EnviroAtlas Terms and Definitions

Below are terms and definitions used throughout the EnviroAtlas website, fact sheets, and other resources. Some terms such as "green space" and "natural land cover" are defined based on the data used in the interactive map and may have alternative definitions outside of EnviroAtlas.

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Nonliving characteristic of the environment; the physical and chemical components that relate to the state of ecological resources. Abiotic materials include non-living environmental media (e.g., water, soils, sediments); abiotic characteristics include such factors as light, temperature, pH, humidity, and other physical and chemical influences.

The geographic area that produces a significant amount of the emissions that contribute to atmospheric deposition in a watershed.

Surrounding, as in the surrounding environment. In EnviroAtlas, ambient air refers to the air surrounding a person through which pollutants can be carried.

Atmospheric Deposition
The process whereby airborne particles and gases are deposited on the earth's surface by wet deposition (precipitation) or by dry deposition (processes such as settling, impaction, and adsorption).


The variability among living organisms (plants, animals, genes, habitats) from all sources, including terrestrial, marine and other aquatic ecosystems, and the ecological complexes of which they are part.

Biological Criteria
Biological criteria or biocriteria are narrative descriptions or numerical values describing the structure and function of aquatic communities at a reference (minimally- or least-disturbed) condition. Biocriteria are necessary to implement water quality standards that protect aquatic life use.

The flora and fauna (plants, animals, fungi, etc.) of a region.

Of or pertaining to living organisms.

Biological poisons (biotoxins) are water pollutants produced by microbes, animals or plants that can cause illness or death in humans, pets, fish, livestock, and birds.

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Carbon Sequestration
Terrestrial, or biologic, carbon sequestration is the process by which trees and plants absorb carbon dioxide, release the oxygen, and store the carbon.

Citizen Science
Scientific projects or in which individual volunteers or networks of volunteers, who may not have specific scientific training, perform or manage research-related tasks such as observation, measurement or computation.

Climate Change
Climate change refers to any significant change in the measures of climate lasting for an extended period of time. In other words, climate change includes major changes in temperature, precipitation, or wind patterns, among others, that occur over several decades or longer.

Climate Forcing
An indicator that measures the "radiative forcing" or heating effect caused by greenhouse gases in the atmosphere. See radiative forcing.

Confined Animal Feeding Operation (CAFO)
An animal feeding operation that (a) confines animals for more than 45 days during a growing season, (b) in an area that does not produce vegetation, and (c) meets certain size thresholds.

Connectivity represents the pattern of core areas of potential habitat or cover that allow the movement of organisms across an intact or fragmented landscape. Landscapes with high connectivity allow species to move freely among core areas, while landscapes with low connectivity tend to isolate species within scattered patches of habitat.

Touching along a boundary or at a point; touching or connected throughout in an unbroken sequence.

Critical Load
The level of atmospheric deposition below which significant harmful effects are not expected to occur in an ecosystem.

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Dasymetric mapping is a technique in which attribute data that is organized by a large or arbitrary area unit is more accurately distributed within that unit by the overlay of geographic boundaries that exclude, restrict, or confine the attribute in question. For example, a population attribute organized by census tract might be more accurately distributed by the overlay of water bodies, vacant land, and other land-use boundaries within which it is reasonable to infer that people do not live (ESRI).

The transformation of nitrates (NO3) to nitrogen oxides or molecular (atmospheric) nitrogen through the respiration of anaerobic bacteria that require low oxygen concentrations plus organic carbon for energy. Denitrification can be used to treat runoff containing excess nutrients. One drawback of denitrification is that it produces nitrous oxide (N2O), a greenhouse gas involved in global warming.

Dry Deposition
The falling of small particles and gases to the Earth without rain or snow. Dry deposition is a component of acid deposition, more commonly referred to as acid rain.

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An interactive system that includes the organisms of a natural community association together with their abiotic physical, chemical, and geochemical environment.

Ecosystem Engineer
Organisms that create, modify, or maintain habitats.

Ecosystem Rarity
A measure of how frequently an ecosystem type is found within a given area.

Ecosystem Service
Outputs of natural ecological functions or processes that directly or indirectly contribute to human welfare, or have the potential to do so in the future (Boyd and Banzhaf, 2007).

Endangered Species
An animal or plant species in danger of extinction throughout all or a significant portion of its range.

Endangered Species Act of 1973
Federal legislation that is intended to provide a means to conserve the ecosystems upon which endangered and threatened species depend and provide programs for the conservation of those species, thus preventing extinction of plants and animals. The law is administered by Interior Department's FWS and Commerce Department's NOAA Fisheries, depending on the species.

The state of a species being limited in range to a geographic area, such as an island or region or a specific habitat type. Endemics are vulnerable to replacement by invasive species or elimination through human disturbance.

