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California Water Science Center Glossary

0-9 A B C D E F G H I J K L M N O P Q R S T U V W X Y Z



The fraction of dissolved organic matter (DOM) that interacts with light in the visible spectrum is referred to as chromophoric dissolved organic carbon (CDOM). The extent to which a specific sample absorbs light depends upon the wavelength of light used. For this reason, spectrophotometry is performed using monochromatic light where all photons have the same wavelength. The intensity of absorbed light at a single wavelength (e.g. A254) can then be related to a sample's DOM concentration and composition.


Absorption is physical or chemical process in which atoms, molecules or ions enter liquid or solid material. This is a different process from adsorption, because molecules being absorbed are taken up by the volume of the material, not only by the surface of the material (as in the case for adsorption).


Acre-feet indicates the amount of water required to cover 1 acre of land (43,560 square feet) to a depth of 1 foot. Equal to 325,851 gallons or 1,233 cubic meters.


Adsorption is a process in which atoms, molecules, or ions accumulate on the surface of materials. For example, pesticides can adsorb to the surface of soil.


Alkaline is a characteristic where water and/or soils contain an amount of basic substances (e.g. HCO3, CO3, or OH) sufficient to raise the pH value above 7.0.


Alluvium are deposits of clay, silt, sand, gravel, or other particulate material that have been deposited by a stream or other body of running water in a streambed, on a flood plain, on a delta, or at the base of a mountain.


Amalgamation is the dissolving or blending of a metal (commonly gold and silver) in mercury to separate it from its parent material.


Anthropogenic is material or a process that originates from human activity.


An aquifer is an underground layer of water-bearing permeable rock, rock fractures, or unconsolidated materials. Groundwater can be extracted from an aquifer using a well. 

Artificial recharge

Artificial recharge is the process where water is put back into groundwater storage from surface-water supplies such as irrigation, or induced infiltration from streams or wells.



The Basin Characterization Model (BCM) is a simple grid-based model that calculates the water balance for any time step or spatial scale by using climate inputs, precipitation, minimum and maximum air temperature.

Base flow

Base flow is the sustained flow of a stream in the absence of direct runoff. It includes natural and human-induced streamflows. Natural base flow is sustained largely by groundwater discharges.

Basin characterization model

The Basin Characterization Model (BCM) is a simple grid-based model that calculates the water balance for any time step or spatial scale by using climate inputs, precipitation, minimum and maximum air temperature.


Bathymetry is the study of the "beds" or "floors" of water bodies, including the ocean, rivers, streams, and lakes.


Bedrock is a general term for consolidated (solid) rock that underlies soils or other unconsolidated material.


Benthic refers to plants or animals that live on the bottom of lakes, streams, or oceans.


Bioaccumulation is the biological build up of a substance at a higher concentration than that at which it occurs in the surrounding environment or medium. Also, the process whereby a substance enters organisms through the gills, epithelial tissues, dietary, or other sources.


Bioavailability is the capacity of a chemical element to be taken up by living organisms, either through physical contact or by ingestion.


Biochemical refers to chemical processes that occur inside or are mediated by living organisms.


Biomass is the amount of living matter, in the form of organisms, present in a particular habitat, usually expressed as weight per unit area.


Biota are living organisms.

Blue carbon

Blue carbon refers to the carbon captured and stored in oceans and coastal ecosystems. Blue carbon can be found in a number of places, including tidal wetlands and marshes. Because blue carbon ecosystems are such effective carbon absorbers (or sinks), play a major role in slowing climate change.


Carbon budget

Carbon budget refers to the quantity of carbon in the ecosystem as well as where in the ecosystem it exists and how it moves in and out of the system. 

Carbon sequestration

Carbon sequestration refers to carbon removed from the atmosphere and retained within the ecosystem.

Carbon sink

A carbon sink is any natural reservoir that absorbs more carbon than it releases, and thereby lowers the concentration of CO2 from the atmosphere. Carbon sinks can play a major role in slowing climate change. However, when drained, they release their enormous carbon stores into the atmosphere.


Channelization is the modification of a stream, typically by straightening the channel, to provide more uniform flow; often done for flood control or for improved agricultural drainage or irrigation.


