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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).

earthen (or earthfill) dam

An embankment dam in which more than 50% of the total volume is formed of compacted fine-grained material. A homogeneous earthen dam is constructed of similar earthen material throughout. These are the most common type of dam because their construction involves using materials in the natural state, requiring little processing.

embankment

Fill material, usually earth or rock, placed with sloping sides and usually with length greater than height. All dams are types of embankments.

energy dissipator

A structure which slows fast-moving spillway flows in order to prevent erosion of the stream channel.

engineer's level

A telescope which is attached to a spirit-tube level, all revolving around a vertical axis and is mounted on a tripod. An engineer's level is used for determining the difference in elevation between two points. The telescope on the level has a vertical cross hair and a horizontal cross hair. Once the instrument is leveled, the sighting through the horizontal cross hair represent a horizontal plane of equal elevation.

ensemble (hydrologic) forecasting

A process whereby a continuous hydrologic model is successively executed several times for the same forecast period by use of varied data input scenarios, or a perturbation of a key variable state for each model run. A common method employed to obtain a varied data input scenario is to use the historical meteorological record, with the assumption that several years of observed data covering the time period beginning on the current date and extending through the forecast period comprises a reasonable estimate of the possible range of future conditions.

equilibrium drawdown

The ultimate, constant drawdown for a steady rate of pumped discharge.

equilibrium surface discharge

The steady rate of surface discharge which results from a long-continued, steady rate of net rainfall, with discharge rate equal to net rainfall rate.

equilibrium time

The time when flow conditions become substantially equal to those corresponding to equilibrium discharge or equilibrium drawdown.

equipotential line

A line, in a field of flow, such that the total head is the same for all points on the line, and therefore the direction of flow is perpendicular to the line at all points.

erosion

Wearing away of the lands by running water, glaciers,winds, and waves, can be subdivided into three process: corrasion, corrosion, and transportation. Weathering, although sometimes included here, is a distant process which does not imply removal of any material.

esturine waters

Deepwater tidal habitats and tidal wetlands that are usually enclosed by land but have access to the ocean and are at least occasionally diluted by freshwater runoff from the land (such as bays, mouths of rivers, salt marshes, lagoons).

esturine zone

The area near the coastline consisting of esturaries and coastal saltwater wetlands.

estuary

The thin zone along a coastline where freshwater systems and rivers meet and mix with a salty ocean (such as a bay, mouth of a river, salt marsh, lagoon).

evaporimeter

An instrument which measures the evaporation rate of water into the atmosphere.

evaporation

The physical process in which liquid water is converted into water vapor.

evaporation pan

A pan used to hold water during observations for the determination of the quantity of evaporation at a given location. Such pans are of varying sizes and shapes, the most commonly used being circular or square.

evaporation rate

The quantity of water, expressed in terms of depth of liquid water, which is evaporated from a given surface per unit of time. It is usually expressed in inches depth, per day, month, or year.

evapotranspiration

Water withdrawn from a land area by evaporation from water surfaces and moist soil and by plant transpiration.

excess rain

Effective rainfall in excess of infiltration capacity.

exclusive flood control storage capacity

The space in a reservoir reserved for the sole purpose of regulating flood inflows to abate flood damage.