Environmental Justice
The fair treatment and meaningful involvement of all people regardless of race, color, national origin, or income with respect to the development, implementation, and enforcement of environmental laws, regulations, and policies.

A complex ecosystem between a river and near-shore ocean waters where fresh and salt water mix. These brackish areas include bays, mouths of rivers, salt marshes, wetlands, and lagoons and are influenced by tides and currents. Estuaries provide valuable habitat for marine animals, birds, and other wildlife.

The process of fertilization that causes high productivity and biomass in an aquatic ecosystem. Eutrophication can be a natural process or it can be a cultural process accelerated by an increase of nutrient loading to a lake by human activity.

Event Mean Concentration (EMC)
A process for representing pollutant concentrations in waterbodies receiving runoff after a precipitation event. The EMC value is determined from a collection of samples taken at various times during the precipitation event combined into a single sample. Event Mean Concentration values are measured as a mass of pollutant per unit volume of water (mg/l).

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Food Chain
A sequence of organisms, each of which uses the next, lower member of the sequence as a food source. Members of a chain are interdependent so that a disturbance to one species can disrupt the entire hierarchy.

Food Web
The complex feeding network occurring within and between food chains in an ecosystem, whereby members of one food chain may belong to one or more other food chains.

Functional Group
A way of categorizing plant or animal species into groups or guilds that describe their habitat, diet, foraging, and nesting type. For studies using wildlife as indicators of human disturbance, species may also be categorized into response guilds, as either tolerant or intolerant of disturbance.

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Gap Status 1
An area having permanent protection from conversion of natural land cover and a mandated management plan in operation to maintain a natural state within which disturbance events (of natural type, frequency, intensity, and legacy) are allowed to proceed without interference or are mimicked through management (USGS-GAP). This status correlates with the IUCN classifications Ia, Ib, and II.

Gap Status 2
An area having permanent protection from conversion of natural land cover and a mandated management plan in operation to maintain a primarily natural state, but which may receive uses or management practices that degrade the quality of existing natural communities, including suppression of natural disturbance (USGS-GAP). This status correlates with IUCN classifications III, IV, V, and VI.

Gap Status 3
An area having permanent protection from conversion of natural land cover for the majority of the area, but subject to extractive uses of either a broad, low-intensity type (e.g., logging, OHV recreation) or localized intense type (e.g., mining). It also confers protection to federally listed endangered and threatened species throughout the area (USGS-GAP). This status does not correlate with any IUCN classifications.

Geospatial Data
The information that identifies the geographic location and characteristics of natural or constructed features and boundaries on the earth. This information may be derived from, among other things, remote-sensing, mapping, and surveying technologies.

Green Infrastructure
Sustainable pollution-reducing practices that also provide other ecosystem services such as reduced greenhouse gas emissions, health benefits, and increased flood control. Examples include grass and forest buffers and use of porous materials for paving.

Green Space
EnviroAtlas defines green space as all vegetated land, including agriculture, lawns, forests, wetlands, and gardens. Barren land and impervious surfaces such as concrete and asphalt are excluded.

Greenhouse Gas
A greenhouse gas is any gas that absorbs infrared radiation in the atmosphere. Greenhouse gases include, but are not limited to, water vapor, carbon dioxide (CO2), methane (CH4), nitrous oxide (N2O), halogenated fluorocarbons (HCFCs), ozone (O3), perfluorinated carbons (PFCs), hydrofluorocarbons (HFCs), and sulfur hexafluoride (SF6).

Groundwater Recharge
Groundwater recharge renews the underground freshwater supply depleted by runoff to springs, streams, and wetlands and human extraction for wells and irrigation. Water for groundwater recharge soaks into the ground and moves through pores in the soil to the water table.

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Short for hydrologic unit code. A hierarchical, numeric code that uniquely identifies hydrologic units. There are six levels of Hydrologic Unit Code classification: 2-digit (region), 4-digit (subregion), 6-digit (accounting unit), 8-digit (cataloguing unit), 10-digit (watershed), and 12-digit (subwatershed). EnviroAtlas national maps summarize data by 12-digit HUC (subwatershed level). See NRCS Hydrologic Units.

Hydrologic Regime
Variations in the state and characteristics of a water body that are regularly repeated in time and space and which go through phases, such as seasonal changes.

Low oxygen content in water.

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Impervious Surface
A hard surface area that either prevents or retards the entry of water into the soil mantle or causes water to run off the surface in greater quantities or at an increased rate of flow. Common impervious surfaces include rooftops, walkways, patios, driveways, parking lots, storage areas, concrete or asphalt paving, and gravel roads.

