Knowledge Base

ECHO SOUNDER — A device for measuring the depth of water or the depth of an object below the surface by sending pressure waves down from the surface and recording the time until the echo returns from the bottom.

ECOLOGICAL IMPACT — The effect that a man-made or natural activity has on living organisms and their non-living (abiotic) environment.

ECOLOGICAL INDICATOR — An individual species or a defined assemblage of organisms that serves as a gauge of the condition of the environment. The term is a collective term for response, exposure, habitat, and stressor indicators. For example, the bacterium Escherichia coli indicates the presence of sewage in water, and the mussel, Mytilus edulis lives in polluted waters.

ECOLOGICAL RISK ASSESSMENT — The application of a formal framework, analytical process, or model to estimate the effects of human actions on a natural resource and to interpret the significance of those effects in light of the uncertainties identified in each component of the assessment process. Such analysis includes initial hazard identification, exposure and dose-response assessments, and risk characteristics.

ECOLOGICAL SUCCESSION — An orderly, directional and therefore predictable process of development that involves changes in species structure and community processes over time. It results from a modification of the physical environment by the community and culminates in a stabilized ecosystem in which maximum biomass and symbiotic functions are maintained.

ECOLOGY — The study of the inter-relationships of living things to one another and to the environment.

ECOSPHERE — The mantle of earth and troposphere inhabited by living organisms; the "bio-bubble" that contains life on earth, in surface waters, and in the air. Also see Biosphere.

ECOSYSTEM — A community of animals, plants, and bacteria, and its interrelated physical and chemical environment. An ecosystem can be as small as a rotting log or a puddle of water, but current management efforts typically focus on larger landscape units, such as a mountain range, a river basin, or a watershed.

ECOSYSTEM FUNCTIONS — Processes that are necessary for the self-maintenance of an Ecosystem such as primary production, nutrient cycling, decomposition, etc. The term is used primarily as a distinction from values.

ECOSYSTEM MANAGEMENT — (Environmental) An approach to managing the nation's lands and natural resources which recognizes that plant and animal communities are interdependent and interact with their physical environment (i.e., soil, water, and air) to form distinct ecological units called Ecosystems. The fact that these ecosystems span jurisdictional and political boundaries necessitates a more comprehensive and unified approach to managing them. Implementing the initial stage of a government-wide approach to ecosystem management typically requires clarifying the policy goals and undertaking certain practical steps to apply the principles being considered to include:
[1] Delineating the ecosystem;
[2] Understanding the system(s) ecologies;
[3] Making management choices;
[4] Unifying disparate data and information needs and sources; and
[5] Adapting management on the basis of new information.

ECOSYSTEM STRUCTURE — Attributes related to instantaneous physical state of an ecosystem; examples include species population density, species richness or evenness, and standing crop Biomass.

EFFECTIVE PRECIPITATION (or Rainfall) — That portion of precipitation which remains on the foliage or in the soil that is available for Evapotranspiration, and reduces the withdrawal of soil water by a like amount. As described by the U.S. Bureau of Reclamation, that part of the precipitation falling on an irrigated area that is effective in meeting the Crop Consumptive Use requirements.

EFFICIENCY — (Irrigation) A measure of a distribution system's ability to transport and apply water to a desired effect with a minimum of effort, expense, or waste. With respect to irrigation project efficiency, the following terms generally apply:
[1] Canal Efficiency — The volume of water diverted into a canal system versus total water available for farm headgate deliveries;
[2] Irrigation Efficiency — The percentage of water applied that can be accounted for in soil moisture increase; and
[3] Farm Efficiency — The amount of water actually required for growing a crop compared to the amount of irrigation water that is diverted at the farm headgate.

EFFICIENT WATER MANAGEMENT PRACTICES (EWMP) — The agricultural water use equivalent of Best Management Practices (BMP) as applied to urban water use, efficient water management practices cover the spectrum of methods to improve both the efficiency and conservation of agricultural water use by (1) enhancing irrigation management services, measurement, and accounting; (2) improving the physical system of irrigation delivery, distribution, and drainage; and (3) promoting the modification of and adjustments to the institutional system of water use by agricultural interests to include information and educational programs.

