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Irrigation Glossary


AC {hertz}: Abbreviation for alternating current.
AC pipe: Asbestos-cement pipe was commonly used for buried pipelines.  It combines strength with light weight and is immune to rust and corrosion. (James, 1988) (No longer made.)
acceleration of gravity: See gravity, acceleration of.
acid: Substance with a pH less than 7.0 (Burt, 1998)

  • Attraction of water molecules for solid surfaces. (Brady, 1990)
  • Physical attraction of unlike substances to one another.  In soils, it is the process that holds water molecules tightly to soil solids at the soil-water interfaces. (Contractor, 1999)

adjusted sodium adsorption ratio [adj. RNa]: Index of permeability problems, based upon water quality. (Burt, 1998)
adsorption: Concentration of a substance at the surface of another, more noticeable with substances of large surface area, such as clay particles. (Hess, 1999)
advance ratio [AR] {-}: Ratio of the time for the water to reach the end of the field to the total set time for an irrigation set on a furrow irrigation system.  The ratio should be less than 0.5 to have a good distribution uniformity. (Burt, Surface Irrigation)
advance time {minutes, sec}:

  • Time required for a given stream of irrigation water to move from the upper end of a field to the lower end. (ASAE, 1998)
  • Time required for a given surface irrigation stream to move from one point in the field to another. (NRCS, 1997)

aeration: To supply or impregnate with air. (Webster, 1981)
aeration capacity {-}: Volume fraction of air filled pores in a soil at field capacity. (Hess, 1999)

  • Group of primary soil particles that cohere to each other more strongly than to other surrounding particles.  (Soil, 1996)
  • Groups of individual soil particles, held together naturally and consisting of particles of sand, silt and clay separated from each other by pores, cracks or planes of weakness.  The term, soil structure, refers to this arrangement of the soil in natural aggregates. Various types of soil structure are recognized (Massive, platy, prismatic, blocky, granular).

air gap: See back flow prevention device.
alfalfa valve: Outlet valve attached to the top of a pipeline riser with an opening equal in diameter to the inside diameter of the riser pipe and an adjustable lid or cover to control water flow...  (ASAE, 1998)
algicide: Substance that will kill or control algae growth. (NRCS, 1997)
alkaline (alkali) soils:

  • Soil with pH greater than 7.0. (Soil, 1996)
  • Soil with an exchangeable sodium percentage greater than 15%. (Burt, 1998)
  • Soil that has sufficient exchangeable sodium (alkali) to interfere with plant growth and cause dispersion and swelling of clay minerals...  Hess, 1999

allowable depletion* [AD] {in.,mm} (7/19/03):

  • Portion of plant available water that is allowed for plant use prior to irrigation based in plant and management considerations. See Fig. 1 at end of document.
  • That part of soil moisture stored in the plant root zone managed for use by plants, usually expressed as equivalent depth of water in acre inches per acre, or inches. (NRCS, 1997)
  • Is sometimes referred to as allowable soil depletion or allowable soil water depletion.

allowable stress factor: See coefficient.
allowable voltage loss* [AVL] {volts}: Voltage loss in a circuit or portion of a circuit which, if not exceeded, will result in the electrical device working correctly.
alternate set irrigation: See irrigation systems.
alternating current [AC]: Current in which the flow of electrons in a circuit flow in one direction and then in the reverse direction. (Reference Manual ch. 5)
ampere* {ampere or amp} (approved via RM):

  • Unit of electrical current.  The unit is used to specify the movement of electrical charge per unit time through a conductor. (Pumps, 1996)
  • 1 ampere = 1 coulomb per second.
  • 1 ampere is the movement of 6.28 billion billion electrons /second.

