Glossary/water dictionary

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abandoned water right

A water right that has not been put to beneficial use for generally five or more years, and in which the owner of the water right states that the water right will not be used or takes such actions that would prevent the water from being used beneficially.^

abandoned well

A well that is no longer used or a well removed from service; a well whose use has been permanently discontinued or is in a state of such disrepair that it cannot be used for its intended purpose. Generally, abandoned wells will be filled with concrete or cement grout to protect underground water from waste and contamination.^

absorption loss

The loss of water from infiltration or seepage into the soil during the process of priming, i.e. during the initial irrigation of a field; generally expressed as flow volume per unit of time.^

abutment (of a dam)

The part of a valley side wall against which a dam is constructed. An artificial abutment is sometimes constructed as a concrete gravity section to take the thrust of an arch dam where there is no suitable natural abutment.^

abutment seepage

Reservoir water that moves through seams or pores in the dam’s natural abutment material and exists as seepage.^

abyssal zone

The bottom of a deep ocean.^


The slow addition to land by deposition of water-borne sediment. An increase in land along the shores of a body of water.^


Gravity-driven waterways, similar in concept to a flume. Most are simple ditches with dirt banks, but they can be lined with concrete. They were important forms of irrigation in the development of agriculture in the American Southwest. The proliferation of cotton, pecans and green chiles as major agricultural staples owe their progress to the acequia system.^


(1) Chemicals that release hydrogen ions (H+ ) in solution and produce hydronium ions (H3O + ). Such solutions have a sour taste, neutralise bases, and conduct electricity. (2) Term applied to water with a pH of less than 7.0 on a pH scale of 0 to 14.^

acid aerosol

Airborne particles composed of sulphates (SOX), sulphuric acid (H2SO4 ), nitrates (NOX), and/or nitric acid (HNO3 ). Dry particle diameters are typically less than 1–2 microns.^

acid deposition

The introduction of acidic material to the ground or to surface waters. Involves a complex chemical and atmospheric phenomenon that occurs when emissions of sulphur and nitrogen compounds and other substances are transformed by chemical processes in the atmosphere, often far from the original sources, and then become deposited on the land or surface waters in either wet or dry forms. Wet deposition (commonly referred to as 'acid rain' or 'acid fog') results from precipitation such as rain, snow or fog. Dry deposition results from particle fallout or acidic gases.^

acid fog

Airborne water droplets containing sulphuric acid and/or nitric acid. Typical diameters are 3–30 microns.^

acid mine drainage (AMD)

Acidic water that flows into streams from abandoned mines or piles of mining waste or tailings. The acid arises from the oxidation of iron sulphide compounds in the mines by air, dissolved oxygen in the water, and chemoautotrophs, which are bacteria that can use the iron sulphide as an energy source. Iron sulphide oxidation products include sulphuric acid, the presence of which has reduced or eliminated aquatic life in many streams in mining regions^

acid neutralising capacity (ANC)

(1) A measure of the ability of water or soil to resist changes in pH. (2) The equivalent sum of all bases or base-producing materials, solutes plus particulates, in an aqueous system that can be titrated with acid to an equivalence point. The term designates titration of an “unfiltered” sample (formerly reported as alkalinity).^

acid precipitation

Atmospheric precipitation that is composed of the hydrolyzed by-products from oxidized halogen, nitrogen, and sulphur substances^

acid rain

Rainfall with a pH of less than 7.0. One of the principle sources is the combining of rain (H2O) and sulphur dioxide (SO2 ), nitrous oxides (NOx ), and carbon dioxide (CO2 ) emissions that are byproducts of the combustion of fossil fuels. These oxides react with the water to form sulphuric (H2SO4 ), nitric (HNO3 ) and carbonic acids (H2CO3 ). Long-term deposition of these acids is linked to adverse effects on aquatic organisms and plant life in areas with poor neutralising (buffering) capacity.^

acid soil (alkaline soil, neutral soil)

A description of one aspect of a soil’s chemical composition. Many plants will grow best within a range of pH rating from slightly acid to slightly alkaline. A pH rating of 7 means that the soil is neutral; a pH below 7 indicates acidity; a pH above 7 indicates alkalinity.^

acid-forming material

Material containing sulphide minerals or other materials, which if exposed to air, water, or weathering processes will form sulphuric acid.^


The condition of water or soil that contains a sufficient amount of acid substances to lower the pH below 7.0.^


Raising the acidity (lowering the pH) of a fluid by adding an acid. ^


A measure of how acid a solution may be. A solution with a pH of less than 7.0 is considered acidic. Solutions with a pH of less than 4.5 contain mineral acidity (due to strong inorganic acids), while a solution having a pH greater than 8.3 contains no acidity.^

activated carbon

A material produced by heating coal or wood in such a manner as to yield a porous structure, creating a very large internal surface area. Activated carbon is available in both powdered and granular forms, and is widely used to adsorb organic compounds from water and wastewater. It provides a means of removing tastes and odors from drinking water.^

activated carbon adsorption

The process of pollutants moving out of water and attaching on to activated carbon.^

activated sludge

The floc produced in raw or settled wastewater due to the growth of bacteria and other organisms in the presence of dissolved oxygen. It is the product that results when primary effluent is mixed with bacteria-laden sludge and then agitated and aerated to promote biological treatment, speeding the breakdown of organic matter in raw sewage undergoing secondary waste treatment.^

activated sludge process

A method of secondary wastewater treatment in which the waste is treated by micro-organisms in a well-aerated tank to degrade the organic material. A sedimentation tank is then used to remove the resultant sludge.^

active conservation storage

Storage of water for later release for purposes, such as municipal and industrial (M&I) uses, hydropower or irrigation.^

active solar water heater

A water heating system in which heat from the sun is absorbed by collectors and transferred by pumps to a storage unit. The heated fluid in the storage unit conveys its heat to the domestic hot water system of the house through a heat exchanger.^

active storage capacity

(1) The total amount of usable reservoir capacity available for seasonal or cyclic water storage. It is gross reservoir capacity minus inactive storage capacity. (2) More specifically, the volume of water in a reservoir below the maximum controllable level and above the minimum controllable level that can be released under gravity. In general, it is the volume of water between the outlet works and the spillway crest. In some instances, minimum pool operating constraints may prevent lowering the reservoir to the level of the outlet works, and the water below the minimum pool level is not considered to be in active storage.^


(1) The effective concentration of a chemical based on thermodynamic considerations. Activity and concentration have the same units and have the same value in very dilute solutions. (2) Adaptation Changes in an organism’s structure or habits that allow it to adjust to its surroundings.^

actual evapotranspiration

Represents a plant's actual rate of water uptake, which is determined by the level of available water in the soil.^

adaptive management

A process for implementing policy decisions as an ongoing activity that requires monitoring and adjustment. Adaptive management applies scientific principles and methods to improve resource management incrementally as managers learn from experience and as new scientific findings and social changes demand.^

adenosine triphosphate (ATP)

An organic, phosphate-rich compound important in the transfer of energy in organisms. Its central role in living cells makes it an excellent indicator of the presence of living material in water. A measure of ATP therefore provides a sensitive and rapid estimate of biomass. ATP is reported in micrograms per litre of the original water sample.^


Molecular attraction that holds the surfaces of two substances in contact, such as water and rock particles. Also, the attraction of water molecules to other materials as a result of hydrogen bonding.^

aline water

Water that contains significant amounts of dissolved solids. ^


Sometimes, water or soils contain an amount of alkali (strongly basic) substances sufficient to raise the pH value above 7.0 and thus be harmful to the growth of crops.^


Deposits of clay, silt, sand, gravel or other particulate material that has 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.^

alternative technology

Technology that aims to use resources sparingly and to do minimum damage to the environment and its native species while permitting the greatest possible degree of control over the technology.^

appropriation doctrine

"A system for allocating water to private individuals used in most western (US) states. A system of 'prior appropriation' was in common use throughout the arid west when early settlers and miners began to develop the land. Prior appropriation is based on the concept of ""first in time, first in fight"". The first person to take a quantity of water and put it to 'beneficial use' has a higher priority of right than a subsequent user. Under drought conditions, higher priority users are satisfied before junior users receive water. Appropriative rights can be lost through non-use; they can also be sold or transferred apart from the land. This is in Contrast with riparian water rights."^


Farming of plants and animals that live in water, such as fish, shellfish, and algae.^


A pipe, conduit or channel designed to transport water from a remote source, usually by gravity.^


A water-bearing geologic formation. A geological formation or structure that stores and/or transmits water, such as to wells and springs. Use of the term is usually restricted to those water-bearing formations capable of yielding water in sufficient quantity to constitute a usable supply.^

aquifer (confined)

Soil or rock below the land surface that is saturated with water. There are layers of impermeable material both above and below it, and it is under pressure so that when the aquifer is penetrated by a well, the water will rise above the top of the aquifer.^

aquifer (unconfined)

An aquifer whose upper water surface (water table) is at atmospheric pressure, and thus is able to rise and fall.^

aquifer properties

The properties of an aquifer that determine its hydraulic behaviour and its response to abstraction.^

aquifer system

A body of permeable and relatively impermeable materials that functions regionally as a water-yielding unit. It comprises two or more permeable units separated, at least locally, by confining units (aquitards) that impede groundwater movement but do not greatly affect the regional hydraulic continuity of the system. The permeable materials can include both saturated and unsaturated sections.^

aquifer test

A test to determine hydrologic properties of an aquifer that involves the withdrawal of measured quantities of water from, or the addition of water to, a well and the measurement of resulting changes in the aquifer both during and after the period of discharge or addition (recharge).^

aquifer, saline / poor quality

An aquifer containing water that is high in total dissolved solids, and is unacceptable for use as drinking water.^

aquifer, sandstone

The type of aquifer supplying groundwater to large parts of the United States (e.g. upper Middle West, Appalachia, Texas). The water-bearing formation is often contained by shale strata, and the water has high levels of iron and magnesium^

aquifer, unconfined

An aquifer made up of loose material, such as sand or gravel, that has not undergone lithification (settling). In an unconfined aquifer the upper boundary is the top of the zone of saturation (water table).^

aquifer, volcanic Rock

An aquifer composed of rock that originated from a volcano, such as basalt. This type of rock may or may not be very permeable. Aquifuse: a formation that has no interconnected openings and hence cannot absorb or transmit water.^


