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The terms below are helpful for understanding the activities of ReVA. Refer to EPA's Terms of the Environment for a more complete glossary of terms related to environmental subjects.




Acidic Deposition: A complex chemical and atmospheric phenomenon that occurs when emissions of sulfur and nitrogen compounds and other substances are transformed by chemical processes in the atmosphere, often far from the original sources, and are then deposited on earth in either wet or dry form. The wet forms, popularly called "acid rain," can fall as rain, snow, or fog. The dry forms are acidic gases or particulates.

Assessment: A study designed to estimate or determine the significance and effects of factors or events




Bioaccumulants: Substances that increase in concentration in living organisms as they take in contaminated air, water, or food because the substances are very slowly metabolized or excreted.

Biofuel: Broadly defined as solid, liquid, or gas fuel consisting of, or derived from biomass. Typically these fuels are termed "renewable" because feedstocks are grown and harvested specifically for fuel (as opposed to traditional sources such as petroleum or natural gas which are considered non-renewable)

Biodiversity: The variety and variability among living organisms and the ecosystems in which they occur. Biodiversity includes the number of different items and their relative frequencies; these items are organized at many levels, ranging from complete ecosystems to the biochemical structures that are the molecular basis of heredity. Thus, the term biodiversity encompasses expressions of the relative abundances of different ecosystems, species, and genes.

Biological Magnification: Refers to the process whereby certain substances such as pesticides or heavy metals move up the food chain, work their way into rivers or lakes, and are eaten by aquatic organisms such as fish, which in turn are eaten by large birds, animals or humans. The substances become concentrated in tissues or internal organs as they move up the chain.




Cellulosic Biofuel: Refers to the process of creating ethanol with a wide variety of feedstocks (corn stover, cereal straws, sawdust, paper pulp) with high contents of cellulose (chief part of the cell walls of plants). The cellulose must be converted from complex carbohydrates into simple sugars as part of the process of creating biofuel.

Community: An assemblage of populations of plants, animals, and microbes that interact with each other and their environment. The community is shaped by populations and their geographic ranges, the types of areas they inhabit, species diversity, species interactions, the flow of energy, and the cycling of nutrients.

Continuous Variable:A variable for which the values do not occur on a continuous scale because of gaps between possible values. Examples are the number of petals on a flower or the number of insects caught in the sweep of a net: only whole numbers (5, 6, 311, etc.) are possible.

Correlation Analysis: An analysis technique quantifying the strength of the relationship of one variable to another.




Discrete Variable: A variable for which the values are not observed on a continuous scale because of the existence of gaps between possible values. Examples are the number of petals on a flower or the number of insects caught in the sweep of a net.

Disturbance: Any event (natural or action by man), that alters the structure, composition, or functions of an ecosystem. Examples of disturbance include forest fires, insect infestations, floods, clearing land for agriculture or urban development, and mountain-top removal for resource extraction.




Ecological Goods and Services: The direct and indirect benefits received from ecological processes such as filtered water and air, pollination of crops, land for recreation, nutrient cycling, raw materials, food production, soil erosion control, and soil formation.

Ecological Risk Assessment: Evaluation of the potential adverse effects that human activities have on the plants and animals that make up ecosystems

Ecological Benefits: Contributions to social welfare of ecological goods and services. The term applies specifically to net improvements in social welfare that result from changes in the quantity or quality of ecological goods and services attributable to EPA policy.

Ecological Benefits Assessment: Evaluation of the expected changes in social welfare resulting from EPA policies via changes in ecological functions or processes; outcomes are described qualitatively and are quantified in physical and monetary terms when possible.

Ecological Functions: Characteristic physical, chemical, and biological activities that influence the flows, storage, and transformations of materials and energy within and through ecosystems, such as the uptake of nitrogen from soil by vegetation.

Ecological Goods and Services: Outputs of ecological functions or processes that directly or indirectly contribute to social welfare or have the potential to do so in the future. Some outputs may be bought and sold, but most are not marketed.

Ecological System: The most complex level of organization is the ecological system. An ecosystem includes the plant, animal and microbial communities in an area, together with the non-living physical environment that supports them. Ecosystems have physically defined boundaries, but they are also dynamic: their boundaries and constituents change over time. They can import and export materials and energy. Thus, an ecological system can interact with and influence other ecosystems. They can also vary widely in size (e.g., ponds, lakes, oceans).

Ecology: The relationships of living things to one another and their environment, or the study of such relationships.

Ecosystem: Biotic and abiotic environment within which most or all nutrients are recycled.

Ecoregions: A large area whose boundaries are fixed by geography, topography, climate, vegetation, and other easily recognized natural features of the landscape. Ecoregions contain many landscapes with different spatial patterns of ecosystems.

Edge Habitat: The outermost belt (ranging from a few feet to several hundred feet) of a patch that has an environment that is very different from the interior of a patch. The term edge habitat can be used, for example, to define the area transitioning from a grass- and shrub-dominated area to a nearby solidly forested area.