California Oil, Gas, and Groundwater (COGG) program was created to identify where and how oil and gas operations are impacting California's protected groundwater resources. The California State Water Resources Control Board is a chief partner in the COGG program

Conceptual Model

A conceptual model is a representation of a system, involving the development of an abstract, graphical, or simplified concepts of interest and how they relate within the larger system.


A contaminant is an undesirable substance, not normally present or an unusually high concentration, of a naturally occurring substance in water or soil.


Cyanobacteria are a group of photosynthetic bacteria, some of which are nitrogen-fixing, that live freshwater, estuarine, and marine water. Some species of cyanobacteria produce toxins, referred to as cyanotoxins, that negatively impact aquatic ecosystem health. Other species produce compounds that create taste and odor issues in drinking water. In addition, low dissolved oxygen concentrations and changes in aquatic food webs can result from an over-abundance of cyanobacteria. The role of cyanobacteria in aquatic foodwebs is not well understood.


Cyanotoxins are toxins produced by some species of cyanobacteria (also referred to as blue-green algae) which can cause acute and chronic illnesses in humans and pets. Not all species of cyanobacteria produce toxins, and species that can produce toxins do not always produce them. There are a number of different kinds of toxins, each of which have specific health concerns. Commonly found cyanotoxins in the U.S. include microcystins, cylindrospermopsin, anatoxins and saxitoxins.



DDT stands for dichloro-diphenyl-trichloroethane. An organochlorine insecticide no longer registered for use in the United States.


Denitrification is a process by which oxidized forms of nitrogen such as nitrate (NO3) are reduced to form nitrites, nitrogen oxides, ammonia, or free nitrogen: commonly brought about by the action of denitrifying bacteria and usually resulting in the escape of nitrogen to the atmosphere.


Desalination is the removal of salts from saline water to provide freshwater. This method is becoming a more popular way of providing freshwater to populations.


Diatoms are single-celled, colonial, or filamentous algae with siliceous cell walls constructed of two overlapping parts.


Discharge is the volume of water that passes a given location within a given period of time. Usually expressed in cubic feet per second.

Dissolved organic carbon (DOC)

Dissolved organic carbon (DOC) is defined operationally as the organic matter in solution that passes through a 0.45-miron filter.

Dissolved organic matter (DOM)

Dissolved organic matter (DOM) is found in every water body, both marine and freshwater. It is a heterogeneous mixture derived primarily from the decomposition products of plant material, bacteria and algae. The character of aquatic DOM varies according to precursor material, which falls broadly into two categories: allochthonous, which is terrestrially-derived and autochthonous, which means it is derived from within the aquatic ecosystem.

Drainage basin

A drainage basin is an area of land that drains all the streams and rainfall to a common outlet such as the outflow of a reservoir, mouth of a bay, or any point along a stream channel.


Drawdown is the difference between the water level in a well before pumping and the water level in the well during pumping. Also, for flowing wells, the reduction of the pressure head as a result of the discharge of water.



An ecoregion is an area of similar climate, landform, soil, potential natural vegetation, hydrology, or other ecologically relevant variables.

Ecosystem engineer

An ecosystem engineer is an organism that creates or significantly changes a habitat to better suit its needs.

Ecosystem services

Ecosystem services are the benefits provided by nature, which directly and indirectly contribute to human well-being. These benefits can range from tangible products such as food and fresh water to cultural services such as recreation and aesthetics.


Effluent is the outflow of liquid waste that flows from a factory or sewage-treatment plant.


Erosion is the process whereby materials of the Earth's crust are loosened, dissolved, or worn away and simultaneously moved from one place to another.


Eutrophication is the process by which water becomes enriched with plant nutrients, most commonly phosphorus and nitrogen.


Evapotraspiration (ET) is the sum of water evaporated from the soil and transpired from plants and is an important component of the hydrologic cycle.


An extensometer measures the compaction and expansion of the aquifer system to a specified depth.


Fecal indicator bacteria

Fecal indicator bacteria are microscopic single-celled organisms found in the wastes of warm-blooded animals. Their presence in water is used to assess the sanitary quality of water for body-contact recreation or for consumption.


Flocculation is a process in which dispersed particles come out of suspension in the form of floc or flake, either spontaneously or due to the addition of a clarifying agent. A hydrological example would be suspended sediment in a body of water clumping together.