anion: Negatively charged ion, which during electrolysis is attracted towards the anode.  The most common anions in soil extracts and waters are bicarbonate, sulphate, carbonate, chloride and nitrate ions. (Hess, 1999)
application efficiency: See efficiency.
application efficiency of lower quarter: See efficiency.
application efficiency low half: See efficiency.
application rate: See precipitation rate.
aquifer: Underground geological formation, or group of formations, containing usable amounts of groundwater that can supply wells or springs for domestic, industrial, and irrigation uses. Removing more groundwater from an aquifer than is naturally replenished is called overdrafting, and can result in a dropping water table, increased pumping costs, land subsidence (which reduces the future recharge capacity), saltwater intrusion, reduced streamflows in interconnected ground- and surface-water systems, and exhaustion of groundwater reserves. Overdrafting groundwater occurs primarily in the Plains States and the West. (Agriculture, 1997)
arc {degrees}: Portion of a full circle (360) covered by a part circle sprinkler.
area* [A] { in.2, ft2,acres,ha} (7/19/03)Surface included within a set of lines (Webster). In irrigation, usually used to describe a surface of land or cross section of pipe.
arid climate: See climate.
atmospheric pressure* [Pa] {psi, , ft water, atmospheres, kPa} (7/19/03)Absolute pressure measured at any location. Standard atmospheric pressure at sea level is defined as 14.7 psi or 34.0 ft of water.
atmospheric vacuum breaker: See backflow prevention device.
automatic drain valve: See valve.
available soil moisture [ASM] {in.,mm} (7/19/03)Difference at any given time between the actual soil moisture content in the root zone soil and the wilting point. (On-Farm Committee, 1979)
available soil moisture capacity [AMC]: See available water.
available water *[AW ] {%, in./ft, mm/mm } (7/19/03)Portion of water in a soil that can be readily absorbed by plant roots. It is the amount of water released between in situ field capacity and the permanent wilting point. (ASAE, 1998)  See Fig. 1 at end of document. See also available water holding capacity.
available water holding capacity [AWHC] {in./ft, mm/mm}: Preferred term is available water.
available water storage capacity [AWSC] {in./ft, mm/mm}: Preferred term is  available water.
average annual precipitation {in., mm}: Long-term historic (generally 30 years or more) arithmetic mean of precipitation (rain, snow, dew) received by an area. (NRCS, 1997)
AVB: See atmospheric vacuum breaker under backflow prevention devices.
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back flow: Any unwanted flow of used or non-potable water or substance from any domestic, industrial or institutional piping system into the pure, potable water distribution system. The direction of flow under these conditions is in the reverse direction from that intended by the system and normally assumed by the owner of the system. (USC, 1998)
backflow prevention device [BPD]: Safety device which prevents the flow of water from the water distribution system back to the water source (ASAE, 1998)