EFFLUENT — (1) Something that flows out or forth, especially a stream flowing out of a body of water. (2) (Water Quality) Discharged wastewater such as the treated wastes from municipal sewage plants, brine wastewater from desalting operations, and coolant waters from a nuclear power plant.

EFFLUENT SEEPAGE — Diffuse discharge of ground water to the ground surface.

EFFLUENT STREAMS — Effluent streams are those leaving a lake.

EJECTOR — (1) A device using a jet of water to withdraw a fluid from a space. (2) A device used to disperse a chemical solution into water being treated.

ELECTRIC POWER WATER (Public Utility) — Water withdrawn by public utilities for hydroelectric power generation and condenser cooling.

ELECTRICAL CONDUCTIVITY — A measure of the salt content of water.

ELUVIATION — (1) The removal of soil material in suspension (or in solution) from a layer or layers of a soil. (2) The transportation of dissolved or suspended material within the soil by the movement of water when rainfall exceeds evaporation.

EMBANKMENT — An artificial deposit of material that is raised above the natural surface of the land and used to contain, divert, or store water, support roads or railways, or for other similar purposes.

EMBANKMENT DAM — A dam structure constructed of fill material, usually earth or rock, placed with sloping sides and usually with a length greater than its height. Types of embankment dams include:
[1] Earthfill or Earth Dam — An embankment dam in which more than 50 percent of the total volume is formed of compacted fine-grained material obtained from a borrow area (i.e., excavation pit);
[2] Fill Dam — Any dam constructed of excavated natural materials or of industrial waste materials;
[3] Homogeneous Earthfill Dam — An embankment dam constructed of similar earth material throughout, except for the possible inclusion of internal drains or drainage blankets; distinguished from a Zoned Earthfill Dam;
[4] Hydraulic Fill Dam — An embankment dam constructed of materials, often dredged, that are conveyed and placed by suspension in flowing water;
[5] Rockfill Dam — An embankment dam in which more than 50 percent of the total volume is comprised of compacted or dumped pervious natural or crushed rock;
[6] Rolled Fill Dam — An embankment dam of earth or rock in which the material is placed in layers and compacted by using rollers or rolling equipment; and
[7] Zoned Embankment Dam — An embankment dam which is composed of zones of selected materials having different degrees of porosity, permeability, and density.

EMERGENCY ACTION PLAN (Dam) — A predetermined plan of action to be taken to reduce the potential for property damage and loss of lives in a downstream area affected by a dam break or excessive spillway discharges.

EMERGENCY SPILLWAY — A dam spillway built to carry runoff in excess of that carried by the principal spillway; a secondary spillway designed to operate only during exceptionally large floods. Also referred to as Auxiliary Spillway.

EMERGENT — Rising above a surrounding medium, especially a fluid. Having part of a plant aerial and the rest submersed; with parts extending out of the water.

EMERGENT PLANT — A plant that grows in shallow water with the root system submerged under the water and the upper vegetation rising above the water surface.

EMERGENT WETLAND — Typically, a wetland classification characterized by erect, rooted, herbaceous, hydrophytes, excluding mosses and lichens, and which is present for most of the growing season.

ENDEMIC — (Ecology) Confined to, or Indigenous in, a certain area or region, as an endemic plant or animal.

ENERGY — The capacity to perform work, or the potential for power and activity; energy may be captured or held in living matter (e.g., food is stored energy). Various forms of energy include kinetic, potential, thermal, nuclear, rotational, and electromagnetic. Hydroelectric power, a form of potential energy, is derived from flowing water, typically by allowing water to be raised to, or maintained at, an elevated height and then release energy as it flows to a lower level.

ENERGY DISSIPATOR — A structure for slowing the fast moving spillway flows of a dam in order to prevent erosion of the stream channel below the dam.

ENERGY GRADIENT — The change in energy per unit length in the direction of flow or motion.

ENTRAINMENT — (Streams) The incidental trapping of fish and other aquatic organisms in the water, for example, used for cooling electrical power plants or in waters being diverted for irrigation or similar purposes.

