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E

E. COLI (ESCHERICHIA COLI)—A bacterial species which inhabits the intestinal tract of man and other warm-blooded animals. Although it poses no threat to human health, its presence in drinking water does indicate the presence of other, more dangerous bacteria. Also see Bacteria.

EAGRE—A high, often dangerous wave caused by the surge of a flood tide upstream in a narrowing Estuary or by colliding tidal currents. Also referred to as a Bore.

EARLY SERAL CONDITION—Synonymous with poor ecological conditions.

EARTHFILL DAM—A dam the main section of which is composed principally of earth, gravel, sand, silt, and clay. Also referred to as Earth Dam. Also see Embankment Dam.

EASEMENT—A legal instrument enabling the giving, selling, or taking or certain land or water rights without transfer of title, such as for the passage of utility lines. An affirmative easement gives the owner of the easement the right to use the land for a stated purpose. A negative easement is an agreement with a private property owner to limit the development of his land in specific ways.

EBB—(1) Ebb Tide. (2) To fall back from the Flood Stage.

EBB TIDE—That period of tide between a high water and the succeeding low water; falling tide. Also see Tides.

ECHARD—Soil water not available for absorption by plants.

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.

ECONOMETRIC MODEL BUILDING—(Statistics) An iterative process for developing a model beginning with some information about the form and structure of the problem and with relevant data. The model building process typically follows a sequence of inter-related steps to include:

[1] Problem Identification and Data Selection—Data is selected, compilation, screened, and analyzed, and the various series tested based on hypotheses of probable causation;
[2] Model Identification (or Specification)—Selection of a general model structure is made based on the nature of the data and the types of outputs desired. Some of these include, for example, a simple single mathematical equation, or multiple (sequential) equations, statistically-based univariate (deterministic) autoregressive functions, multivariate analysis, simple ordinary least squares (OLS) regression, multiple regression, simultaneous equation, etc.;
[3] Estimation (Model Fitting)—Based on the selection of a model structure, the data is used to best describe the behavior of the variable under observation, e.g., stream flows, reservoir levels, runoff, economic output, employment, consumer spending, etc.;
[4] Model Testing (and Refinement, as Necessary)—The model's structure and variables chosen are then validated by applying the data and observing forecast errors with respect to know (sample) values;
[5] Forecasting—Based upon the ability of the model to accurately "fit" or predict historical values, the model is used to forecast beyond the last data point as prescribed by scenarios under analysis.

Also see Econometrics, Regression Analysis, Stochastic Process, and Deterministic Process.

ECONOMETRICS—(Statistics) The application of statistical and mathematical methods to the analysis of economic data, with a purpose of giving empirical content to economic theories and verifying them or refuting them. Also see Econometric Model Building, Regression Analysis, Stochastic Process, and Deterministic Process.

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. Also see Biodiversity.

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.

ECOTONE—(1) A habitat created by the juxtaposition of distinctly different habitats; an edge habitat; or an ecological zone or boundary where two or more ecosystems meet. (2) A transition line or strip of vegetation between two communities, having characteristics of both kinds of neighboring vegetation as well as characteristics of its own.

ECOTYPE—A locally adopted population of a species which has a distinctive limit