  • air gap: Physical separation of the supply pipe by at least two pipe diameters (never less than one inch) vertically above the overflow rim of the receiving vessel. In this case line pressure is lost. Therefore, a booster pump is usually needed downstream, unless the flow of the water by gravity is sufficient for the water use. With an air gap there is no direct connection between the supply main and the equipment. An air gap may be used to protect against a contaminant or a pollutant, and will protect against both back-siphonage and backpressure. An air gap is the only acceptable means of protecting against lethal hazards. (USC, 1998)
  • atmospheric vacuum breaker [AVB]: Backflow device configured with a single moving part, a float, which moves up or down to allow atmospheric air into the piping system. (Rochester, 1995). The AVB is always placed downstream from all shut-off valves. Its air inlet valve closes when the water flows in the normal direction. But, as water ceases to flow the air inlet valve opens, thus interrupting the possible back-siphonage effect. If piping or a hose is attached to this assembly and run to a point of higher elevation, the backpressure will keep the air inlet valve closed because of the pressure created by the elevation of water. Hence, it would not provide the intended protection. Therefore, this type of assembly must always be installed at least six (6) inches above all downstream piping and outlets. Additionally, this assembly may not have shut-off valves or obstructions downstream. A shut-off valve would keep the assembly under pressure and allow the air inlet valve (or float check) to seal against the air inlet port, thus causing the assembly to act as an elbow, not a backflow preventer. The AVB may not be under continuous pressure for this same reason. An AVB must not be used for more than twelve (12) out of any twenty-four (24) hour period. It may be used to protect against either a pollutant or a contaminant, but may only be used to protect against a back-siphonage condition. (USC, 1998)
  • double check assembly [DC]: Two internally loaded, independently operating check valves together with tightly closing resilient seated shut-off valves upstream and downstream of the check valves. Additionally, there are resilient seated test cocks for testing of the assembly. The DC may be used to protect against a pollutant only. However, this assembly is suitable for protection against either backsiphonage or backpressure.  (USC, 1998) 
  • pressure vacuum breaker [PVB]: Backflow device configured with a spring loaded float and an independent spring loaded check valve. (Rochester, 1995) Check valve which is designed to close with the aid of a spring when flow stops. It also has an air inlet valve which is designed to open when the internal pressure is one psi above atmospheric pressure so that no non-potable liquid may be siphoned back into the potable water system. Being spring loaded it does not rely upon gravity as does the atmospheric vacuum breaker. This assembly includes resilient seated shut-off valves and test cocks. The PVB must be installed at least twelve (12) inches above all downstream piping and outlets. The PVB may be used to protect against a pollutant or contaminant, however, it may only be used to protect against backsiphonage. It is not acceptable protection against backpressure. (USC, 1998)
  • reduced pressure principle assembly [RP, RPA, RPZ]: Consists of two internally loaded independently operating check valves and a mechanically independent, hydraulically dependent relief valve located between the check valves. This relief valve is designed to maintain a zone of reduced pressure between the two check valves at all times. The RP also contains tightly closing, resilient seated shut-off valves upstream and downstream of the check valves along with resilient seated test cocks. This assembly is used for the protection of the potable water supply from either pollutants or contaminants and may be used to protect against either backsiphonage or backpressure. (USC, 1998)

back pressure: Increase of pressure in the downstream piping system above the supply pressure at the point of consideration which would cause, or tend to cause, a reversal of the normal direction of flow. (ASAE, 1998)
back siphonage:

  • Reversal of flow (backflow) due to a reduction in system pressure which causes a negative or sub-atmospheric pressure to exist at a site in the water system.  (ASAE, 1998)
  • Technically, if one siphons a fluid out of a container or a pipeline, one causes that fluid to flow up over the rim of the container or top of the pipe and then down into a lower elevation through a piece of tubing or, in this case a piece of pipe that is part of the distribution system. In the vernacular, the unwanted fluid is "sucked" into the potable water line. It is important to understand that it is not necessary for the system main to be under a true vacuum (i.e., zero psia) for backsiphonage to occur. All that is required is a negative difference in pressure and a piece of tubing or pipe that is completely full of fluid.  (USC, 1998)

basic intake rate: See intake rate.
basin irrigation: See irrigation systems
beneficial uses: See uses.
best efficiency point: See efficiency.
best management practice [BPM] {}: An irrigation BMP is a voluntary irrigation practice that is both economical and practical and is designed to reduce water consumption and protect water quality while maintaining a healthy, functional landscape. (John Ossa, Committee Chair, IA Water Management Committee. Nov. 2000)
black water: Water containing liquid and solid human body waste generated through toilet usage. (ASAE, 1998)
Blaney-Criddle Method: Air temperature based method to estimate crop evapotranspiration. (NRCS, 1997)
bog coefficient* [BC] {-}: Inverse of scheduling coefficient but using the wettest 1%window instead of the driest.  Gives no indication of the location of dry areas.  (Contractor, 1999)
border dike: Earth ridge or small levee built to guide or hold irrigation or recharge water in a field. (ASAE, 1998)
border ditch: Small excavation used as a border of an irrigated strip or plot with water being spread from one or both sides. (ASAE, 1998)
border irrigation: See irrigation systems.
brake horsepower: See horsepower.
brake horsepower hour: See work.
British thermal units {BTU}: Amount of heat required to raise the temperature of one pound of water from 63 to 64 degrees Fahrenheit. ...  (Pumps, 1996)
bubbler: Water emission device that tends to bubble water directly to the ground or that throw water a short distance, on the order of one foot, (300 mm) before water contacts the ground surface. (Smith, 1997)
bubbler irrigation: See irrigation systems.
bulk density (of soil)*[Is,BDsoil] {lb/ft3}{g/cc} (11/6/99)Mass of dry soil per unit bulk volume ... (generally ranging in value from 1.3 to 1.6 g/cc)  (ASAE, 1998)
(bulk) density of water* [Iw, BDwater] {lb/ft3}{g/cc} (11/6/99): Same as density of water. Mass of water per unit bulk volume. (approximately 1.0  g/cc, 62.4 lb/ft3)
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cable tow traveler: See irrigation systems.
cablegation: See irrigation systems.
capillary radius [Cr] {ft, mm}: Additional wetted radius in soil profile beyond surface wetted radius for point source emitters. (Landscape, 1996)
capillary water: Water held in the capillary, or small pores of the soil, usually with soil water pressure (tension) greater than 1/3 bar. Capillary water can move in any direction. (NRCS, 1997)
carryover soil moisture {in., mm}: Moisture stored  in the soil within the root zone during the winter, at times when the crop is dormant, or before the crop is planted.  This moisture is available to help meet water needs of the next crop to be grown. (NRCS, 1997)
catch can grid: Containers spaced at regular intervals for collecting water for use in a water audit (sprinkler profile test). (Contractor, 1999).
cation: Positively charged ion which during electrolysis is attracted towards the cathode.  Sodium, potassium, calcium and magnesium are the most common cations in waters and soil extracts. (Hess, 1999)
cation exchange capacity* [CEC] {cmol/kg}:

  • The measure of the positively charged ions in a soil matrix. (Fertility and Fertilizers)
  • The sum of exchangeable cations  (usually Ca, Ma, K, Na, Al, H) that the soil constituent or other material can adsorb at a specific pH, usually expressed in centimoles of charge per Kg of exchanger (cmol/Kg), or milli equivalents per 100 grams of soil at neutrality (pH = 7.0), meq/100g. (NRCS, 1997)


  • Process when pressure on a liquid falls momentarily below the vapor pressure of that liquid, causing a phase change from liquid to vapor, and then a pressure increase causes another phase change and the gas becomes a liquid again. (Richard Neff, 2000, unpublished)
  • Process where pressure in the suction line falls below the vapor pressure of the liquid, vapor is formed and moves with the liquid flow.  These vapor bubbles or "cavities" collapse when they reach regions of higher pressure on their way through pumps. (Pumps, 1998)

CEC: See cation exchange capacity.
center pivot irrigation: See irrigation systems.
centrifugal pump: See pumps.
Certified Agricultural Irrigation Specialist (CAIS): The Certified Agricultural Irrigation Specialist is involved in the management and operation of on-farm irrigation systems. These systems include surface irrigation methods, as well as pressurized systems like microirrigation and sprinklers.
Certified Golf Irrigation Auditor (CGIA): The Certified Golf Irrigation Auditor is involved in the analysis of turf irrigation water use tailored to the unique conditions found on golf courses. Golf Auditors collect site data, make maintenance recommendations and perform water audits on golf courses. Through their analytical work at the site, these irrigation professionals develop base schedules for greens/tees, fairways and roughs.
Certified Irrigation Contractor (CIC): The Certified Irrigation Contractor is an irrigation professional whose principle business is the execution of contracts and subcontracts to install, repair and maintain irrigation systems. The CIC must conduct business in such a manner that projects meet the specifications and requirements of the contract. (IA Water Management Committee, 2001)
Certified Irrigation Designer (CID): The IA Certified Irrigation Designer engages in the preparation of professional irrigation designs.  They evaluate site conditions and determine net irrigation requirements based on the needs of the project.  The designer is then responsible for the selection of the most effective irrigation equipment and design methods.  The objective of a CID is to establish specifications and design drawings for the construction of an irrigation project. (IA Water Management Committee, 2001)
Certified Landscape Irrigation Auditor (CLIA): The Certified Landscape Irrigation Auditor is involved in the analysis of landscape irrigation water use.  Auditors collect site data, make maintenance recommendations and perform water audits.  Through their analytical work at the site, these irrigation professionals develop monthly irrigation base schedules.  Prior to certification examination, auditors are required to take an Irrigation Association approved preparatory course. (IA Water Management Committee, 2001)
Certified Landscape Water Manager (CLWM) 
check, check structure: Structure to control water depth in a canal, ditch or irrigated field. (NRCS, 1997)
check basin irrigation: See irrigation systems.
check irrigation: See irrigation systems.
check valve: See valve, check.
chemigation: Application of chemicals (including fertilizers) to crops through an irrigation system by mixing them with the irrigation water. (ASAE, 1998)
Christiansen's uniformity coefficient: See uniformity coefficient.
circular mil*[CM] {circular mils} (approved via RM): Unit of measure used to report the cross sectional area of a wire conductor (Reference Manual, ch. 5)
class (pipe):