ENTRANCE HEAD — The Head required to cause flow into a conduit or other structure, including both entrance loss and Velocity Head.

ENTRAPMENT ZONE — An area of an estuary or other watercourse where seaward-flowing fresh water overlays more dense, saline ocean water resulting in a two-layer mixing zone characterized by Flocculation, aggregation, and accumulation of suspended materials from upstream.

ENVIRONMENT — All of the external factors, conditions, and influences which affect the growth, development, and survival of organisms or a community. The components of an environment include climate, physical, chemical, and biological factors, nutrients, and social and cultural conditions. These influences affect the form and survival of individuals and communities.

ENVIRONMENTAL ANALYSIS — (1) An analysis of alternative actions and their predictable short and long-term environmental effects, which may include physical, biological, economic, social and environmental design factors and their interaction. (2) (NEPA) Systematic process for considering environmental factors in resource management actions.

ENVIRONMENTAL ASSESSMENT (EA) — An environmental analysis prepared pursuant to the National Environmental Policy Act (NEPA) that presents the first thorough examination of alternative plans to positively demonstrate that the environmental and social consequences of an applicable project or action were considered. If it is shown that such activities do, in fact, significantly impact the environment or are otherwise deemed controversial, then an Environmental Impact Statement (EIS) will normally be required.

ENVIRONMENTAL AUDIT — (1) An internal investigation of company compliance with environmental regulations. (2) A study of a site prior to a real estate transaction to uncover potential environmental liability associated with the property, such as the prior improper disposal of hazardous wastes into the ground. (3) An independent assessment of the current status of a party's compliance with applicable environmental requirements or of a party's environmental compliance policies, practices, and controls.

ENVIRONMENTAL EVALUATION — That part of the planning process by governmental agencies that inventories and estimates the potential effects on the human environment of alternative solutions to resource problems, determines the need for an Environmental Assessment (EA) or an Environmental Impact Statement (EIS), and aids in the consideration of alternatives and the identification of available resources.

ENVIRONMENTAL INDICATOR — A measurement, statistic or value that provides a proximate gauge or evidence of the effects of environmental management programs or of the state or condition of the environment.

ENVIRONMENTALISM — Advocacy for or work toward protecting the natural environment from destruction or pollution.

ENVIRONMENTAL MANIPULATION — Actions taken directly or indirectly by man to alter the natural characteristics and evolving patterns of an Ecosystem through alterations to plant or animal life, or habitat conditions.

ENVIRONMENTAL WATER — The water for wetlands, the instream flow for a major river (based on the largest fish flow specified in an entire reach of that river) or, for wild and scenic rivers, the amount of water based on unimpaired natural flow.

EPHEMERAL STREAM — A stream that flows only in direct response to precipitation, and thus discontinues its flow during dry seasons. Such flow is usually of short duration. Most of the dry washes of more arid regions may be classified as ephemeral streams.

EROSION — The wearing away and removal of materials of the earth's crust by natural means. As usually employed, the term includes weathering, solution, corrosion, and transportation. The agents that accomplish the transportation and cause most of the wear are running water, waves, moving ice, and wind currents. Most writers include under the term all the mechanical and chemical agents of weathering that loosen rock fragments before they are acted on by the transportation agents; a few authorities prefer to include only the destructive effects of the transporting agents. Various types of water erosion include:
[1] Accelerated — Erosion much more rapid than normal, natural, or geologic erosion, primarily as a result of the influence of the activities of man or, in some cases, of other animals or natural catastrophes that expose bare surfaces, for example, forest fires;
[2] Geological — The normal or natural erosion caused by geological processes acting over long geologic periods and resulting in the wearing away of mountains, the building up of floodplains, coastal plains, etc., and also referred to as natural erosion;
[3] Gross — A measure of the potential for soil to be dislodged and moved from its place of origin, not necessarily the amount of soil that actually reaches a stream or lake, but the amount of soil that can be calculated from water and wind equations;
[4] Gully — The erosion process whereby water accumulates in narrow channels and, over short periods of time, removes soil from this narrow area to considerable depths, ranging from 1-2 feet (0.3-0.6 meters) to as much as 75-100 feet (23-31 meters); [5] Natural — The wearing away of the earth's surface by water, ice, or other natural agents under natural environmental conditions of climate, vegetation, etc., undisturbed by man, and also referred to as geological erosion;
[6] Normal — The gradual erosion of land used by man that does not greatly exceed natural erosion;
[7] Overfall — Erosion caused by water flowing over an overfall;
[8] Rill — An erosion process in which numerous small channels only several inches deep are formed; occurs mainly on recently cultivated soils and/or recent cuts and fills;
[9] Sheet — The removal of a thin, fairly uniform layer of soil from the land surface by runoff waters;
[10] Shore — Removal of soil, sand, or rock from the land adjacent to a body of water due to wave action;
[11] Splash — The spattering of small soil particles caused by the impact of raindrops on wet soils. The loosened and spattered particles may or may not be subsequently removed by surface runoff;
[12] Streambank — Scouring of material and the cutting of channel banks by running water;
[13] Streambed — Scouring of material and cutting of channel beds by running water;
[14] Undercutting — Removal of material at the base of a steep slope, overfall, or cliff by falling water, a stream, wind erosion, or wave action; the removal steepens the slope or produces an overhanging cliff.