Fluorescence occurs when a molecule absorbs energy causing an electron to be excited to a higher energy level, and as the electron returns to ground state, energy is lost as an emission of a photon. Thus, the excitation and emission wavelengths (e.g. Ex260Em450) at which fluorescence occurs are characteristic to specific molecular structures.


A fungicide is type of pesticide used to control unwanted fungus (parasitic fungi or their spores).



The Groundwater Ambient Monitoring and Assessment Program’s Priority Basin Project (GAMA-PBP) was established by the California State Water Resources Control Board (SWRCB). Its current focus is assessing groundwater quality tapped by domestic wells and small community-supply systems while continuing a long-term trend monitoring effort.


The Groundwater Ambient Monitoring and Assessment Program’s Priority Basin Project (GAMA-PBP) was established by the California State Water Resources Control Board (SWRCB). Its current focus is assessing groundwater quality tapped by domestic wells and small community-supply systems while continuing a long-term trend monitoring effort.


A geographical information system (GIS) integrates hardware, software, and data for capturing, managing, analyzing, and displaying all forms of geographically referenced information. A GIS allows one to view, understand, question, interpret, and visualize data in many ways that reveal relationships, patterns, and trends in the form of maps, globes, reports, and charts.


Geomorphology is the science of landforms and the processes responsible for their origin and evolution.


In general, groundwater is any water that exists beneath the land surface. The term is most commonly applied to water in fully saturated soils and geologic formations.


Harmful algal blooms

Harmful Algal Blooms (HABs): An algal bloom may be called harmful because of resulting reductions in dissolved oxygen concentrations, alterations in aquatic food webs, unsightly scums along shorelines, production of taste-and-odor compounds that cause unpalatable drinking water and fish flesh, or the production of toxins potent enough to poison aquatic and terrestrial organisms. Many different types of algae can cause harmful algal blooms in freshwater ecosystems. However, the most frequent and severe blooms typically are caused by cyanobacteria, the only freshwater “algae” with the potential for production of toxins potent enough to adversely affect human health.


An herbicide is a type of pesticide used to control unwanted plants.


Heterogeneity is the characteristic of a substance in which material properties vary from point to point.

Hydrogeologic Framework

A hydrogeologic framework is a representation of the subsurface geology of an area of interest in order to describe the groundwater system and how water moves through aquifer units.


A hydrograph is a graph relating stage, flow, velocity, or other characteristics of water with respect to time.

Hydrologic cycle

The hydrologic cycle is the cyclic transfer of water vapor from the Earth's surface via evaporation into the atmosphere, from the atmosphere via precipitation back to earth, and through runoff into streams, rivers, and lakes, and ultimately into the oceans.


Igneous rock

Igneous rocks form when hot, molten rock crystalllizes and solidifies. Igneous rocks are divided into two groups, intrusive and extrusive, depending on where the molten rock solidies.

In situ

In situ is a Latin phrase that translates literally to "on site" or "in position."


Infiltration is the downward entry of water into the soil or rock.

Injection well

An injection well is a well used for injecting fluids into the subsurface.


A type of pesticide used to control unwanted insects.



Landsat is a partnership between the National Aeronautics and Space Administration (NASA) and the U.S. Geological Survey (USGS). Since 1972, Landsat data have provided a unique resource for those who work in agriculture, geology, forestry, regional planning, education, mapping, and global-change research.


A leachate is any material removed by the process of leaching (water carrying soluble substances or small particles through soil or rock).


Marco Hidrogeológico

Marco Hidrogeológico: una representación de la geología subterránea de una zona de interés para describir el sistema de aguas subterráneas y cómo se mueve el agua a través de las unidades acuíferas.

Maximum contaminant level

Maximum contaminant level (MCL) is the maximum permissible level of a contaminant in water that is delivered to any user of a public water system. MCLs are enforceable standards established by the U.S. Environmental Protection Agency.


Metamorphic refers to rock that Rock that has formed in the solid state in response to pronounced changes of temperature, pressure, and chemical environment.


Microcystins—or cyanoginosins—are a class of toxins produced by certain freshwater cyanobacteria, commonly known as blue-green algae.