Invasive species
A non-indigenous plant or animal species that can harm the environment, human health, or the economy.

IUCN Category Ia
Strict Nature Reserve - Category Ia are strictly protected areas set aside to protect biodiversity and also possibly geological/geomorphological features, where human visitation, use and impacts are strictly controlled and limited to ensure protection of the conservation values. Such protected areas can serve as indispensable reference areas for scientific research and monitoring (IUCN).

IUCN Category Ib
Wilderness Area - Category Ib protected areas are usually large unmodified or slightly modified areas, retaining their natural character and influence, without permanent or significant human habitation, which are protected and managed so as to preserve their natural condition (IUCN).

IUCN Category II
National Park - Category II protected areas are large natural or near natural areas set aside to protect large-scale ecological processes, along with the complement of species and ecosystems characteristic of the area, which also provide a foundation for environmentally and culturally compatible spiritual, scientific, educational, recreational and visitor opportunities (IUCN).

IUCN Category III
Natural monument or feature - Category III protected areas are set aside to protect a specific natural monument, which can be a landform, sea mount, submarine cavern, geological feature such as a cave or even a living feature such as an ancient grove. They are generally small protected areas and often have high visitor value (IUCN).

IUCN Category IV
Habitat/species management area - Category IV protected areas aim to protect particular species or habitats and management reflects this priority. Many category IV protected areas will need regular, active interventions to address the requirements of particular species or to mintain habitats, but this is not a requirement of the category (IUCN).

IUCN Category V
Protected landscape/seascape - A protected area where the interaction of people and nature over time has produced an area of distinct character with significant ecological, biological, cultural and scenic value: and where safeguarding the integrity of this interaction is vital to protecting and sustaining the area and its associated nature conservation and other values (IUCN).

IUCN Category VI
Protected area with sustainable use of natural resources - Category VI protected areas conserve ecosystems and habitats, together with associated cultural values and traditional natural resource management systems. They are generally large, with most of the area in a natural condition, where a proportion is under sustainable natural resource management and where low-level non-industrial use of natural resources compatible with nature conservation is seen as one of the main aims of the area (IUCN).

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Keystone Species
A species whose loss from an ecosystem would cause a greater than average change in other species populations or ecosystem processes; species that have a disproportionately large effect on other species in a community.

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Land cover
The observed biophysical cover on the earth's surface.

Leaf Area Index
The ratio of the total upper leaf surface of the plant community to the corresponding ground area expressed as a proportion.

Long-lived Gas
A chemically stable gas that persists in the atmosphere over time scales of a decade to centuries or longer (half-life greater than 100 years) and has a long-term influence on climate.

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A climate in a small area that varies significantly from the overall climate of a region. Microclimates are formed by natural or man-made geography and topography, such as hills, buildings, and the presence or absence of trees and vegetation.

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National Ambient Air Quality Standards (NAAQS)
A level of outdoor air quality set by the EPA to protect human health and public welfare. Standards have been set for six common pollutants: carbon monoxide, lead, nitrogen dioxide, particulate matter, ozone, and sulfur dioxide.

Natural Land Cover
In EnviroAtlas, natural land cover includes barren lands, forests, shrublands, herbaceous land, and wetlands as classified by the 2006 National Land Cover Database.

Nonpoint Source Pollution
Pollution that is not released through pipes, but rather originates from multiple sources over a relatively large area. Nonpoint sources can be divided into source activities related to either land or water use including failing septic tanks, improper animal-keeping practices, forest practices, and urban and rural runoff.

Nutrient Loading
The amount of nutrients, typically nitrogen or phosphorus, entering into a defined geographic area, such as a watershed. Nutrients may enter water from runoff, groundwater, or the air.

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Organic Nitrogen
Organic nitrogen is the byproduct of living organisms. It includes proteins, amino acids, urea, and particles of plant and animal debris. Wastewater-derived dissolved organic nitrogen may be a significant source of available nitrogen to surface waters in disturbed watersheds.

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A land lot or piece of land.

Point Source Pollution
Pollutant loads discharged at a specific location from pipes, outfalls, and conveyance channels from either municipal wastewater treatment plants or industrial waste treatment facilities. Point sources can also include pollutant loads contributed by tributaries to the main receiving water stream or river.

Primary Consumer
A primary consumer is an herbivore, an animal that has an all-plant diet.

Provisioning Services
Provisioning services are the products or ecosystem goods obtained from ecosystems, such as food, water, genetic, and biochemical resources.

Public Health
Public health refers to all organized measures (whether public or private) to prevent disease, promote health, and prolong life among the population as a whole. Its activities aim to provide conditions in which people can be healthy and focus on entire populations, not on individual patients or diseases (World Health Organization).