A saturated but poorly permeable bed that impedes groundwater movement and does not yield water freely to wells, but which may transmit appreciable water to or from adjacent aquifers and, where sufficiently thick, may constitute an important groundwater storage unit. Aquitards are characterised by values of leakance that may range from relatively low to relatively high. Really extensive aquitards of relatively low leakance may function regionally as boundaries of aquifer flow systems.^

area (sub-area), hydrographic

Primarily these are sub-drainage systems, typically valleys, within a more comprehensive drainage basin. Hydrographic areas (valleys) may be further subdivided into hydrographic sub-areas based on unique hydrologic characteristics (e.g., differences in surface flows) within a given valley or area.^

area flooded

Area of a floodplain that is flooded in a specific stream reach, watershed or river basin; may be for a single flood event, but is usually expressed as an average, annual value based on the sum of areas from all individual flood events over a long period of time, such as 50 to 100 years, and adjusted to an average value.^

area of influence

The area surrounding a pumping or recharging well within which the water table or potentiometric surface has been changed due to the well’s pumping or recharge.^

area of origins protection

(1) Generally, laws, regulations, or policies that provide some form of protection to states, counties or regions from which an interbasin transfer of water is made. (2) US state and federal laws, dating back to 1931, enacted to guarantee that the counties that contribute water to state and federal water projects will get priority for water when it is needed to match future growth. As yet, these statutes have not received close legal scrutiny by the courts.^

area of review

The area around an underground injection well that may be influenced adversely by fluid injection. Typically, the extent of this area may be calculated by using the specific gravity and rate of introduction of the injected fluids, the size, storage capacity and hydraulic conductivity of the injection zone, and certain underground formation pressures.^

area-capacity curve

A graph showing the relation between the surface area of the water in a reservoir and the corresponding volume.^


A commonly used expression, generally synonymous with 'confined aquifer' and referring to subsurface (ground) bodies of water which, due to underground drainage from higher elevations and confining layers of soil material above and below the water body (referred to as an artesian aquifer), result in underground water at greater-than-atmospheric pressures.^

artesian aquifer

A commonly used expression, generally synonymous with (but a generally less favored term than) 'confined aquifer'. An artesian aquifer is an aquifer that is bounded above and below by formations of impermeable or relatively impermeable material. An aquifer in which groundwater is under pressure significantly greater than atmospheric and its upper limit is the bottom of a bed of distinctly lower hydraulic conductivity than that of the aquifer itself.^

artesian basin

A body of groundwater more or less compact, moving through soils with more or less resistance.^

artesian pressure

The pressure under which artesian water in an artesian aquifer is subjected, generally significantly greater than atmospheric pressure.^

artesian water

Groundwater that is under pressure when tapped by a well and is able to rise above the level at which it is first encountered. It may or may not flow out at ground level. The pressure in such an aquifer commonly is called artesian pressure, and the formation containing artesian water is an artesian aquifer or confined aquifer. ^

artesian well

1) A well bored down to the point, usually at great depth, at which the water pressure is so great that the water is forced out at the surface. The name is derived from the French region of Artois, where the oldest well in Europe was bored in 1126. (2) A well tapping a confined or artesian aquifer in which the static water level stands above the top of the aquifer. The term is sometimes used to include all wells tapping confined water. wells with water levels above the unconfined water table are said to have positive artesian head (pressure) and those with a water level below the unconfined water table (i.e. a 'negative artesian head'). If the water level in an artesian well stands above the land surface, the well is a 'flowing artesian well'. If the water level in the well stands above the water table, it indicates that the artesian water can and probably does discharge to the unconfined water body.^

artesian zone

A zone where water is confined in an aquifer under pressure so that the water will rise in the well casing or drilled hole above the bottom of the confining layer overlying the aquifer.^

artificial recharge (1)

(1) The addition of surface water to a groundwater reservoir by human activity, such as putting surface water into a spreading basin. (2) The designed replenishment of groundwater storage from surface water supplies, such as irrigation or induced infiltration from streams or wells. There are five common techniques to effect artificial recharge of a groundwater basin: [1] water Spreading, consisting of the basin method, stream-channel method, ditch method, and flooding method, all of which tend to divert surface water supplies to effect underground infiltration; [2] recharge pits designed to take advantage of permeable soil or rock formations; [3] recharge wells, which work directly opposite of pumping wells, although they generally have limited scope and are better used for deep, confined aquifers; [4] induced recharge, which results from pumping wells near surface supplies, thereby inducing higher discharge towards the well; and [5] wastewater disposal, which includes the use of secondary treatment wastewater in combination with spreading techniques, recharge pits, and recharge wells to reintroduce the water into deep aquifers thereby both increasing the available groundwater supply and also further improving the quality of the wastewater. ^

artificial recharge (2)

a process by which water is put back into groundwater storage from surface water supplies such as irrigation, or induced infiltration from streams or wells.^

artificial substrate

(1) A device placed in the water for a specified period of time that provides living spaces for a multiplicity of organisms; for example, glass slides, concrete blocks, multi-plate samplers, or rock baskets; used primarily to collect organisms in areas where the physical habitat is limiting or cannot be adequately sampled using conventional methods. (2) A device that is purposely placed in a stream or lake for colonisation of organisms. The artificial substrate simplifies the community structure by standardising the substrate from which each sample is taken. Examples of artificial substrates are basket samplers (made of wire cages filled with clean streamside rocks) and multiplate samplers (made of hardboard) for benthic organism collection, and plexiglass strips for periphyton collection.^

artificially developed water

Water that would not have reached a stream if left to flow in accordance with natural laws. If developed by an individual, that person may acquire rights to such waters superior to adjudicated rights of earlier appropriators of natural waters of the stream into which such waters are diverted.^


A mineral fibre that can pollute air or water and cause cancer or asbestosis when inhaled. The U.S. Environmental Protection Agency (EPA) has banned or severely restricted its use in manufacturing and construction.^


A disease associated with inhalation of asbestos fibres. The disease makes breathing progressively more difficult and can be fatal.^

attached ground water

The portion or amount of alkali substances in the ground sufficient to raise the pH value above 7.0 or to be harmful to the growth of crops, a condition called 'alkaline'.^

auxiliary 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 'emergency spillway'. ^

available nutrients

Nutrient ions or compounds in forms that plants can absorb and utilise in growth.^

available soil water storage

Amount of water storable in the root zone at the time of irrigation.^

available water

The portion of water in a soil that can be absorbed by plant roots, usually considered to be that water held in the soil against a tension of up to approximately 15 atmospheres. ^

available water content (AWC)

A measure of the relative amount of water available in the upper levels of the soil strata that is available for use by plants.^

available water holding capacity

The capacity of a soil to hold water in a form available to plants. Also, the amount of moisture held in the soil between field capacity, or about one-third atmosphere of tension, and the wilting coefficient, or about 15 atmospheres of tension.^

average annual flood damages

The weighted average of all flood damages that would be expected to occur yearly under specified economic conditions and development. Such damages are computed on the basis of the expectancy in any one year of the amounts of damage that would result from floods throughout the full range of potential magnitude. ^

average annual recharge

The amount of water entering an aquifer on an average annual basis. In many, if not most, hydrologic conditions, 'average' has little significance for planning purposes because 'average' years might in fact be rare occurrences. ^

average annual runoff (yield)

The average water-year (Oct. 1 to Sept. 30) runoff or the supply of water produced by a given stream or water development project for a total period of record; measured in cubic feet per second or acre-feet.^

average conditions

The conditions under which a numerical value for a hydrologic variable, such as precipitation or streamflow, is equal to the arithmetic mean for a selected time period. ^

average discharge

In the annual series of the U.S. Geological Survey’s (USGS) reports on surface-water supply, the arithmetic average of all complete water years of record whether or not they are consecutive. The term 'average' is generally reserved for average of record, and 'mean' is used for averages of shorter periods, namely daily mean discharge.^

average water year

A term denoting the average annual hydrologic conditions based upon an extended or existing period of record. Because precipitation, runoff and other hydrologic variables vary from year to year, planners typically project future scenarios based on hydrologic conditions that generally include average, wet (high water), and drought (low water) years. ^

average year water demand

The demand for water under average hydrologic conditions for a defined level of development.^

average year water supply

The average annual supply of a water development system over a long period. For a dedicated natural flow, it is the long-term average natural flow for wild and scenic rivers, or it is 'environmental flows' as required for an average year under specific agreements, water rights, court decisions and congressional directives.^

backwater effect

The rise in surface elevation of flowing water upstream from and as a result of an obstruction to flow, such as a narrow bridge opening, buildings or fill material that limits the area through which the water must flow. In stream gaging, a rise in stage produced by a temporary obstruction such as ice or weeds, or by flooding of the stream below. The difference between the observed stage and that indicated by the stage-discharge relation is reported as backwater. Also referred to as 'heading up'. ^

backwater pools

A pool type formed by an eddy along channel margins downstream from obstructions such as bars, rootwads, or boulders, or resulting from backflooding upstream from an obstructional blockage. Backwater pools are sometimes separated from the channel by sand or gravel bars.^

balance of water resources and needs

The usable water resource of a certain water management unit in a given period of investigation, and the assessment and comparison of quantitative and qualitative characteristics of the water requirements to be supplied by this resource.^

balanced groundwater scenario (BGS)

A term referring to the development of a scenario exploring changes in cropping patterns such that long-term ground water withdrawals do not exceed long-term groundwater recharge rates. ^

balanced operation

Operation of a canal system where the water supply exactly matches the total flow demand.^

ball cock

A self-regulating device controlling the supply of water in a tank, cistern or toilet by means of a float connected to a valve that opens or closes with a change in water level.^

ball valve

A valve regulated by the position of a free-floating ball that moves in response to fluid or mechanical pressure.^


Heavy material, often seawater, placed in the hold of a ship to gain stability. Periodic discharges of this ballast water from oil tankers constitute a significant portion of the oil introduced into the oceans of the world each year.^