Modelo conceptual

Modelo conceptual: una representación de un sistema, que implica el desarrollo de un concepto abstracto, gráfico o simplificado de interés y cómo se relacionan dentro del sistema más amplio.


The U.S. Geological Survey’s (USGS) Modular Ground-Water Flow Model (MODFLOW-2005) is a computer program that simulates groundwater flow by using finite differences.



The National Water-Quality Assessment (NAWQA) program was created to address the need for consistent and scientifically sound information for managing the Nation's water resources. This program is unique compared to other national water-quality assessment studies in that it integrates the monitoring of the quality surface and ground-waters with the study of aquatic ecosystems.


Neonicotinoids are a class of neuro-active insecticides chemically similar to nicotine. The neonicotinoid family includes acetamiprid, clothianidin, imidacloprid, nitenpyram, nithiazine, thiacloprid and thiamethoxam.


Nitrate is an ion consisting of nitrogen and oxygen (NO3). Nitrate is a plant nutrient and is very mobile in soils.


A nutrient is an element or compound essential for animal and plant growth. Common nutrients in fertilizer include nitrogen, phosphorus, and potassium.


Optical properties

Optical properties such as absorbance and fluorescence are used to gain insight into the composition of dissolved organic matter (DOM) - these techniques are relatively rapid and inexpensive and allow for the comprehensive tracking of DOM dynamics in watersheds.


An organochlorine compound is an organic compound containing chlorine that has an effect on the chemical behavior of the molecule.



A particulate is a suspended microscopic particle of solid or liquid matter.


A pathogen is a disease-producing agent; usually applied to a living organism. Generally, pathogens are any viruses, bacteria, or fungi that cause disease.


Pelagic fish live primarily in the pelagic zone of ocean or lake waters – being rarely close to the bottom or near the shore.


Percolation is the movement of water through the openings in rock or soil.


Permeability is the quality of a material to allow liquids or gases to pass through it.


Pesticides are substances that are meant to control pests, including weeds. The term pesticide includes all of the following: herbicide, insecticide, nematicide, molluscicide, piscicide, avicide, rodenticide, bactericide, insect repellent, animal repellant, antimicrobial, and fungicide.


Phenols are a class of organic compounds containing phenol (C6H5OH) and its derivatives. Used to make resins, weed killers, and as a solvent, disinfectant, and chemical intermediate. Some phenols occur naturally in the environment.


Phosphorus is a nutrient essential for growth. It can play a key role in stimulating aquatic growth in lakes and streams.


Photosynthesis is the synthesis of chemical compounds by organisms with the aid of light. Carbon dioxide is used as raw material for photosynthesis and oxygen is a product.


Physiography is the description of the surface features of the Earth, with an emphasis on the origin of landforms.


Phytoplankton are microscopic marine algae. Phytoplankton is the base of several aquatic food webs.


A piezometer is a specialized well used to measure water levels at specific depths.


A playa is an area of flat, dried-up land, especially a desert basin from which water evaporates quickly.


A pollutant is an undesirable substance, not normally present or an unusually high concentration, of a naturally occurring substance in water or soil.


Porosity is a measure of the water-bearing capacity of subsurface rock.


Porous describes the attribute of rock (or other material) of having minute spaces or holes through which liquid or air may pass.

Potable water

Potable water is water of a quality that is suitable for drinking.

Presupuesto de agua

Un presupuesto de agua simplemente establece que la tasa de cambio en el agua almacenada en un área, como una cuenca, se equilibra con la tasa a la que el agua entra y sale del área.



Recharge is a hydrologic process, where water moves downward from surface water to groundwater. Recharge is the primary method through which water enters an aquifer. In addition to natural processes, such as rainfall, recharge can also be accomplished by methods, such as the use of injection wells.


A riffle is a shallow part of the stream where water flows swiftly over completely or partially submerged obstructions (such as rocks) to produce surface agitation (such as rapids).


Riparian means relating to or situated next to a lake or river.

Riparian water rights

Riparian water rights are the rights of an owner whose land is next to a body water, such as a lake or river.


Runoff is that part of the precipitation, snow melt, or irrigation water that appears in uncontrolled surface streams, rivers, drains or sewers.