  • Term generally used to describe the pressure rating of SDR-PR (standard dimension ratio-pressure rated) PVC pipe. For example, a class 200 pipe has a pressure rating of 200 psi. (colloquial)
  • Term used to identify the physical characteristics of thermoplastic pipe. (ASTM standard D1784-81)

class, soil: Group of soils defined as having a specific range in one or more particular properties such as acidity, degree of slope, texture, structure, land- use capability, degree of erosion, or drainage. (Soil, 1996)

  • Soil separate consisting of particles less than 0.002 mm in equivalent diameter. (Brady, 1990; Soil, 1996)
  • Soil textural class. (Soil, 1996)
  • Naturally occurring material composed primarily of fine-grained minerals, which is generally plastic at appropriate water contents and will harden when dried or fired. ...  (Soil, 1996)

clay loam: Soil textural class. See also texture, soil. (Soil, 1996)

  • arid climate: Climate characterized by low rainfall and high evaporation potential. A region is usually considered as arid when precipitation averages less than 10 inches per year. (NRCS, 1997)
  • humid climate: Climate characterized by high rainfall and low evaporation potential. A region generally is considered as humid when precipitation averages more than 40 inches per year. (NRCS, 1997)
  • semiarid climate: Climate characterized as neither entirely arid nor humid, but intermediate between the two conditions. A region is usually considered as semiarid when precipitation averages between 10 and 20 inches per year. (NRCS, 1997)
  • subhumid climate: Climate characterized by moderate rainfall and moderate to high evaporation potential. A region is usually considered subhumid when precipitation averages more than 20 inches per year, but less than 40 inches per year. (NRCS, 1997)

coarse sand: 

  • Coarse sand is a soil separate.  Its physical size varies according to the classification system being utilized. (Brady, 1990; Soil, 1996)
  • Soil textural class. (Soil, 1996)

coarse sandy loam: Soil textural class. (Soil, 1996)
coefficient [k, K] {-}: Various forms of "k" are used to describe constants, coefficients and factors.

  • allowable stress factor *[Kas] {-}: Coefficient used to modify reference evapotranspiration to reflect the water use of a particular plant or group of plants particularly with reference to the water stress. (paraphrased from Landscape, 1996)
    Ratio of the actual crop evapotranspiration to it potential (or reference) evapotranspiration. (NRCS, 1997)
  • crop coefficient* [Kc] {-} (11/6/99): Coefficient used to modify reference evapotranspiration  to reflect the water use of a particular plant or group of plants particularly with reference to the plant species. Ratio of the actual crop evapotranspiration to its potential (or reference) evapotranspiration. (NRCS, 1997)
  • density factor *[Kd] {-}] (approved via RM): Coefficient used to modify reference evapotranspiration  to reflect the water use of a particular plant or group of plants particularly with reference to the density of the plant material.  (paraphrased from Landscape, 1996)
  • landscape coefficient *[KL] {-} (approved via RM): Coefficient used to modify reference ET which includes species factor, density factor and microclimate factor. (Landscape, 2000)
  • microclimate factor* [Kmc] {-} (approved via RM): Factor or coefficient used to adjust reference evapotranspiration to reflect the microclimate of an area. (paraphrased from Landscape, 1996)
  • pan coefficient [ ] {}: Factor to relate actual evapotranspiration of a crop to the rate water evaporates from a free water surface in a shallow pan.  The coefficient usually changes by crop growth stage. (NRCS, 1997)
  • species factor* [Ks] {-} (approved via RM): Factor or coefficient used to adjust reference evapotranspiration to reflect plant species. (Landscape, 1996)