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Radiative Forcing
A measure of the influence of a particular factor (e.g. greenhouse gas [GHG], aerosol, or land use change) on the net change in the Earth's energy balance.

Reactive Nitrogen
All chemically, biologically, and radiatively active forms of nitrogen on Earth.

The land adjacent to a stream or other water body.

Riparian Buffer
A vegetated area of varying widths adjacent to streams, rivers, lakes, ponds, or wetlands that is important for providing shade, filtering, and habitat benefits. A fixed-distance buffer along stream networks of 15, 30, or 50m width is often used in Geographic Information Systems (GIS) map analysis of water quality research questions.

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Salt water intrusion
Displacement of fresh or ground water by the advance of salt water due to its greater density, usually in coastal and estuarine areas.

Secondary Consumer
A secondary consumer is a carnivore that feeds on plant-eating (herbivorous) animals.

Shannon-Weiner Index
The Shannon-Weiner Index is a measure of the diversity of land cover types throughout the reporting unit. The index value increases with the number of land cover types in the reporting unit.

Standardized Shannon-Weiner Index
The Shannon-Weiner index can be standardized to account for the variety of distinct land cover classes present in the reporting unit.

Short-lived gas
A chemically reactive gas that is generally removed by natural oxidation processes in the atmosphere, by removal at the surface, or by washout in precipitation. Their half-life is less than 100 years and their concentrations are highly variable.

Simple diversity or Richness Index
The Simple diversity or Richness Index is a landscape diversity index that is a simple count of the number of distinct land cover classes present in the reporting unit.

Simpson's index
Simpson's index is a landscape diversity index that measures the evenness of the distribution of land cover classes throughout the reporting unit. Simpson's index is most sensitive to the presence of common land cover types. Simpson's index values range from 0 to 1, with 1 representing perfect evenness of all land cover types.

Any process, activity or mechanism which removes a greenhouse gas, an aerosol, or a precursor of a greenhouse gas or aerosol from the atmosphere.

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The Eco-Health Relationship Browser
An interactive, relational browser that illustrates the linkages between human health and ecosystem services. Eco-Health Browser

Threatened Species
An animal or plant species likely to become endangered within the foreseeable future throughout all or a significant portion of its range.

Total Maximum Daily Load (TMDL)
A calculation of the maximum amount of a pollutant that a waterbody can receive and still safely meet water quality standards.

Total Suspended Solids (TSS)
Total suspended solids (TSS) are floating solids in water that can be trapped by a filter, typically a 2-micron filter. TSS include silt and clay soil particles, algae, fine organic debris, and sewage.

Trophic Cascade
A disruption in the balance among members of a food chain occurring from the top-down (removal of one or more predators) or from the bottom-up (removal of one or more herbivores). For example, removal of a predator may allow herbivores to increase until their habitat is degraded, negatively affecting other species dependent on the same habitat.

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Urban Heat Island Effect
A measurable increase in ambient urban air temperatures resulting primarily from the replacement of vegetation with buildings, roads, and other heat-absorbing infrastructure. The heat island effect can result in significant temperature differences between rural and urban areas.

Urban Sprawl
Patterns of urban growth which includes large acreage of low-density residential development, rigid separation between residential and commercial uses, residential and commercial development in rural areas away from urban centers, minimal support for non-motorized transportation methods, and a lack of integrated transportation and land use planning.

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The locations visible from one or more specified points. In EnviroAtlas, potential viewsheds are estimated based on whether a window is present and if the landscape allows for the line of sight.

Volatile Organic Compound
Emitted as gases from certain solids or liquids, VOCs include substances - some of which may have short- and long-term adverse health effects - such as benzene, toluene, methylene chloride, and methyl chloroform.

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The land area that drains into a river, stream, or lake. Watershed boundaries follow the highest ridgeline around the stream drainage area. The bottom of the watershed or pour point is the lowest point of the land area where water flows out of the watershed.

Web Services
Web services are used by geographic information system (GIS) professionals to share geospatial data. An online representation of spatial data by a server is accessible to other computers or applications. It eliminates the need to download the data and then upload it in order to use it in a different application. EnviroAtlas implements Esri's ArcGIS Server technology to create, publish, and share web services for data developed for EnviroAtlas. EnviroAtlas web services are used to display data in the EnviroAtlas Interactive Map and are also available for public use in desktop mapping applications or web-based mapping applications.

Wet deposition
Atmospheric deposition that occurs when rain, snow, or fog carry gases and particles to the earth's surface.

Willingness to Pay (WTP)
A measure of value based on the premise that the value of a good is simply what it is worth to those who consume it or benefit from it. The total value of an avoided illness is what the otherwise-afflicted individual would be willing to pay to avoid it plus what others would be willing to pay for him or her to avoid it (EPA).

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