EROSION, BANK — Destruction of land areas bordering rivers or water bodies by the cutting or wearing action of waves or flowing water.

EROSION, BEACH — The retrogression of the shore line of large lakes and coastal waters caused by wave action, shore currents, or natural causes other than Subsidence.

EROSION CONTROL — Materials, structures, and actions utilized and taken to reduce or prevent erosion.

EROSION, GROSS — The total of all sheet, gully, and channel erosion in a drainage basin, usually expressed in units of mass.

EROSION CONTROL — The application of necessary measures including artificial structures, vegetative manipulation, water control, or physical soil changes to minimize soil erosion.

EROSION FLOOD PLAIN — A flood plain that has been created by the lateral erosion and the gradual retreat of the valley walls.

ESTUARINE — (1) Of, pertaining to, or formed in, an Estuary. (2) One of the classification systems under the Wetlands and Deepwater Habitats classification system.

ESTUARINE 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, etc.).

ESTUARINE ZONE — The area near the coastline that consists of estuaries and coastal saltwater wetlands.

ESTUARY — An area where fresh water meets salt water; for example, bays, mouths of rivers, salt marshes, and lagoons. The Coastal Zone Management Act of 1972 defines an estuary as "that part of a river or stream or other body of water having unimpaired connection with the open sea, where the sea-water is measurably diluted with freshwater derived from land drainage." These brackish water ecosystems shelter and feed marine life, birds, and wildlife.

EUTROPHIC (WATER) — Pertaining to a lake or other body of water characterized by large nutrient concentrations such as nitrogen and phosphorous and resulting high productivity. Such waters are often shallow, with algal blooms and periods of oxygen deficiency. Slightly or moderately eutrophic water can be healthful and support a complex web of plant and animal life. However, such waters are generally undesirable for drinking water and other needs. Degrees of Eutrophication typically range from Oligotrophic water (maximum transparency, minimum chlorophyll-a, minimum phosphorus) through Mesotrophic, Eutrophic, to Hypereutrophic water (minimum transparency, maximum chlorophyll-a, maximum phosphorus).

EUTROPHICATION — The process of enrichment of water bodies by nutrients. Degrees of Eutrophication typically range from Oligotrophic water (maximum transparency, minimum chlorophyll-a, minimum phosphorus) through Mesotrophic, Eutrophic, to Hypereutrophic water (minimum transparency, maximum chlorophyll-a, maximum phosphorus). Eutrophication of a lake normally contributes to its slow evolution into a Bog or Marsh and ultimately to dry land. Eutrophication may be accelerated by human activities and thereby speed up the aging process.

EVAPORATION — The physical process by which a liquid (or a solid) is transformed to the gaseous state. In Hydrology, evaporation is vaporization that takes place at a temperature below the boiling point.

EVAPORATION, LAND — Evaporation from land surfaces, in contrast to evaporation from free water surfaces.