coefficient, consumptive use [ICUC][{%}: Ratio of volume of irrigation water consumptively used to the total volume of irrigation water that has left the region, both in a specified period of time. (Burt et al, 1997)
coefficient of retardation* [C, f] {-} (approved via RM): Values describing the hydraulic frictional characteristics of a pipe material.
coefficient of runoff [C] {-}: Runoff coefficient used in the rational method of predicting a design peak runoff rate. It helps to characterize runoff rate from a watershed area. (Schwab, et al., 1993)
coefficient of manufacturing variation: See manufacturer's coefficient of variation.
Coefficient of uniformity: See Uniformity coefficient

  • Attraction of water molecules to each other. (Brady, 1990)
  • Bonding strength between soil particles. (Hess, 1999)
  • Attraction of like substances to one another.  In soils, it is the process that forms a film of water around soil solids. (Contractor, 1999)

compensating emitter: See emitter.
consumptive use: See uses.
continuous-flow irrigation: System of irrigation water delivery where each irrigator receives the allotted quantity of water continuously. (ASAE, 1998)
continuous flushing emitter: See emitter.
control structure: Water regulating structure, usually for open channel flow conditions.  (NRCS, 1997)

  • An electric timing device that operates each (irrigation) zone for a predetermined time and frequency. (Keesen, 1995)
  • An automatic timing device that sends an electric signal for automatic valves to open or close according to a set irrigation schedule.  (Reaves, Lower Colorado River Authority.)

conveyance efficiency: See efficiency.
conveyance loss {? ft3}: Loss of water from a channel or pipe during transport, including losses due to seepage, leakage, evaporation, and transpiration by plants growing in or near the channel.  (ASAE, 1998)
corporation stop (valve): See valve.
corrugation irrigation: See irrigation systems.
coupler (sprinkler): Device, either self-sealed or mechanically sealed, that connects the ends of two lengths of pipe or pipe to a hose. (ASAE, 1998)
crop coefficient: See coefficient.
crop evapotranspiration: See evapotranspiration.
cross connection: Any actual or potential connection or structural arrangement between a public or private potable water system and any other source or system through which it is possible to introduce into any part of the potable system any used water, industrial fluids, gas, or substance other than the intended potable water with which the potable system is supplied. By-pass arrangements, jumper connections, removable sections, swivel or change-over arrangements or other “temporary” arrangements through which backflow could occur are considered to be cross-connections.  (USC, 1998) See also backflow.
crop growth stages: Periods of like plant function during the growing season. Usually four or more periods are identified  (NRCS, 1997):

  • initial – Between planting or when growth begins and approximately 10 percent ground cover.
  • crop development – Between about 10 percent ground cover and 70 or 80 percent ground cover.
  • mid season – From 70 or 80 percent ground cover to beginning of maturity.
  • late – From beginning of maturity to harvest.

crop rooting depth: See root zone.
crop water stress index [CWSI] {-}: Index of moisture in a plant compared to a fully watered plant, measured and calculated  by a CWSI instrument.  Relative humidity, solar radiation, ambient air temperature, and plant canopy temperature are measured. (NRCS, 1997)
crop water use: Same as plant water requirement and evapotranspiration.
cumulative intake {in.,mm}: Depth of water absorbed by soil from the time of initial water application to the specified elapsed time. (NRCS, 1997)
curb stop (valve): See valve.
current*[I] {amperes, amps} (11/6/99):

  • Movement or flow of electrons. (Derryberry, 1994).
  • The flow of electrons in a conductor. (Reference Manual)

cutback irrigation: See irrigation systems / furrow.
cycle time {min, h}: Length of water application periods, typically used with surge irrigation. (NRCS, 1997)