A major landform comprising distinctively round topped ridgeline remnants of fan alluvium. The ridge’s broadly rounded shoulders meet from either side to form a narrow crest and merge smoothly with the concave backslopes. In ideal examples, the slightly concave footslopes of adjacent ballenas merge to form a smoothly rounded drainage-way.^

bank and channel stabilisation

Implementation of structural features along a stream bank to prevent or reduce bank erosion and channel degradation.^

bank storage

The water absorbed into the banks of a stream, lake, or reservoir when the stage rises above the water table in the bank formations, then returns to the channel as effluent seepage when the stage falls below the water table. Bank storage may be returned in whole or in part as seepage back to the water body when the level of the surface water returns to a lower level.^

bank, banks

The slope of land adjoining a body of water, especially adjoining a river, lake or a channel. With respect to flowing waters, banks are either right or left as viewed facing in the direction of the flow. As 'banks', a large elevated area of a sea floor.^

bankfull channel

The stream channel that is formed by the dominant discharge, also referred to as the active channel, which meanders across the floodplain as it forms pools, riffles and point bars.^

bankfull stage

The stage at which a stream first begins overflows its natural banks. More precisely, an established river stage at a given location along a river which is intended to represent the maximum safe water level that will not overflow the river banks or cause any significant damage within the river reach. 'Bankfull stage' is a hydraulic term, whereas 'flood stage' implies resultant damage.^


(1) A Christian sacrament marked by ritual use of water and admitting the recipient to the Christian community. (2) A non-Christian rite using water for ritual purification.^


(1) A sand or gravel deposit found on the stream bed that is often exposed during low-water periods. (2) An elongated landform generated by waves and currents, usually running parallel to the shore, composed predominantly of unconsolidated sand, gravel, stones, cobbles or rubble and with water on two sides. (3) A component landform comprised of elongate, commonly curving, low ridges of well-sorted sand and gravel that stand above the general level of a 'bolson floor' and were built by the wave action of a Pleistocene lake. (4) A unit of pressure equal to 106 dynes per cm2 , 100 kilopascals, or 29.53 inches of mercury.^

bar racks

Closely spaced rods, often in the form of a screen, that remove large solids from the wastewater entering a sewage treatment plant. ^

bar screen

In wastewater treatment, a device used to remove large solid materials. ^


An instrument that records simultaneous barometric pressure and temperature on the same chart. ^


An artificial obstruction, such as a dam or an irrigation channel, built in a watercourse to increase its depth or to divert its flow either for navigation or irrigation. Sometimes the purpose is to control peak flow for later release.^


A dark volcanic rock composed of microscopic grains of augite, feldspar and olivine. Some basalts have many holes that give the rock a swiss-cheese-like appearance. As the lava cools, gases escape, leaving holes of different sizes. ^

basalt aquifers

Aquifers found in basalt rock in areas of past volcanic activity, particularly in the Pacific Northwest region of the United States and in Hawaii.^

base (1)

(1) Any of various typically water-soluble and bitter tasting compounds that in solution have a pH greater than 7, are capable of reacting with an acid to form a salt, and are molecules or ions able to take up a proton from an acid or able to give up an unshared pair of electrons to an acid. (2) Chemicals that release hydroxide ions (OH– ) in solution. Such solutions have a soapy feel, neutralise acids, and conduct electricity.^

base (2)

A substance that is neutral and has a pH of more than 7. A base has fewer free hydrogen ions (H+) than hydroxyl ions (OH-).^

base flow (1)

The sustained or dry weather flow of streams resulting from the outflow of permanent or perched groundwater, and from the drainage of lakes and swamps. Also included are waters from glaciers, snow and other possible sources not resulting from direct runoff.^

base flow (2)

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.^

base flow (3)

(1) The flow characteristic to which a perennially flowing stream reduces during the dry season. It is supported by groundwater seepage into the channel. (2) The fair-weather or sustained flow of streams; that part of stream discharge not attributable to direct runoff from precipitation, snowmelt or a spring. Discharge entering streams channels as effluent from the groundwater reservoir. (3) The volume of flow in a stream channel that is not derived from surface runoff. ^

base level

(1) The elevation to which a stream-channel profile has developed. (2) The lowest level to which a land surface can be reduced by the action of running water.^

base period

A period of time specified for the selection of data for analysis. The base period should be sufficiently long to contain data representative of the averages and deviations from the averages that must be expected in other periods of similar and greater length. For example, the U.S. Weather Bureau computes values of average, heavy and light monthly precipitation from data observed during the base period of 1931–1960. For groundwater studies, the base period should both begin and end at the conclusion of a dry trend so that the difference between the amount of water in transit in the soil at the ends of the base period is minimal.^

base runoff

Sustained or fair weather runoff. In most streams, base runoff is composed largely of groundwater effluent. The term 'base flow' is often used in the same sense as base runoff. However, the distinction is the same as that between streamflow and runoff. When the concept in the terms base flow and base runoff is that of the natural flow in a stream, base runoff is the more appropriate term.^

base width

(1) The time interval between the beginning and end of the direct runoff produced by a storm. (2) The time period covered by a unit hydrograph. (Baseline) The condition that would prevail if no action were taken.^

baseline (data)

A quantitative level or value from which other data and observations of a comparable nature are referenced. Information accumulated concerning the state of a system, process, or activity before the initiation of actions that may result in changes. (Basic) Describing a solution, sediment, or other material that has a pH greater than 7.0.^

baseline study

Study of the environmental conditions and organisms existing in a region prior to unnatural disturbances. ^

basic hydrologic data

Includes inventories of features of land and water that vary only from place to place (e.g., topographic and geologic maps), and records of processes that vary with both place and time (e.g. records of precipitation, streamflow, groundwater, and quality-of-water analyses). 'Basic hydrologic information' is a broader term that includes surveys of the water resources of particular areas and a study of their physical and related economic processes, interrelations and mechanisms.^


(1) (Hydrology) A geographic area drained by a single major stream; consists of a drainage system comprised of streams and often natural or man-made lakes. Also referred to as 'drainage basin', 'watershed' or 'hydrographic region'. (2) (Irrigation) A level plot or field, surrounded by dikes, which may be flood irrigated. (3) (Erosion control) A catchment constructed to contain and slow runoff to permit the settling and collection of soil materials transported by overland and rill runoff flows. (4) (Nautical) A naturally or artificially enclosed harbor for small craft, such as a yacht basin.^

basin irrigation

An irrigation method in which crops are surrounded by a border to form a submersion check called basin of round, square or any other form. Irrigation water generally comes directly from the supply ditch/canal or from other basins.^


The solid rock beneath the soil and superficial rock. A general term for solid rock that lies beneath soil, loose sediments, or other unconsolidated material^


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


The increase in concentration of a chemical in organisms that reside in environments contaminated with low concentrations of various organic compounds. Also used to describe the progressive increase in the amount of a chemical in an organism resulting from rates of absorption of a substance in excess of its metabolism and excretion.^


A method for quantitatively determining the concentration of a substance by its effects on the growth of a suitable animal, plant or microorganism under controlled conditions.^

biochemical oxidation

The process by which bacteria and other micro-organisms feed on complex organic materials and decompose them. Self-purification of waterways and activated sludge and trickling filter wastewater treatment processes depend on this principle.^

biochemical oxygen demand (BOD)

(1) A measure of the quantity of dissolved oxygen, in milligrams per liter, necessary for the decomposition of organic matter by micro-organisms, such as bacteria. (2) A measure of the amount of oxygen removed from aquatic environments by aerobic micro-organisms for their metabolic requirements. Measurement of BOD is used to determine the level of organic pollution of a stream or lake. The greater the BOD, the greater the degree of water pollution. Also referred to as 'biological oxygen demand' (BOD).^

biochemical oxygen demand (BOD) loading

BOD content, commonly expressed in pounds/day, of wastewater passing into a waste treatment system or a body of water. The greater the BOD content, the greater the degree of pollution. ^


A chemical substance that kills living organisms. Typically used to include materials that can kill desirable as well as undesirable organisms^

bioclimatic zones

Also referred to as 'biomes', these constitute the earth’s 10 zones differentiated by climate, soil, water, and plant and animal life. ^


A community of animal and plant life. ^


(1) The process by which an individual organism directly concentrates a substance from the surrounding air, water, or soil. (2) The increase in concentration of a chemical in an organism resulting from absorption levels exceeding the rate of metabolism and excretion.^

bioconcentration factor (BCF)

Used to describe the accumulation of chemicals in aquatic organisms that live in contaminated environments.^

bioconcentration. bioconcentration potential (BCP)

The maximum concentration of a chemical in an organism resulting from the rate of absorption equaling the rate of metabolism and excretion.^


The conversion of organic materials, such as plant or animal waste, into usable products or energy sources by biological processes or agents, such as certain microorganisms.^


Capable of being decomposed by biological agents or microorganisms, especially bacteria. The property of a substance that permits it to be broken down by micro-organisms into simple, stable compounds such as carbon dioxide and water. ^


The metabolic breakdown of materials into simpler components by living organisms. ^


The controlled use of microbes, usually bacteria, to reduce level of nitrates (NO3 – ) and thereby reclaim contaminated water or wastewater. The process consists of several stages to decompose the nitrates first into nitrites and then into nitrogen gas, N2 . Upon entering the treatment process, sodium sulfite (Na2SO3 ) is added as a reducing agent to the wastewater to remove the oxygen from the water. To break down the nitrates, the bacteria must have a carbon food source, and typically ethanol is added for the bacteria to feed on. In order to survive, however, the bacteria need oxygen, which they obtain by breaking down the nitrate ions, first to nitrite and then to harmless nitrogen gas. Also referred to as 'endogenous respiration'. ^


A large, rotating cylinder possessing surface features that allow for the growth of attached micro-organisms. The cylinder revolves and contacts the wastewater along one side, while the other side is exposed to air, thereby maximising the oxygenation of the water and stimulating decomposition of dissolved or suspended organic material.^


The gradual accumulation of waterborne organisms (as bacteria and protozoa) on the surfaces of engineering structures in water that contributes to corrosion of the structures and to a decrease in the efficiency of moving parts. ^


Methane gas produced during the anaerobic decomposition of the remains of plants or animal wastes by bacteria. ^


A living organism that denotes the presence of a specific environmental condition. For example, the presence of coliform bacteria identifies water that is contaminated with human fecal material. ^

biological activated carbon (BAC) process

The combination of ozonation and granular activated carbon (GAC) for the removal of dissolved organics, particularly dissolved organic carbon (DOC) from drinking water. This water treatment method has seen more widespread use in Europe primarily due to: (1) the generally poorer quality of surface waters there; (2) the greater concern and more stringent standards for chlorination byproducts; and (3) strict aesthetic demands of European consumers. Also referred to as 'biologically enhanced activated carbon process'.^

biological additives

Cultures of bacteria, enzymes or nutrients that are introduced into an oil discharge or other wastes to promote decomposition.^

biological community

All of the living things in a given environment.^

biological control

The direct human introduction of living organisms – predators, parasites, or pathogens – to eliminate or control undesirable species. The practice is usually considered an ecologically sound alternative to the application of chemical pesticides. ^

biological diversity

The number and kinds of organisms per unit area of volume; the composition of species in a given area at a given time. ^

