater rates equals the total direct runoff.

infiltration rate

1. The rate at which infiltration takes place expressed in depth of water per unit time.
2. The rate (usually expressed in cubic feet per second or million gallons per day per mile of waterway) at which ground water enters an infiltration ditch or gallery, drain, sewer, or other underground conduit.

influent seepage

Movement of gravity water in the aeration zone from the ground surface toward the water table.

influent stream

Any watercourse in which all or a portion of the surface water flows back into the ground through the zone of aeration.

initial detention

The volume of water on the ground, either in depressions or in transit, at the time active runoff begins.

initial loss

Rainfall preceding the beginning of surface runoff. It includes interception, surface wetting, and infiltration unless otherwise specified.

initial moisture deficiency

The quantity, usually expressed in depth of water upon a unit area, by which the actual water content of a given soil zone (usually the root zone) in a given area is less than the field capacity of such zone at the beginning of the rainy season. Also called initial water deficiency.

initial water deficiency

The quantity, usually expressed in depth of water upon a unit area, by which the actual water content of a given soil zone (usually the root zone) in a given area is less than the field capacity of such zone at the beginning of the rainy season. Also called initial moisture deficiency.

inland freshwater wetlands

Swamps, marshes, and bogs found inland beyond the coastal saltwater wetlands.

instantaneous unit hydrograph

The theoretical, or ideal, unit hydrograph which has a infinitesimal duration.

instream use

The use of water which doesn't require withdrawal or diversion from its natural watercourse. Examples include the use of water for navigation, recreation, and support of fish and wildlife.

intangible flood damage

Estimates of the damage done by disruption of business, danger to health, shock, and loss of life and in general all costs not directly measurable which require a large element of judgment for estimating.

Integrated Flood Observing and Warning System (IFLOWS)

A wide area network utilizing UHF/VHF radio and landline communications. IFLOWS components include rainfall and stage sensors, transceivers, store-forward repeaters and computer base stations.

interbasin transfer

The physical transfer of water from one watershed to another.

intercepting drain

A drain constructed at the upper end of the area to be drained, to intercept surface or ground water flowing toward the protected area from higher ground, and carry it away from the area. Also called a curtain drain.

interception

The process by which precipitation is caught and held by foliage, twigs, and branches of trees, shrubs, and other vegetation, and lost by evaporation, never reaching the surface of the ground. Interception equals the precipitation on the vegetation minus streamflow and through fall.

interception storage requirements

Water caught by plants at the onset of a rainstorm. This must be met before rainfall reaches the ground.

interflow

The lateral motion of water through the upper layers of soil until it enters a stream channel. This usually takes longer to reach stream channels than runoff. This also called subsurface storm flow.

intermeidate zone

The subsurface water zone below the root zone and above the capillary fringe.

intermittent stream

A stream which flows periodically. Compare perennial stream.

inundation

To be covered with flood waters.

inundation map

A map delineating the area which would be inundated in the event of a dam failure.

irrigated area

The gross farm area upon which water is artificially applied for the production of crops, with no reduction for access roads, canals, or farm buildings.

irrigation

The controlled application of water to arable lands to supply water requirements not satisfied by rainfall.

irrigation efficiency

The percentage of water applied that can be accounted for in soil moisture increase for consumptive use.

irrigation requirement

The quantity of water, exclusive of precipitation, that is required for crop production. It includes surface evaporation and other economically unavoidable wastes.

isobath

An imaginary line on the earth's surface or a line on a map connecting all points which are the same vertical distance above the upper or lower surface of a water-bearing formation or aquifer.

isohyet

A line connecting points of equal rainfall.