Effector: A physical, chemical, or biological factor that indirectly causes stress. It indicates the possibility for future stress, but does not directly cause stress.

Endpoints: A technical term used to describe the environmental value that is to be protected. An environmental value is an ecological unit and its characteristics. For example, salmon are valued ecological units; reproduction and age class structure are some of their important characteristics. Together "salmon reproduction and age class structure" form an endpoint.

Environmental Stressor: Any event or situation that requires a nonroutine change in adaptation or behavior of the environment

Estuary: Partially enclosed body of water formed where freshwater from rivers and streams flows into the ocean, mixing with the salty sea water

Euclidean distance: The straight line distance between two points.  ReVA calculates euclidean distances in multivariate space and thus uses a generalized equation.  In N dimensions, the multivariate Euclidean distance between two points p and q is √(∑i=1N (pi-qi)²) where pi (or qi) is the coordinate of p (or q) in dimension i.

Exposure: The contact or co-occurrence of a stressor with an ecological unit.




Forest Productivity: The conversion of light energy and carbon dioxide into living organic material (energy stored in carbon-based compounds), often measured by forest inventories by species, and typically expressed in terms of energy per unit area per unit time




Geographic Information System (GIS): A GIS is a system of hardware and software used for storing, retrieving, mapping, and analyzing geographic data. It is a computer technology that brings together all types of information based on geographic location for the purpose of query, analysis, and generation of maps and reports. GIS is both a database designed to handle geographic data, and a set of computer operations ("tools") that can be used to analyze the data. In a sense, GIS can be thought of as a higher-order map.

Ground Water: Water occurring in the soil or in an aquifer




Hydrography: The description, study, and mapping of the waters of the Earth's surface (the seas, lakes and rivers), including their forms and physical features.

A member of the hierarchical system for identifying and subdividing river-basin units within the United States. Hydrologic units are used for the collection and organization of hydrologic data. The levels of the hierarchy, listed in order of largest to smallest in area, are: region, subregion, accounting unit, and subbasin. Each hydrologic unit is identified uniquely with a hydrologic unit code.

Hydrologic Unit Code (HUC): A hierarchical, numeric code that uniquely identifies hydrologic units. The first two digits identify the region, the first four digits identify subregions, the first six digits identify accounting units, and the full eight digits identify subbasins. From the above example, (definition of hydrologic unit), the hydrologic unit codes are:

02 - the region (Mid-Atlantic)

0206 - the subregion (Upper Chesapeake)

020600 - the accounting unit (Upper Chesapeake. Delaware, Maryland, Virginia, and Pennsylvania)

02060002 -

Zeroes in the two-digit accounting unit field indicate that the accounting unit and the subregion are the same. Zeroes in the two-digit subbasin field indicate that the subbasin and the accounting unit are the same.




Index: A composite of metrics that synthesizes characteristics among these metrics into one value to represent environmental condition or environmental vulnerability, or some other quality or feature.

Indicator:  An organism or metric whose characteristics show the presence of specific environmental conditions, good or bad (i.e.  it "indicates" environmental condition, no more).

Interior Habitat: Habitat necessary for insulation from edge effects (e.g., noise, wind, sun, predation) that occurs within the interior of a patch.




Land Cover: Anything that is visible from above the Earth's surface. Examples include vegetation, exposed or barren land, water, snow, and ice.

Land Use: The way land is developed and used with respect to the kinds of anthropogenic (human-induced) activities that occur (e.g., agriculture, residential uses, industrial uses).

Land Use Planning: The process of organizing the use of lands and their resources to best meet people's needs over time, according to the land's capabilities.

Landscape: A large land area composed of interacting ecosystems that are repeated due to factors such as geology, soils, climate, and human impacts.

Landscape Characterization: Use of historical and current information to describe land cover types and other landscape features for the purpose of evaluating their change over time

Landscape Metric: A characteristic of the environment that is measured to provide evidence of the biological condition of one or more resources at the ecosystem level.




Migratory Bird Stopovers: Areas where birds rest and feed for a short period of time during their seasonal movements. Large-scale examples include the Lake Erie islands and the Salton Sea. Wetlands, pinelands, and vernal pools also can be important stopover locations for migratory birds.

Model: A representation of reality used to simulate a process, understand a situation, predict an outcome, or analyze a problem. Models can range from very simple (e.g., the relationship between fish length and fish weight) to extremely complex (e.g., global change models, used to make predictions about the effects of climate change).

Monetization: Valuation in monetary (dollar) terms. Also “economic valuation” or “monetary valuation.”




Natural State: The set of conditions under which an ecosystem evolved.

Non-Indigenous Species (NIS): Nonnative plant, animal, or microbe species introduced into a region that then overwhelms, crowds out, or disrupts relationships among native species, degrades habitats, and contaminates the gene pools of indigenous species. Examples include the wooly adelgid (an insect damaging hemlock trees in the Smoky Mountains), kudzu, and fire ants in southern U.S., and more than 160 known aquatic species in the Great Lakes.