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. ^

biological oxidation

Decomposition of complex organic materials by micro-organisms. Occurs in the self-purification of water bodies and in activated sludge wastewater treatment processes.^

biological oxygen demand (BOD)

1) The amount of oxygen required to stabilise decomposable matter by aerobic action. (2) An indirect measure of the concentration of biologically degradable material present in organic wastes. It usually reflects the amount of oxygen consumed in five days by biological processes breaking down organic waste. Also referred to as 'biochemical oxygen demand' (BOD).^

biological processes

Processes characteristic of, or resulting from, the activities of living organisms.^

biological treatment

A treatment technology that uses bacteria to consume organic wastes. In biological wastewater treatment, the use of bacteria to degrade and decompose organic materials in wastewater.^


(1) The science of life and of living organisms, including their structure, function, growth, origin, evolution and distribution. It includes botany and zoology and all their subdivisions. (2) The life processes or characteristic phenomena of a group or category of living organisms. (3) The plant and animal life of a specific area or region. ^


The process by which a substance is passed up the food chain, resulting in an especially high level of the substance at upper levels of the food chain. A biological process wherein a contaminant’s concentration increases at each level up the food chain, including humans. Thus, the availability of such contaminants, even in the seemingly insignificant parts per trillion range, often are ecologically important.^


(1) The total mass of living matter within a given unit of environmental area, typically expressed as mass per unit area or volume of habitat. (2) Plant material, vegetation or agricultural waste used as a fuel or energy source.^


Water that contains animal, human or food wastes; wastewater from toilets, latrines and sinks used for food preparation or disposal of chemical or chemical-biological ingredients. ^


To scald or parboil in water or steam in order to remove the skin from, whiten, or stop enzymatic action (as with, for example, food for freezing).^

blue-green algae

A group of phytoplankton that often cause nuisance conditions in water, so called because they contain a blue pigment in addition to chlorophyll. ^

blue water

The open sea.^

bog hole

A hole containing soft mud or quicksand.^


To change from a liquid to a vapour by the application of heat. ^

boiling point

The temperature at which the vapour pressure of a liquid is equal to the pressure exerted on the liquid. (Water) When the atmospheric pressure is 86 centimeters of mercury (sea level), the boiling point of water is, by definition, 100 degrees Celsius, or 212 degrees Fahrenheit. The boiling point decreases with elevation. ^

boiling water reactor (BWR)

A nuclear reactor in which water, used as both coolant and moderator, is allowed to boil in the core. The resulting steam can be used directly to drive a turbine generating electric power. ^


The vaporisation of liquid. Bolson — An alluvium-floored basin, depression, or wide valley, mostly surrounded by mountains and drained by a system that has no surface outlet; an undrained or an internally drained intermontane basin. Bolson fill is the alluvial Detritus that fills a bolson; also commonly called bolson deposits. Also see Semi-Bolson.^


An alluvium-floored basin, depression, or wide valley, mostly surrounded by mountains and drained by a system that has no surface outlet; an undrained or an internally drained intermontane basin. Bolson fill is the alluvial detritus that fills a bolson; also commonly called 'bolson deposits'. ^


A floating device used to contain oil on a body of water. ^

border ditch

A ditch used as a border of an irrigated strip or plot, water being spread from one or both sides of the ditch along its entire length.^

border irrigation

A sub-system of controlled flood irrigation in which the land is divided into parallel border strips demarcated from one another by earth ridges. Water is successively delivered into each strip from a head or field ditch at its upper end. On the upstream part of each strip is a flat zone, the level portion from which the stream of water spreads evenly across the entire downstream portion.^


A solution that is resistant to pH changes, or a solution or liquid whose chemical makeup tends to neutralise acids or bases without a great change in pH. Surface waters and soils with chemical buffers are not as susceptible to acid deposition as those with poor buffering capacity.^

buffer strips

(1) Strips of grass or other erosion-resisting vegetation between or below cultivated strips or fields. (2) Grassed or planted zones that act as a protective barrier between an area which experiences livestock grazing or other activities and a water body. Also referred to as a 'buffer zone'. ^

buffer zone

(1) A protective, neutral area between distinct environments. (2) An area that acts to minimise the impact of pollutants on the environment or public welfare. For example, a buffer zone may be established between a compositng facility and nearby neighborhoods to minimise odor problems.^

calibrated orifices

A water control structure whose orifice, perforated with great accuracy in a thin wall, allows the passage of relatively large discharges of 50-150 litres, meant to be temporarily stored in a basin or furrow.^

canal evaporation Losses

Losses due to evaporation from the water surfaces of canals. They are generally accounted for as part of the total losses occurring in an irrigation system.^

capillary action

The means by which liquid moves through the porous spaces in a solid, such as soil, plant roots and the capillary blood vessels in our bodies due to the forces of adhesion, cohesion and surface tension. Capillary action is essential in carrying substances and nutrients from one place to another in plants and animals.^

capital cost

The total expenditure incurred on a work since the beginning of its construction, excluding cost of operation, maintenance and repairs, but including cost of investigations and of all extensions and improvements.^

case study

(General) A record of the history of a specific study or project performed. (Specific) A written account of an event or situation to which participants are to react, with an emphasis is on decision making. A case study can be used to start a general session, or as part of a small group session.^


(1) The catching or collecting of water, especially rainfall. (2) A reservoir or other basin for catching water. (3) The water thus caught. ^

catchment area

(1) The intake area of an aquifer and all areas that contribute surface water to the intake area. (2) The areas tributary to a lake, stream, sewer, or drain. (3) A reservoir or basin developed for flood control or water management for livestock and/or wildlife. (4) The area from which a lake, a reservoir or a chosen crosssection of a stream or waterway receives water. ^

catchment area (basin)

The area draining into a river, reservoir, or other body of water.^

centre pivot sprinkler

A sprinkler system in which the water source is in the centre, and a system of pipes and sprinkler heads rotates or pivots about the central point to water a given circular area.^


An underground catch basin for combined liquid and solid waste, such as household sewage, so designed as to retain the organic matter and solids but permitting the liquids to seep through the bottom and sides.^


(1) (Watercourse) A natural stream that conveys water; a natural or artificial watercourse with definite bed and banks to confine and conduct flowing water; a ditch or channel excavated for the flow of water. River, creek, run, branch, anabranch and tributary are some of the terms used to describe natural channels, which may be single or braided. Canal, aqueduct, and floodway are some of the terms used to describe artificial (man-made) channels. (2) (Landform) The bed of a single or braided watercourse that commonly is barren of vegetation and is formed of modern alluvium. Channels may be enclosed by banks or splayed across and slightly mounded above a fan surface and include bars and dumps of cobbles and stones. Channels, excepting floodplain playas, are landform elements.^

chemical oxygen demand (COD)

(Water quality) (1) A measure of the chemically oxidizable material in the water which provides an approximation of the amount of organic and reducing material present. The determined value may correlate with biochemical oxygen demand (BOD) or with carbonaceous organic pollution from sewage or industrial wastes. (2) A chemical measure of the amount of organic substances in water or wastewater. A strong oxidizing agent together with acid and heat are used to oxidize all carbon compounds in a water sample. Non-biodegradable and recalcitrant (slowly degrading) compounds, which are not detected by the test for BOD, are included in the analysis. The actual measurement involves a determination of the amount of oxidizing agent (typically, potassium dichromate) that is reduced during the reaction. ^


The sum total of the meteorological elements that characterise the average and extreme conditions of the atmosphere over a long period of time at any one place or region of the earth’s surface. The collective state of the atmosphere at a given place or over a given area within a specified period of time. ^

climatic cycle (1)

Actual or supposed cyclic recurrences of such weather phenomena as wet and dry years, hot and cold years, at more or less regular intervals in response to long-range terrestrial and solar influences, such as volcanic dust and sunspots.^

climatic cycle (2)

The periodic changes of climate, including a series of dry years following a series of years with heavy rainfall.^

climatic year

A period used in meteorological measurements. A continuous 12-month period during which a complete annual cycle occurs, arbitrarily selected for the presentation of data relative to hydrologic or meteorologic phenomena. The climatic year in the United States begins on October 1st and runs through September 30th. ^

climatology, climatological

The science and study dealing with climate and climatic phenomena as exhibited by temperature, winds and precipitation.^

cluster sampling

Where clusters of observation are formed on a basis that reduces cost (e.g. within a limited area). Care is taken to ensure heterogeneity and to avoid redundancy of information.^

commercial water use

Water used for motels, hotels, restaurants, office buildings, other commercial facilities, and institutions. Water for commercial uses comes both from public-supplied sources, such as a county water department, and self-supplied sources, such as local wells.^


The process of water vapour in the air turning into liquid water. Water drops on the outside of a cold glass of water are condensed water. Condensation is the opposite process of evaporation.^

conjunctive irrigation planning

Planning an irrigation project in a given area having groundwater resources and surface water, so that both the surface and groundwater resources yield the most economical and suitable combination of the use of water from the two sources.^

consumptive use

That part of water withdrawn that is evaporated, transpired by plants, incorporated into products or crops, consumed by humans or livestock, or otherwise removed from the immediate water environment. ^

consumptive water use

The quantity of water used by the vegetative growth of a given area in transpiration or building of plant tissue and that evaporated from the soil or from intercepted precipitation on the area in any specific time. It is expressed in water depth per unit of time. ^

continuous flow irrigation system

A system in which individual irrigators receive the quantity of water to which they are entitled in the form of a continuous flow.^

continuous supply

Continuous and constant discharge to inlets of the individual farms or fields.^

contour ditch irrigation

A sub-system of controlled irrigation in which water flows through openings in ditches (which more or less follow the contours) or over the ditch banks as sheet flow across the fields. The delivery is controlled by the spacing between ditches and by the size and site of the openings of each ditch.^

control structure

A stage-discharge regulating device of a spillway. It may be of any form, viz. weir, side channel, glory spillway, orifice, tube, pipe or channel.^

controlled flooding

Water is diverted to levelled lands and in a sequential manner in such a way as to deliver the desired dose to all points; it includes flooding from ditches, border irrigation and corrugation irrigation.^

conventional technology

Technology based on a long history of experience without making use of later developments. Losses of water in transit from the source of supply to the point of service, whether in natural channels or in artificial ones, such as canals, distributaries, ditches or watercourses.^

conveyance loss (1)