EVAPORATION, NET RESERVOIR — The evaporative water loss from a reservoir after making allowance for precipitation on the reservoir and runoff that would have occurred from that precipitation from the land area covered by the reservoir. Net reservoir evaporation equals the total evaporation minus the precipitation on the reservoir plus the runoff from the land area covered by the reservoir.

EVAPORATION OPPORTUNITY (Relative Evaporation) — The ratio of the rate of evaporation from a land or water surface in contact with the atmosphere, to the Evaporativity under existing atmospheric conditions. It is the ratio of actual to potential rate of evaporation, generally expressed as a percentage. The opportunity for a given rate of evaporation to continue is determined by the available moisture supply.

EVAPORATION PONDS — (Water Quality) Shallow ponds in which sewage sludge is placed to dry and then be removed for further treatment and/or disposal. Also, shallow ponds used to extract through evaporation various chemicals in solution or suspension, e.g., salt evaporation ponds. Also see Evaporites.

EVAPORATION RATE — The quantity of water which evaporates from a given surface per unit of time, usually expressed in inches or depth per day, month, or year.

EVAPORATION, TOTAL — The sum of water lost from a given land area during any specific period of time by transpiration from vegetation and the building of plant tissue; by evaporation from water surfaces, moist soil, and snow; and by interception. It has been variously termed Evaporation, Evaporation from Land Areas, Evapotranspiration, Total Loss, Water Loss, and Fly Off.

EVAPORATIVE COOLING — Cooling of a liquid, such as water, by allowing a portion to evaporate. The process is important in the operation of cooling towers used to cool heated effluents from power plants as well as in the cooling of the human body through the evaporation of perspiration. The process is more effective than convection cooling.

EVAPORATIVITY (Potential Rate of Evaporation) — The rate of evaporation under the existing atmospheric conditions from a surface of water that is chemically pure and has the temperature of the atmosphere.

EVAPORITES — Sediments deposited from an aqueous (water) solution as a result of extensive or local evaporation of a solvent, such as salts in the Great Salt Lake in the western United States.

EVAPOTRANSPIRATION (ET) — The combined processes by which water is transferred from the earth surface to the atmosphere; evaporation of liquid or solid water plus transpiration from plants. Evapotranspiration occurs through evaporation of water from the surface, evaporation from the capillary fringe of the groundwater table, and the transpiration of groundwater by plants (Phreatophytes) whose roots tap the capillary fringe of the groundwater table. The sum of evaporation plus transpiration.

EVAPOTRANSPIRATION, ACTUAL — The evapotranspiration that actually occurs under given climatic and soil-moisture conditions.

EVAPOTRANSPIRATION OF APPLIED WATER (ETAW) — The portion of the total Evapotranspiration which is provided by irrigation.

EVAPOTRANSPIRATION, POTENTIAL — (1) The maximum quantity of water capable of being evaporated from the soil and transpired from the vegetation of a specified region in a given time interval under existing climatic conditions, expressed as depth of water. (2) The water loss that will occur if at not time there is a deficiency of water in the soil for use by vegetation.

EVAPOTRANSPIROMETER — An instrument designed to measure Evapotranspiration as related to a particular place, soil type, and vegetation. The device consists of a block of soil with some planted vegetation enclosed in a container. Evapotranspiration is determined by maintaining a Water Budget for the container, that is, accounting for the water applied, water drained off the bottom, and the change in the moisture content of the soil. If there is a provision for drainage of the soil water, the device is referred to as a Lysimeter.

EXCESS RAINFALL — Effective rainfall in excess of infiltration capacity, resulting in runoff.

EXEMPT LAND (USBR) — Irrigation land in a district to which the acreage limitation and pricing provisions of reclamation law do not apply.

EXTREME HIGH WATER OF SPRING TIDES — The highest tide occurring during a lunar month, usually near the new or full moon. This is equivalent to extreme higher high water of mixed semidiurnal tides.

EXTREME LOW WATER OF SPRING TIDES — The lowest tide occurring during a lunar month, usually near the new or full moon. This is equivalent to extreme lower low water of mixed semidiurnal tides.

EXTREME VALUE SERIES — Hydrological series which includes the largest or smallest values, with each value selected from an equal time interval in the record.