SCHISM (Semi-implicit Cross-scale Hydroscience Integrated System Model) is an open-source community-supported modeling system used to addresses a wide range of physical and biological processes.


Salinity is the saltiness or amount of salt dissolved in a body of water.

Saltwater intrusion

Saltwater intrusion is the movement of salt water into freshwater aquifers.


The word sediment usually applies to material in suspension in water or recently deposited from suspension. In the plural (sediments) the word is applied to all kinds of deposits from the waters of streams, lakes, or seas.

Sedimentary rock

Sedimentary rock is formed of sandstone and shale, fragments of other rock transported from their sources, and/or rocks formed from secretions of organisms, such as most limestone.


A seep is an area, generally small, where fluids percolate slowly to the land surface.


Seepage is the amount of fluid discharged at a seep. A seep is an area, generally small, where fluids percolate slowly to the land surface.


Sequestration the process by which carbon dioxide is removed from the atmosphere and stored in solid or liquid form.


A solute is a substance dissolved in a solvent that, when combined,forms a solution.

Specific conductance

Specific conductance is a measure of the ability of water to conduct an electrical current.


Stratigraphy is a branch of geology concerned with the study of rock layers and layering.


A stressor is a chemical or biological agent, environmental condition, external stimulus or an event seen as causing stress to an organism.


Subsidence is a dropping of the land surface. One of the ways this happens is as a result of groundwater being pumped. As a result, cracks, fissures, and other forms of surface deformation can appear in the land.

Superfund site

A "superfund site" is an area that the U.S. Environmental Protection Agency (EPA) has determined to be contaminated by hazardous waste and requiring cleanup because it poses a risk to human health and/or the environment.

Suspended sediment

Suspended sediment is very fine soil particles that remain in suspension in water for a considerable period of time without contact with the bottom. Such material remains in suspension due to the upward components of turbulence and currents and/or by suspension.



The terms “taxa” and “taxon” are related to the word “taxonomy." They refer to scientifically classified groups or entities: a taxonomic unit (such as a genus or order).

Total dissolved solids

Total dissolved solids (TDS) comprise inorganic salts (principally calcium, magnesium, potassium, sodium, bicarbonates, chlorides, and sulfates) and some small amounts of organic matter that are dissolved in water.


A chemical, physical, or biological agent that may cause an adverse effect or effects on biological systems.

Trace element

A trace element is an element found in only minor amounts in water or sediment. Trace elements include arsenic, cadmium, chromium, copper, lead, mercury, nickel, and zinc.


A tracer is a stable, easily detected substance that is present in or is added to a material that is then used to follow the location of that material in the environment or to detect any physical or chemical changes it undergoes.


Transpiration is the loss of water vapor from above-ground portions of plants, including leaves and stems.


A tributary is a river or stream flowing into a larger river, stream or lake.


Turbidity is the amount of solid particles that are suspended in water and that cause light rays shining through the water to scatter. Thus, turbidity makes the water cloudy or even opaque in extreme cases.


Unsaturated zone

The unsaturated zone is the zone immediately below the land surface where the pores contain both water and air, but are not totally saturated with water. These zones differ from an aquifer, where the pores are saturated with water.


Water Budget

A water budget simply states that the rate of change in water stored in an area, such as a watershed, is balanced by the rate at which water flows into and out of the area. (See: Healy, R.W., Winter, T.C., LaBaugh, J.W., and Franke, O.L., 2007, Water budgets: Foundations for effective water-resources and environmental management: U.S. Geological Survey Circular 1308, 90 p.)

Water budget

A water budget is an accounting of the inflow, outflow, and storage changes of water in a hydrologic unit, such as a lake or river.

Water column

A water column is a conceptual column of water from the surface of a sea, river,or lake to the bottom sediment. Water columns are used chiefly for environmental studies evaluating the mixing of thermal or chemically stratified layers in a lake, stream or ocean.

Water table

The water table is the point below the land surface where groundwater is first encountered and below which the earth is saturated. Depth to the water table varies widely across the country.


A watershed is the land area where precipitation runs off into streams, rivers, lakes, and reservoirs.


Wetlands are ecosystems whose soil is saturated for long periods seasonally or continuously, including marshes, swamps, and ponds.



Zooplankton are a diverse group of animals (usually tiny), that drift in aquatic environments.