Water that is lost in transit from a pipe, canal or ditch by leakage or evaporation. Generally, the water is not available for further use; however, leakage from an irrigation ditch may percolate to a groundwater source and be available for further use.^

conveyance loss (2)

Comprises evaporation from the water surface, seepage and incidental transpiration by vegetation growing in the water or along the banks of natural channels, canals or watercourses.^

conveyance structures

Structures built to help provide general control and conveyance of the flow from the intake structures to the area to be irrigated.^


Democratic organisation for service provision controlled by its members, who contribute equitably to the capital of the cooperative.^

corrugation irrigation

A sub-system of controlled flood irrigation. Corrugations between crop rows are fed at the head by flows from a furrow long enough to wet laterally the ridges situated between the corrugations. In soils with a large natural slope, corrugations with a small longitudinal slope appear to be parallel to the contours.^

cost-benefit analysis

Quantitative and qualitative evaluation of the positive and negative impacts of a project.^

crop irrigation requirement

Consumptive use minus effective precipitation.^

crop water requirement

The total water needed for evapotranspiration from planting to harvest for a given crop in a specific climatic regime when adequate soil water is maintained by rainfall and/or irrigation so that it does not limit plant growth and crop yield.^

cross-irrigable area

The total area within the extreme limits set for irrigation by a project, supply system or canal, minus areas excluded because of their unsuitability for irrigation. ^

cut-off drainage system

A drainage system for draining seepy hillsides. Tiles are placed along the hillside to intercept the seep water and prevent it from reaching the bottom land, or an open channel is dug along the hillside to achieve the same effect.^

data transmission

The sending of data from one place to another, or from one part of the system to another, utilising different transmission facilities, such as telephone or telegraph lines, direct radio links, or satellite transmission systems.^

decision support system

Systems for helping planners make decisions using models to select the most favourable solution from among alternatives that are available according to established criteria.^

delivery structures

All the structures (canals or pipes and their appurtenant works, such as intakes, distributors, drops and discharge structures) that ensure delivery of water to the irrigators of an irrigation area from the main canal.^

demand management

A programme adopted to achieve effective management of the use of water resources in order to meet the general objectives of: economic efficiency, environmental conservation, and community and consumer satisfaction.^

depreciation and renewal funds

The provision for interventions necessitated by the changing of spare parts is covered by a renewal fund that capitalises the depreciation stocks normally constituted for this purpose. However, it is possible to practise a less expensive policy by capitalising only lesser amounts based on the technical depreciation periods of equipment, which always exceed those for legal or fiscal depreciation.^

depth integrating sediment sampler

A device to take a sample of suspended sediment in a stream. It is lowered from the stream surface to the bed and raised to the surface again at a constant rate. The sample of sediment laden water is drawn in continuously, thus obtaining a composite sample which is integrated over the depth of the stream.^

derived data

Records of observations and measurements of physical facts, occurrences and conditions that have been developed from basic data by means of standard methods of computation and estimation.^


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


The volume of water that passes a given location within a given period of time. Usually expressed in cubic feet per second.^

discharge rating curve

A curve that expresses graphically the relation between the discharge and its corresponding stage (or elevation of water surface) in a stream or conduit at a given point.^

domestic water use

Water used for household purposes, such as drinking, food preparation, bathing, washing clothes, dishes, and dogs, flushing toilets, and watering lawns and gardens. About 85% of domestic water (in the US) is delivered to homes by a public-supply facility, such as a county water department; about 15% of the US population supply their own water, mainly from wells.^

drainage basin

Land area where precipitation runs off into streams, rivers, lakes and reservoirs. It is a land feature that can be identified by tracing a line along the highest elevations between two areas on a map, often a ridge. Large drainage basins, like the area that drains into the Mississippi River, contain thousands of smaller drainage basins. Also called a 'watershed'.^


A lowering of the groundwater surface caused by pumping.^

drip irrigation

A common irrigation method where pipes or tubes filled with water slowly drip onto crops. Drip irrigation is a low-pressure method of irrigation and less water is lost to evaporation than with high-pressure spray irrigation.^

drop structure

A structure designed to secure the lowering of the water surface in a channel in a short distance, and the safe destruction of the liberated surplus energy. ^

dry spell

A sustained period of time with insufficient precipitation.^

duty of water

The relation between the area irrigated, or to be irrigated, and the quantity of water used, or required, to irrigate it for the purpose of maturing its crop. Duty is stated with reference to a base period and the place of its reckoning or measurement. It is usually expressed in volume of water or rate of flow per unit area (litres per hectare).^

ecological impact

The total impact of an environmental change, either natural or man-made, on the ecology of the area.^

economic value of unit of irrigation water

The value of a crop raised by a unit of irrigation water if run continuously throughout the life of the crop.^

effective rainfall

(1) In general, rain that produces runoff. (2) In irrigation practice, that portion of the total precipitation which is retained by the soil so that it is available for use for crop production. (3) In geohydrology, that part of the total precipitation that reaches the groundwater.^

effective rooting depth

Soil depth from which the crop extracts most of the water needed for evapotranspiration (also called 'design rooting depth').^

effective water holding capacity

The amount of water held in the soil after the excess gravitational water has drained away and after the rate of downward movement of water has materially decreased, preferably given as a percentage by volume and not by weight. ^


Water that flows from a sewage treatment plant after it has been treated.^

environmental assessment

A formal process to predict the environmental consequences of human development activities and to plan appropriate measures to eliminate or reduce adverse effects and augment positive effects (also, environmental impact assessment (EIA)).^

environmental impact

A change in effect on an environmental resource or value resulting from human activities including project development, often called an 'effect'.^

environmental monitoring

Observation of effects of development projects on environmental resources and values.^


The process by which a material is worn away by a stream of liquid (water) or air, often due to the presence of abrasive particles in the stream.^

erosion control

The application of necessary measures to control accelerated erosion of land surfaces by vegetation or artificial structures, such as terraces, dams or bunds.^


A place where fresh and salt water mix, such as a bay, salt marsh, or where a river enters an ocean.^


The process of a water body becoming anaerobic, i.e. without oxygen. Human activities that add nutrients to a water body can accelerate this process.^

evaluation of alternatives

The searching out of comparative advantages and disadvantages of alternatives in an attempt to find the one that ‘fits’ best. The criterion of fit is sometimes called measure of effectiveness. During the planning process, the original goals and objectives are frequently modified and refined. Thus, the measure of effectiveness also needs to be altered. As the planner approaches the design stage, evaluation becomes more precise.^


The process of liquid water becoming water vapour, including vapourisation from water surfaces, land surfaces, and snow fields, but not from leaf surfaces. ^


The sum of evaporation and transpiration.^

exchangeable sodium percentage (ESP)

The degree of saturation of the soil exchange complex with sodium. It may be calculated by the formula: ESP = exchangeable sodium (meq/100 g soil)/cation exchange capacity (mec/100 g soil) x 100.^

expert system

Software that applies the knowledge, advice and rules defined by experts in a particular field to a user’s data to help solve a problem.^

farm budget analysis

Analysis of a farm’s income (including that from off-farm employment opportunities) and expenditure.^

farm irrigation efficiency

The ratio or percentage of the irrigation water consumed by crops of an irrigated farm to the water diverted from the source of supply, measured at the farm head-gate.^

farmer’s labour return

Gross farm income minus interest on average farm capital.^

financial rate of return

The ratio of the net revenue and the sum-at-charge, expressed as a percentage, attained or estimated to be attained after the development period of a project.^


An overflow of water onto lands that are used or usable by man and not normally covered by water. Floods have two essential characteristics: the inundation of land is temporary; and the land is adjacent to and inundated by overflow from a river, stream, lake or ocean.^

flood control

The use of techniques to change the physical characteristics of floods. These techniques include purpose built-in river control structures. Management of inflow of floodwater into a region, as well as its outflow, in such a manner that any flooding is either kept to a minimum (return period, extent) or occurs at a planned moment and during a planned period.^

flood control works

Engineering structures: built to protect land and property from damage by flood, e.g. levees, banks or other works along a stream; designed to confine floodwater to a particular channel or direct it along planned pathways; or a flood control reservoir.^

flood frequency analysis

The estimation of the frequency of occurrence of floods at a site. Flood frequency analysis attempts to fit a probability distribution to flood discharge data and generalise the results for use at sites with no flood data. Many techniques are in use.^

flood irrigation

All types of irrigation that make use of rising water from flood for inundating areas without major structural works, e.g. flood recession, spate irrigation and wild flooding.^

flood plain

A strip of relatively flat and normally dry land alongside a stream, river or lake that is covered by water during a flood.^

flood spreading

The flooding of gravelly or otherwise relatively pervious lands in order to recharge a groundwater basin.^

flood stage

The elevation at which overflow of the natural banks of a stream or body of water begins in the reach or area in which the elevation is measured.^

flood, 100-year

A 100-year flood does not refer to a flood that occurs once every 100 years, but to a flood level with a 1% chance of being equaled or exceeded in any given year.^


The channel of a river or stream and the parts of the floodplain adjoining the channel that are reasonably required to efficiently carry and discharge the flood water or flood flow of a river or stream.^

flowing well/spring

A well or spring that taps groundwater under pressure so that water rises without pumping. If the water rises above the surface, it is known as a flowing well.^


Water that contains less than 1,000 milligrams per litre (mg/L) of dissolved solids; generally, more than 500 mg/L of dissolved solids is undesirable for drinking and many industrial uses.^

fully automatic irrigation system

An irrigation system or network on a farm, whereby the water requirements of the plants are met automatically. It makes use of devices that measure soil moisture (e.g. tensiometer) or other indicators of irrigation need (e.g. time elapsed since rainfall), and trigger a series of operations to convey the necessary water through the network at the proper time.^

furrow irrigation

A method similar to corrugation irrigation used in permeable soils. It consists in feeding narrow furrows very close to one another with small discharges so as to wet more easily all the soil situated between two rows of crops (often orchards). Furrows parallel to the rows may be laid mechanically with a drill plough.^

gage height

The height of the water surface above the gage datum (zero point). Gage height is often used interchangeably with the more general term, 'stage', although gage height is more appropriate when used with a gage reading.^

gaging station

A site on a stream, lake reservoir or other body of water where observations and hydrologic data are obtained. The U.S. Geological Survey measures stream discharge at gaging stations.^

geographic information system (GIS)