Non-Point Source (NPS) Pollution: Pollution with a non-specific location. The sources of the pollutant(s) are dispersed, not well defined, and typically not constant. Rainstorms and snowmelt often make this type of pollution worse. Examples include sediments from construction sites and chemical-bearing runoff from road surfaces and agricultural fields.

Non-renewable Energy: Energy from a natural resource that cannot be re-made, re-grown or regenerated on a scale comparative to its consumption.




Parameter: A quantity that characterizes a statistical population and can be estimated by calculations from sample data.

Pathogens: Microorganisms (e.g., bacteria, viruses, fungi or protozoans) that can cause disease in humans, animals, or plants. Some pathogens are highly specific to particular species; others can require two or more types of organisms to complete their life cycles, while others can opportunistically move among several or many host species.

Population: An aggregate of individuals of a species within a specified location in space and time.




Quality of Life: The level of enjoyment and fulfillment derived by humans from the life they live within their local economic, cultural, social, and environmental conditions.

Quantification: Expression of benefits in numerical units.




Receptor: A receptor is an ecological entity exposed to a stressor. This term may refer to tissues, organisms, populations, communities, and/or ecosystems. While either "ecological component" (U.S. EPA, 1992a) or "biological system" (Cohrssen and Covello, 1989) are alternative terms, "receptor" is usually clearer in discussions of exposure where the emphasis is on the ecological entity exposed to the stressor.

Reflectance: The ratio of reflected power to incident power. In ecological uses, reflectance is a measure of the ability of a surface to reflect light or other electromagnetic radiation. Examples include the amount of light energy “bouncing off” of a leaf surface, compared to the amount of light striking the leaf surface, or, at a larger scale, the amounts of light (in different wavelengths, measured by satellite sensors) “bouncing off” of water surfaces or land areas dominated by snow or ice cover, forests, grasslands, agricultural activities, or urban structures.

Region: In the ReVA context, a region is a large, multi-state geographic area corresponding to an EPA region such as the Mid-Atlantic, Northeast, Southeast, or Pacific Northwest regions within the United States.

Relative Vulnerability: The vulnerability of an ecological entity in relation to another ecological entity. A tropical rainforest ecosystem, for example, can be more vulnerable than a temperate forest to temporal variation in rainfall.

Renewable Energy: Utilizes natural resources such as sunlight, wind, tides and geothermal heat, which are naturally replenished

Risk: A measure of the probability that damage to human life, health, property, and/or the environment will occur as a result of a given hazard.

Risk Management: The process of evaluating and selecting alternative regulatory and non-regulatory responses to a given hazard. It requires the consideration of legal, economic, and behavioral factors.

Runoff: The flow of water, usually from rainfall and snow melt, which is not absorbed into the ground. It flows across the land and eventually runs to stream channels, lakes, oceans, or other depressions in the Earth's surface. The characteristics that affect the rate of runoff include rainfall duration and intensity, as well as the ground's slope, soil type, and amount and types of vegetation present. Runoff can intercept pollutants from the air and land and carry them into streams and lakes.




Scale: The spatial or temporal dimension over which an object or process exists, as in, for example, a landscape, or a forest ecosystem or community

Social Welfare: Human well-being, measured at some aggregate level.

Spatial Scale: Also referred to as Spatial Distribution, a geographic area bounded by geopolitical boundaries (state, county,

Spatial Variability: The different ways in which things are arranged on a map. For example, the number, size, shape, and distance between patches can be used to describe the pattern of forest patches. The spatial pattern exhibited by a map also can be described in terms of its overall texture, complexity, connectity of units, and other indicators.

Stressors: A physical, chemical or biological factor that can disrupt, change or otherwise alter ecosystem health and human health in a negative way. For example, pesticides used in agriculture are stressors both to ecosystem health and human health.

Surface Water: Water found over the land surface in streams, ponds, marshes, lakes, or other fresh (not salty) sources

Synergistic Effects: The effect of two or more variables whose combined effects are greater than the sum of each individual effect.




Temporal Scale: Duration or period of time

Threshold: An ecological threshold is a condition beyond which there is an abrupt change in a quality or property of the ecosystem. Ecosystems often do not respond smoothly and steadily to gradual changes in forcing variables. Instead, they often respond with abrupt, discontinuous shifts to an alternative state as the ecosystem exceeds a threshold for one or more of its key variables or processes. A threshold is the point at which such an effect can be seen.




Valuation: Process of estimating the worth, merit, or desirability of something. In the EBASP, the term is used more specifically to mean estimating the worth of a wide variety of environmental conditions in common units that can be aggregated and compared.

Vulnerability: Susceptibility to degradation or damage from adverse factors or influences.




Watershed: A watershed is an area of land that is drained by a single stream, river, lake, or other body of water. Ridges form the dividing lines between watersheds. Water on one side of the ridge flows into one stream, and water on the other side of the ridge flows into a different stream. Thus, a watershed is a natural unit defined by the landscape


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