All information concerning a point or a group of points georeferenced on Earth’s surface, such as maps or satellite images, digitally stored, processed and manipulated by a computer program (e.g. IDRIS, MAPINFO, ARCINFO).^


A geothermal feature of the Earth where there is an opening in the surface that contains superheated water that periodically erupts in a shower of water and steam.^


A disease that results from an infection by the protozoan parasite Giardia intestinalis, caused by drinking water that is either not filtered or not chlorinated. The disorder is more prevalent in children than in adults and is characterised by abdominal discomfort, nausea and alternating constipation and diarrhea.^

gravity irrigation

A method of operating a system or part of a system using gravity alone, water being available at a sufficient level (or pressure) to ensure its conveyance or delivery to the fields or its distribution in the fields.^

greenhouse effect

The warming of the Earth’s atmosphere caused by the buildup of carbon dioxide, which allows light from the sun’s rays to heat the earth but prevents the loss of heat.^


Wastewater from clothes washing machines, showers, bathtubs, hand washing, lavatories and sinks.^


(1) Water that flows or seeps downward and saturates soil or rock, supplying springs and wells. The upper surface of the saturate zone is called the 'water table'. (2) Water stored underground in rock crevices and in the pores of geologic materials that make up the Earth's crust.^

groundwater balance

A systematic review of inflow, outflow and storage as applied to the computation of groundwater changes. It is based on the concept that all inputs of water in a defined space and time are equal to the sum of all outputs of water, and the changes of water storage, in the same space and time.^

groundwater inventory

A detailed estimate of the amount of water added to the groundwater reservoir of a given area (recharge) balanced against estimates of amounts of withdrawals from the groundwater reservoir of the area during a specific period.^

groundwater recharge (1)

Replenishment of groundwater supply in the zone of saturation, or the addition of water to groundwater storage by natural processes or artificial methods for subsequent withdrawal for beneficial use or to check saltwater intrusion in coastal areas.^

groundwater recharge (2)

Inflow of water to a groundwater reservoir from the surface. Infiltration of precipitation and its movement to the water table is one form of natural recharge. Also, the volume of water added by this process.^

groundwater, confined

Groundwater under pressure significantly greater than atmospheric, with its upper limit the bottom of a bed with hydraulic conductivity distinctly lower than that of the material in which the confined water occurs.^

groundwater, unconfined

Water in an aquifer that has a water table that is exposed to the atmosphere.^


A water-quality indication of the concentration of alkaline salts in water, mainly calcium and magnesium. If the water you use is 'hard', then more soap, detergent or shampoo is necessary to raise a lather.^


(1) The source and upper reaches of a stream; also the upper reaches of a reservoir. (2) The water upstream from a structure or point on a stream. (3) The small streams that come together to form a river. Also may be thought of as any and all parts of a river basin except the mainstream river and main tributaries.^

hydroelectric power water use

The use of water in the generation of electricity at plants where the turbine generators are driven by falling water.^

hydrologic cycle

The circulation of water from the sea through the atmosphere to the land and then, often with many delays, back to the sea or ocean. The water passes through various stages and processes, such as precipitation, interception, runoff, infiltration, percolation, groundwater storage, evaporation and transpiration; also the many shortcuts of the water that is returned to the atmosphere without reaching the sea.^

hydrologic cycle

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

hydrologic studies

Collection of hydrological data and appraisal of available water supply for various phases of water resources development, and appurtenant works and operations related thereto, including the determination of the extremes, such as floods and droughts. It includes methods and techniques for installing and using hydro-meteorological observation stations.^

impermeable layer

A layer of solid material, such as rock or clay, through which water is not allowed to pass.^


An operationally measurable term used in logical framework analyses, such as objectively verifiable indicator (OVI) for the verification of project objectives and results with regard to quantity, quality, time, place and beneficiaries.^

individual irrigation system

Systems located downstream of the outlets served by the collective irrigation system and meant to deliver water to the farms or fields of an individual area.^

industrial water use

Water used for industrial purposes in such industries as steel, chemical, paper, and petroleum refining. Inthe US, water for industrial uses comes mainly (80%) from self-supplied sources, such as a local wells or withdrawal points in a river, but some water comes from public-supplied sources, such as the county/city water department.^


Flow of water from the land surface into the subsurface.^

infiltration rate

The rate at which water penetrates the surface of a soil. The term usually refers to water occurring as precipitation, but it is also applied to water flowing or standing upon soil^

injection well

Refers to a well constructed for the purpose of injecting treated wastewater directly into the ground. Wastewater is generally forced (pumped) into the well for dispersal or storage into a designated aquifer. Injection wells are generally drilled into aquifers that do not deliver drinking water, unused aquifers, or below freshwater levels.^

institutional arrangements

An interrelated set of entities and rules that serves to organise societies’ activities for achieving social goals. The institutional arrangements should ensure that the integrated approach is included in decisions and policies regarding river basin management and irrigation development.^


A structure placed in a surface water source to permit water withdrawal.^

integrated river basin management

The process of formulating and implementing a course of action to achieve specific objectives involving the natural, agricultural and human resources of a river basin, and taking into account the social, economic and institutional factors operating in a river basin and their impact on the environment. It signifies the interactions of components and the dominance of components in the particular area.^


The controlled application of water for agricultural purposes through man-made systems to supply water requirements not satisfied by rainfall. ^

irrigation cycle

Successive deliveries of water on all the units of a network in such a way as to achieve a given irrigation on the entire field concerned.^

irrigation efficiency

The ratio or percentage of the irrigation water consumed by crops of an irrigated farm, field or project to the water diverted from the source of supply. It is called 'farm irrigation efficiency' or 'farm delivery efficiency' when measured at the farm head-gate; 'field irrigation efficiency' when measured at the field or plot; and 'water conveyance and delivery efficiency' or 'overall efficiency' when measured at the source of supply.^

irrigation potential

Total possible area brought under irrigation, plus that which can be planned for irrigation in a river basin, region or country, from available water resources, with designs based on good technical practice at the time of assessing the potential.^

irrigation requirements

The quantity of water exclusive of precipitation, i.e. quantity of irrigation water, required for normal crop production. It includes soil evaporation and some unavoidable losses under the given conditions. It is usually expressed in water-depth units (millimetres) and may be stated in monthly, seasonal or annual terms, or for a crop period.^

irrigation water quality table

This indicates guidelines for the interpretation of water quality for crop production. The table was adapted from the University of California Committee of Consultants, the United States, in 1974 and revised in 1979. It emphasises the longterm influence of water quality on crop production and farm management.^

irrigation water use

Water application on lands to assist in the growing of crops and pastures or to maintain vegetative growth in recreational lands, such as parks and golf courses.^

land reclamation

Making land capable of more intensive use by changing its general character, as by drainage of excessively wet land, irrigation of arid or semi-arid land, or recovery of submerged land from seas, lakes and rivers.^

land use capability

Division of agricultural land into classes of similar production potential with a view to making the best use of each piece of land without causing excessive erosion, or loss of productivity.^


The process by which soluble materials in the soil, such as salts, nutrients, pesticide chemicals or contaminants, are washed into a lower layer of soil or are dissolved and carried away by water.^

leaching requirement

The fraction of water entering the soil that must pass through the root zone in order to prevent soil salinity from exceeding a specified value. Leaching requirement is used primarily under steady-state or long-term average conditions.^

lentic waters

Ponds or lakes (standing water).^


A natural or man-made earthen barrier along the edge of a stream, lake or river. Land alongside rivers can be protected from flooding by levees.^

livestock water use

Water used for livestock watering, feed lots, dairy operations, fish farming and other on-farm needs.^

lotic waters

Flowing waters, as in streams and rivers.^

main drainage system

System which conveys drainage water from the field drainage system to an outlet.^

management of an irrigation development project

Series of actions to design, implement, perpetuate and improve an irrigation development project for the achieving of set objectives.^

maximum contaminant level (MCL)

Yhe designation given by the U.S. Environmental Protection Agency (EPA) to water-quality standards promulgated under the Safe Drinking Water Act. The MCL is the greatest amount of a contaminant that can be present in drinking water without causing a risk to human health.^

method of water delivery

Way of making an irrigation system function to convey water from the source of supply to each field served by the system.^

micro-basin irrigation

A water harvesting method used in small fields on gentle slopes. A moulding of the land surface enables the creation on very small slopes of an artificial micro-relief in each small field. Here the runoff water from the upper portion (runoff area) becomes concentrated in the lower portion (crop area) where the crops are raised, and which are sometimes reduced to a single tree (Negarin method).^

micro-irrigation with mini-diffusers

A micro-irrigation system in which water is emitted in small sprinklings through fixed small diffusers in the form of fine droplets distributed over a certain area, or by individual low pressure jets localising the water on the soil in separate spots. Their discharge is generally limited (20-60 litres/h at 1 bar) and often emitted in the form of circular sectors either to avoid wetting the neck of the trees or to limit the range on the sides of the space between rows, which should remain dry. Their use is limited to orchards^

milligrams per litre (mg/l)

A unit of the concentration of a constituent in water or wastewater. It represents 0.001 gram of a constituent in 1 litre of water. It is approximately equal to one part per million (PPM).^

million gallons per day (Mgd)

A rate of flow of water equal to 133,680.56 cubic feet per day, or 1.5472 cubic feet per second, or 3.0689 acre-feet per day. A flow of one million gallons per day for one year equals 1,120 acre-feet (365 million gallons).^

mining water use

Water use during quarrying rocks and extracting minerals from the land.^


To render or become mild or milder; to modify; to reduce negative impacts of projects, structures or any action on the socio-economy and environment.^

mobile microirrigation

An irrigation machine (generally frontal nozzle-line) in which the mobile nozzle-line functioning at low pressure applies water directly to the space between rows of annual crops. Suspended flexible pipes fitted with mouthpieces at their end feed continuously into small basins dug beforehand or simple partitioned corrugations.^

moisture-holding capacity

The amount of water required to fill all the pore spaces between the soil particles, i.e. the upper limit of the possible moisture content. It is usually expressed as the percentage of the soil volume (1 percent equals 1 mm/dm of soil depth), or sometimes as the percentage of the dry weight of the soil.^

monitoring programme

A programme designed to measure quantitatively or qualitatively the level of a substance over a period of time.^

monitoring well (observation well)

A non-pumping well used for observing the elevation of the water table; or a well used for some anticipated, generally undesirable, conditions, such as encroachment of a saltwater front or a pollutant introduced to groundwater.^

multi-criterion decision making

A multi-criterion decision-making problem means a multi-attribute or a multi-objective decision problem or both. Multicriterion decision making is used to indicate the general field of study which includes decision making in the presence of two or more conflicting objectives and/or decision analysis processes involving two or more attributes.^

municipal water system

A water system that has at least five service connections or which regularly serves 25 individuals for 60 days; also called a 'public water system'.^

nephelometric turbidity unit (NTU)

Unit of measure for the turbidity of water. Essentially, a measure of the cloudiness of water as measured by a nephelometer. Turbidity is based on the amount of light that is reflected off particles in the water.^

non-beneficial consumptive use

The water consumed by native vegetation, evaporated from bare and idle land surfaces and from water surfaces.^

non-point source (NPS) pollution

Pollution discharged over a wide land area, not from one specific location. These are forms of diffuse pollution caused by sediment, nutrients, organic and toxic substances originating from land-use activities, which are carried to lakes and streams by surface runoff. Non-point source pollution is contamination that occurs when rainwater, snowmelt, or irrigation washes off plowed fields, city streets or suburban backyards. As this runoff moves across the land surface, it picks up soil particles and pollutants, such as nutrients and pesticides.^

non-renewable resource

Those resources which do not regenerate themselves or maintain a sustained yield after being utilised or destroyed.^


(Climate) The climate characteristic of land lares near oceans that contribute to the humidity, and at the same time have a moderating influence on temperature and the range of temperature variation. ^

open channel drainage

Drainage accomplished by open channels or ditches.^

operation and maintenance (O&M)

Operation' is the organised procedure for causing a piece of equipment, a treatment plant or other facility or system to perform its intended function, but not including the initial building or installation of the unit. 'Maintenance' is the organised procedure for keeping the equipment, plant, facility or system in such condition that it is able to perform its intended function continually and reliably.^

organic matter

Plant and animal residues, or substances made by living organisms. All are based upon carbon compounds.^

orographic precipitation

Precipitation resulting from the lifting of moist air over an orographic barrier, e.g. a mountain range.^


The movement of water molecules through a thin membrane. The osmosis process occurs in our bodies and is also one method of desalinating saline water.^


The place where a sewer, drain or stream discharges; the outlet or structure through which reclaimed water or treated effluent is finally discharged to a receiving water body.^

overall efficiency

The ratio or percentage of the irrigation water consumed by crops to the water diverted from the source of supply (measured at the source of supply).^

oxygen demand

The need for molecular oxygen to meet the needs of biological and chemical processes in water. Even though very little oxygen will dissolve in water, it is extremely important in biological and chemical processes.^

particle size

The diameter, in millimeters, of suspended sediment or bed material. ^


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

peak flow

The maximum instantaneous discharge of a stream or river at a given location. It usually occurs at or near the time of maximum stage.^

per capita use

The average amount of water used per person during a standard time period, generally per day.^


 The value below which a given percentage of observations in a group of observations fall. For example, the 20th percentile is the value below which 20% of the observations may be found.^


(1) The movement of water through the openings in rock or soil. (2) The entrance of a portion of the streamflow into the channel materials to contribute to groundwater replenishment.^


The ability of a material to allow the passage of a liquid, such as water through rocks. Permeable materials, such as gravel and sand, allow water to move quickly through them, whereas unpermeable material, such as clay, do not allow water to flow freely.^


A measure of the relative acidity or alkalinity of water. Water with a pH of 7 is neutral; lower pH levels indicate increasing acidity, while pH levels higher than 7 indicate increasingly basic solutions.^

pilot project

An early, usually small, project set up to gain experience for operating the actual project.^

point-source pollution

Water pollution coming from a single point, such as a sewage-outflow pipe.^


A measure of the water-bearing capacity of subsurface rock. With respect to water movement, it is not just the total magnitude of porosity that is important, but the size of the voids and the extent to which they are interconnected, as the pores in a formation may be open, or interconnected, or closed and isolated. For example, clay may have a very high porosity with respect to potential water content, but it constitutes a poor medium as an aquifer because the pores are usually so small.^

portable flume (portable weir)

A portable flume for measuring small discharges, such as in farm laterals or watercourses, and consisting of a miniature broad-crested weir made of wooden or iron sheets.^

potable water

Water of a quality suitable for drinking.^

potentiometric surface / piezometric surface

"The imaginary line where a given reservoir of fluid will ""equalize out to"" if allowed to flow; a potentiometric surface is based on hydraulic principles."^


Rain, snow, hail, sleet, dew and frost.^

primary wastewater treatment

The first stage of the wastewater treatment process where mechanical methods, such as filters and scrapers, are used to remove pollutants. Solid material in sewage also settles out in this process.^

prior appropriation doctrine

"The system for allocating water to private individuals used in most Western US states. The doctrine of 'prior appropriation' was in common use throughout the arid West as early settlers and miners began to develop the land. The prior appropriation doctrine is based on the concept of ""First in Time, First in Right"". The first person to take a quantity of water and put it to beneficial use has a higher priority of right than a subsequent user. The rights can be lost through non-use; they can also be sold or transferred apart from the land. Contrasts with 'riparian water rights'."^

public supply

Water withdrawn by public governments and agencies, such as a county water department, and by private companies that is then delivered to users. Public suppliers provide water for domestic, commercial, thermoelectric power, industrial and public water users. Most people's household water is delivered by a public water supplier. The systems have at least 15 service connections (such as households, businesses, or schools) or regularly serve at least 25 individuals daily for at least 60 days out of the year.^

public water use

Water supplied from a public-water supply and used for such purposes as firefighting, street washing, and municipal parks and swimming pools.^

rainfall intensity

The rate at which rainfall occurs expressed in depth units per unit of time. It is the ratio of the total amount of rain to the length of the period in which the rain falls.^

rating curve

A drawn curve showing the relation between gage height and discharge of a stream at a given gaging station.^


Water added to an aquifer; for instance, rainfall that seeps into the ground.^

reclaimed wastewater

Wastewater treatment plant effluent that has been diverted for beneficial uses such as irrigation, industry, or thermoelectric cooling instead of being released to a natural waterway or aquifer.^


Act or process of reclaiming swampy, marshy, deteriorated, desert and virgin lands and making them suitable for cultivation or habitation; also, conversion of foreshores into properly drained land for any purpose, either by enclosure and drainage, or by deposition of material thereon.^

recycled water

Water that is used more than once before it passes back into the natural hydrologic system.^

renewable resource

A resource capable of being continuously renewed or replaced through such processes as organic reproduction (biomass), groundwater recharge (water) or weathering of parent material (soil).^


A pond, lake, or basin, either natural or artificial, for the storage, regulation and control of water.^

resource management

The introduction and enforcement of restraints, including specific technical practices, to safeguard the future of renewable resources and uphold the principle of sustained yield.^

return flow

(1) That part of a diverted flow that is not consumptively used and returned to its original source or another body of water. (2) (Irrigation) Drainage water from irrigated farmlands that re-enters the water system to be used further downstream.^

return flow (irrigation)

Irrigation water that is applied to an area and which is not consumed in evaporation or transpiration and returns to a surface stream or aquifer.^

reverse osmosis

(1) (Desalination) The process of removing salts from water using a membrane. With reverse osmosis, the product water passes through a fine membrane that the salts are unable to pass through, while the salt waste (brine) is removed and disposed. This process differs from electrodialysis, where the salts are extracted from the feedwater by using a membrane with an electrical current to separate the ions. The positive ions go through one membrane, while the negative ions flow through a different membrane, leaving the end product of freshwater. (2) (Water Quality) An advanced method of water or wastewater treatment that relies on a semi-permeable membrane to separate waters from pollutants. An external force is used to reverse the normal osmotic process resulting in the solvent moving from a solution of higher concentration to one of lower concentration.^

riparian water rights

The rights of an owner whose land abuts water. They differ from state to state (US) and often depend on whether the water is a river, lake, or ocean. The doctrine of riparian rights is an old one, having its origins in English common law. Specifically, persons who own land adjacent to a stream have the right to make reasonable use of the stream. Riparian users of a stream share the streamflow among themselves, and the concept of priority of use (prior appropriation doctrine) is not applicable. Riparian rights cannot be sold or transferred for use on non-riparian land.^


A natural stream of water of considerable volume, larger than a brook or creek.^


(1) That part of the precipitation, snow melt, or irrigation water that appears in uncontrolled surface streams, rivers, drains or sewers. Runoff may be classified according to speed of appearance after rainfall or melting snow as direct runoff or base runoff, and according to source as surface runoff, storm interflow, or groundwater runoff. (2) The total discharge described in (1), above, during a specified period of time. (3) Also defined as the depth to which a drainage area would be covered if all of the runoff for a given period of time were uniformly distributed over it.^

salinity control

Abatement or prevention of saltwater contamination of agricultural, industrial and municipal water supplies, or reducing alkaline salts and preventing deterioration of cultivable lands.^

seasonal irrigation

An irrigation is termed seasonal when the lands of the area are irrigated only during a part of the year, called 'watering season'.^

secondary wastewater treatment

Treatment (following primary wastewater treatment) involving the biological process of reducing suspended, colloidal and dissolved organic matter in effluent from primary treatment systems and which generally removes 80% to 95% of the biochemical oxygen demand (BOD) and suspended matter. Secondary wastewater treatment may be accomplished by biological or chemical-physical methods. Activated sludge and trickling filters are two of the most common means of secondary treatment. It is accomplished by bringing together waste, bacteria, and oxygen in trickling filters or in the activated sludge process. This treatment removes floating and settleable solids and about 90% of the oxygen-demanding substances and suspended solids. Disinfection is the final stage of secondary treatment.^


Usually applied to material in suspension in water or recently deposited from suspension. In the plural the word is applied to all kinds of deposits from the waters of streams, lakes, or seas.^

sediment yield

The total sediment outflow from a watershed or past a given location in a specified period of time. It includes bedload as well as suspended load including dissolved solids. Usually expressed as load per unit of time (e.g. tonnes per year or kilograms per second).^

sedimentary rock

Rock formed of sediment, and specifically: (1) sandstone and shale, formed of fragments of other rock transported from their sources and deposited in water; and (2) rocks formed by or from secretions of organisms, such as most limestone. Many sedimentary rocks show distinct layering, which is the result of different types of sediment being deposited in succession.^

sedimentation tanks

Wastewater tanks in which floating wastes are skimmed off and settled solids are removed for disposal.^


(1) The slow movement of water through small cracks, pores, interstices etc. of a material into or out of a body of surface or subsurface water. (2) The loss of water by infiltration into the soil from a canal, ditches, laterals, watercourse, reservoir, storage facilities, or other body of water, or from a field.^

self-supplied water

Water withdrawn from a surface water or groundwater source by a user rather than being obtained from a public supply. An example would be homeowners getting their water from their own well.^

septic tank

A tank used to detain domestic wastes to allow the settling of solids prior to distribution to a leach field for soil absorption. Septic tanks are used when a sewer line is not available to carry them to a treatment plant. A settling tank in which settled sludge is in immediate contact with sewage flowing through the tank, and wherein solids are decomposed by anaerobic bacterial action.^

settling pond (water quality)

An open lagoon into which wastewater contaminated with solid pollutants is placed and allowed to stand. The solid pollutants suspended in the water sink to the bottom of the lagoon and the liquid is allowed to overflow out of the enclosure.^

sewage treatment plant

A facility designed to receive the wastewater from domestic sources and to remove materials that damage water quality and threaten public health and safety when discharged into receiving streams or bodies of water.  ^


A system of underground pipes that collect and deliver wastewater to treatment facilities or streams.^


A depression in the Earth's surface caused by dissolving of underlying limestone, salt or gypsum. Drainage is provided through underground channels that may be enlarged by the collapse of a cavern roof.^

social benefits

Benefits as a result of the project, during and after construction, consisting mainly of opportunities for: (i) employment of labour; and (ii) employment of capital.^

sodium adsorption rate (SAR)

A ratio for soil extracts and irrigation waters used to express the relative activity of sodium ions in exchange reactions with soil: SAR = Na+ x [(Ca++ + Mg++)/2] -0.5 where the ionic concentrations are expressed in meq/litre.^

soil classification systems

System of classification of soils based on recognition of the type and predominance of the constituents of soil considering grain size, gradation, plasticity and compressibility. Among the widely used soil classification systems are the US Soil Taxonomy and the FAO Soil Classification System.^

soil moisture deficit

The amount of water that must be applied to the soil to cause thorough drainage.^

soil moisture tension

The equivalent negative pressure or suction in the soil moisture; expressed in pressure units (bar or pascal).^


A substance that is dissolved in another substance, thus forming a solution.^


A mixture of a solvent and a solute. In some solutions, such as sugar water, the substances mix so thoroughly that the solute cannot be seen. But in other solutions, such as water mixed with dye, the solution is visibly changed.^


A substance that dissolves other substances, thus forming a solution. Water dissolves more substances than any other, and is known as the 'universal solvent'.^

specific conductance

A measure of the ability of water to conduct an electrical current as measured using a 1cm cell and expressed in units of electrical conductance, i.e., Siemens per centimeter at 25 degrees Celsius. Specific conductance can be used for approximating the total dissolved solids content of water by testing its capacity to carry an electrical current. In water quality, specific conductance is used in groundwater monitoring as an indication of the presence of ions of chemical substances that may have been released by a leaking landfill or other waste storage or disposal facility. A higher specific conductance in water drawn from downgradient wells when compared to upgradient wells indicates possible contamination from the facility.^

spray irrigation

An common irrigation method where water is shot from high-pressure sprayers onto crops. Because water is shot high into the air onto crops, some water is lost to evaporation.^


A water body formed when the side of a hill, a valley bottom or other excavation intersects a flowing body of groundwater at or below the local water table, below which the subsurface material is saturated with water.^

sprinkler irrigation

A method of irrigation under pressure in which water is sprinkled in the form of artificial rain through lines carrying distribution components: rotary sprinklers, diffusers with permanent water streams, perforated pipes. In comparison with surface and drip irrigation, the length of the path of travel by the water drops through the air causes: (1) great sensitivity to wind, which reduces the uniformity of distribution; or (2) ‘air conditioning’ effects on the crops if used in antifrost sprinkling or spraying.^

storm sewer

A sewer that carries only surface runoff, street wash and snow melt from the land. In a separate sewer system, storm sewers are completely separate from those that carry domestic and commercial wastewater (sanitary sewers).^


A general term for a body of flowing water; natural water course containing water at least part of the year. In hydrology, it is generally applied to the water flowing in a natural channel as distinct from a canal.^


The water discharge that occurs in a natural channel. A more general term than runoff, streamflow may be applied to discharge whether or not it is affected by diversion or regulation.^


A dropping of the land surface as a result of groundwater being pumped. Cracks and fissures can appear in the land. Subsidence is virtually an irreversible process.^

subsurface drainage system

Any drainage system (drainage wells, open ditches or drain pipes) that is designed to control the groundwater table.^

surface drainage system

Shallow ditches or open drains that serve to receive surface flow or drainage water.^

surface irrigation

A method of irrigation in which water is applied to the land by allowing it to flow by simple gravity, before infiltrating. It includes various systems depending upon the relative magnitude of the surface flooding phase and infiltration phase after accumulation (submersion).^

surface water

Water that is on the Earth's surface, such as in a stream, river, lake or reservoir.^

suspended sediment

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.^

suspended solids

Solids that are not in true solution and that can be removed by filtration. Such suspended solids usually contribute directly to turbidity. Defined in waste management, these are small particles of solid pollutants that resist separation by conventional methods.^

suspended-sediment concentration

The ratio of the mass of dry sediment in a water-sediment mixture to the mass of the water-sediment mixture. Typically expressed in milligrams of dry sediment per liter of water-sediment mixture.^

suspended-sediment discharge

The quantity of suspended sediment passing a point in a stream over a specified period of time. When expressed in tonnes per day, it is computed by multiplying water discharge (in cubic feet per second) by the suspended-sediment concentration (in milligrams per liter) and by the factor 0.0027.^

system modernisation

This operation consists in replacing certain structures by using a new or improved technology, e.g. replacing channels and ditches by underground pipes.^

tertiary wastewater treatment

Selected biological, physical, and chemical separation processes to remove organic and inorganic substances that resist conventional treatment practices; the additional treatment of effluent beyond that of primary and secondary treatment methods to obtain a very high quality of effluent. The complete wastewater treatment process typically involves a three-phase process: (1) first, in the primary wastewater treatment process, which incorporates physical aspects, untreated water is passed through a series of screens to remove solid wastes; (2) second, in the secondary wastewater treatment process, typically involving biological and chemical processes, screened wastewater is then passed a series of holding and aeration tanks and ponds; and (3) third, the tertiary wastewater treatment process consists of flocculation basins, clarifiers, filters, and chlorine basins or ozone or ultraviolet radiation processes.^

thermal pollution

A reduction in water quality caused by increasing its temperature, often due to disposal of waste heat from industrial or power generation processes. Thermally polluted water can harm the environment because plants and animals can have a hard time adapting to it.^

thermoelectric power water use

Water used in the process of the generation of thermoelectric power. Power plants that burn coal and oil are examples of thermoelectric-power facilities.^

topographic divide

A divide that demarcates the boundary of the area from which surface runoff is derived.^

transmissibility (groundwater)

The capacity of a rock to transmit water under pressure. The coefficient of transmissibility is the rate of flow of water, at the prevailing water temperature, in gallons per day, through a vertical strip of the aquifer one foot wide, extending the full saturated height of the aquifer under a hydraulic gradient of 100%. A hydraulic gradient of 100% means a one foot drop in head in one foot of flow distance.^


Process by which water that is absorbed by plants, usually through the roots, is evaporated into the atmosphere from the plant surface, such as leaf pores. ^


A smaller river or stream that flows into a larger river or stream. Usually, a number of smaller tributaries merge to form a river.^


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. Turbidity is measured in nephelometric turbidity units (NTU).^

unconfined aquifers

An aquifer in which the groundwater table is free to rise and fall according to hydraulic gradients.^

underground drainage

Drainage, either natural or artificial, beneath the surface of the Earth.^

unsaturated zone

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.^

user fees

All financial contributions to be borne by the users (or possibly their farmer in the case of renting). The fees pay for the supply of water and accompanying services.^

volumetric water rate

The charge levied according to the quantity of water actually delivered to the outlet or turnout.^

wash load

Suspended material of very small size (generally clay and colloids) originating primarily from erosion on the land slopes of the catchment area and present to a negligible degree in the river bed itself.^


Water that has been used in homes, industries and businesses that is not for reuse unless it is treated.^

wastewater treatment return flow

Water returned to the environment by wastewater treatment facilities.^

water balance

Mathematical calculations for infow and outflow of water components for a given area or soil profile.^

water budget (water balance)

Accounting of inflow and outflow of water components for a given area or soil profile.^

water control

The physical control of water by measures such as conservation practices on the land, channel improvements and installation of structures for reducing water velocity and trapping sediments.^

water conveyance and delivery efficiency

The ratio or percentage of the irrigation water delivered at the irrigation plot to the water diverted from and measured at the source of supply.^

water cycle

The circuit of water movement from the oceans to the atmosphere and to the Earth and return to the atmosphere through various stages or processes such as precipitation, interception, runoff, infiltration, percolation, storage, evaporation and transportation.^

water quality

A term used to describe the chemical, physical and biological characteristics of water, usually in respect to its suitability for a particular purpose.^

water user association (WUA)

Association of water users combining both governance and management functions (they are not the owners of the infrastructure).^

water years

Continuous 12-month period selected to present data relative to hydrologic or meteorological phenomena during which a complete annual hydrologic cycle normally occurs. The water year used by the U.S. Geological Survey runs from October 1 through September 30, and is designated by the year in which it ends.^

well capacity

The rate at which a well will yield water, in litres per second or cubic metres per second.^


A method of landscaping that uses plants that are well adapted to the local area and are drought-resistant. Xeriscaping is becoming more popular as a way of saving water at home.^


Mass per unit time per unit area.^