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Irrigation Glossary


deep percolation* [DP] {in., mm}:

  • Movement of water downward through the soil profile below the root zone that cannot be used by plants. (ASAE, 1998)
  • Infiltrated water, which moves below the root zone. (Burt et al, 1997)

deficit irrigation: Irrigation water management alternative where the soil in the plant root zone is not refilled to field capacity in all or part of the field. (NRCS, 1997)
deep percolation percentage [DP] {%}: Ratio of the average depth of irrigation water infiltrated and drained out of the root zone to the average depth of irrigation water applied. (ASAE, 1998)
degree of hazard: The type of backflow preventer used to prevent backflow from occurring at the point of a cross-connection depends on the type of substance which may flow into the potable water supply. A pollutant is considered to be any substance which would affect the color or odor of the water, but would not pose a health hazard. This is also considered a non-health hazard. A substance is considered a health hazard if it causes illness or death if ingested. This health hazard is called a contaminant.(USC, 1998)
delivery box (irrigation):

  • Structure diverting water from a canal to a farm unit often including measuring devices.Also called "turnout". (ASAE, 1998)
  • Water control structure for diverting water from a canal to a farm unit often including a measuring device.Also called delivery site, delivery facility, and turnout. (NRCS, 1997)

delivery loss: Preferred term is conveyance loss.
demand irrigation (system )(delivery): Procedure where each irrigator may request irrigation water in the amount needed and at the time desired. (ASAE, 1998)
density (of water)* [Iw] {g/cc, lb/ft3}: Mass of water per unit volume. Same as bulk density of water.
density factor: See coefficient.

  • Pattern of dots that shows the expected coverage from a particular combination of sprinklers, nozzles, pressure and spacing. (Solomon, 1988)
  • Graphical representation of precipitation rates. ... (Contractor, 1999)

depth* [d, D] {in., mm} (11/6/99)General term relating to depth of soil, water, or similar.
depth of irrigation {acre inches per acre, in., ft, mm}:

  • Depth of water applied
  • Depth of soil affected by an irrigation event. (NRCS, 1997)

diameter* [D, d] {in., mm} (11/6/99):

  • Dimension / size of a circular pipe, usually but not always the inside diameter [ID]. 
  • Inside diameter [ID, D] {in.} (approved via RM) 
  • Outside diameter [OD, D] {in.}
  • Nominal diameter (ND is, mm)

diameter of throw* [Dt] {ft, m} (11/6/99)Average diameter of the area wetted by an irrigation sprinkler operating in still air. (ASAE, 1998)
dielectric union: Pipe connection (union) having an insulator between the two sides of the union for the purpose of reducing corrosion caused by galvanic action. (Smith, 1997)
dimension ratio* [DR] {-}: Ratio of the average pipe diameter to the minimum wall thickness. The pipe diameter may be either outside or inside diameter. (ASAE, 1998) See related term standard dimension ratio.
direct current [DC] {}: Current in which electrons flow constantly in one direction. (Reference Manual, ch 5)
distribution system: System of ditches, or conduits and their controls, which conveys water from the supply canal to the farm points of delivery. (ASAE, 1998)
distribution uniformity* [DU] {%, decimal} (11/6/99)Measure of the uniformity of irrigation water over an area. (ASAE, 1998).
distribution uniformity of lower quarter* [DUlq] {%, decimal}:

  • Ratio of the average low quarter depth of irrigation water infiltrated to the average depth of irrigation water infiltrated. (On-Farm Committee, 1979)
  • Ratio of the average of the lowest one-fourth of measurements of irrigation water infiltrated (or applied) to the average depth of (the total) irrigation water infiltrated (applied). (ASAE, 1998)

disturbed (manipulated) soils: Soils with soil profiles that have been altered because of earth-moving activities (or soil amendment). (Contractor, 1999)
diversion box: Structure built into a canal or ditch for dividing the water into predetermined portions and diverting it to other canals or ditches. (ASAE, 1998)
diversion dam: Barrier built in a stream for the purpose of diverting part or all of the water from the stream into a canal. (ASAE, 1998)
double check valve assembly: See backflow prevention device.
drip irrigation: See irrigation system, drip.
drought: Period of dryness especially when prolonged that causes extensive damage to crops or prevents their successful growth. (Webster's Collegiate Dictionary 10th Edition)
dry weight (of soil sample) *[DW] {lbs} {grams} (11/6/99)Oven dry weight of a soil sample. (Reference Manual)
ductile iron: Form of iron used to make pipe and fittings.
dynamic head: See head, dynamic.
dynamic pressure {psi, kPa}: Measure of water pressure with the water in motion (also known as working pressure). (Monroe, 1993)

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effective precipitation [Pe] {in., mm}: Portion of total precipitation which becomes available for plant growth. (ASAE, 1998)
effective rainfall {in., mm}: Portion of total rainfall which becomes available for plant growth.. (ASAE, 1998)
effective rooting depth: See rooting depth, effective.
efficiency [E] {%, fraction} (11/6/99):

  • application efficiency* [Ea, AE] (approved via RM): Ratio of the average depth of the irrigation water stored in the root zone to the average depth of irrigation water applied. (On-Farm Committee, 1979) Ratio of the average depth of irrigation water infiltrated and stored in the root zone to the average depth of irrigation water applied. (NRCS, 1997) Amount of water stored in the root zone that is available for plant use divided by the average amount of water applied during an irrigation. (Scheduling, 1998) Ratio of the average depth of irrigation water contributing to target, to the average depth of irrigation water applied. (Burt, et al, 1997) 
  • (water) application efficiency of lower half* [Eh, ]: Ratio of the average of the lowest one-fourth of measurements of irrigation water infiltrated to the average depth of water applied. (NRCS, 1997) 
  • (water) application efficiency of lower quarter* [Eq]: Ratio of the average low quarter depth of irrigation water infiltrated and stored in the root zone to the average depth of irrigation water applied. (On-Farm Committee, 1979) Ratio of the average of the lowest one-fourth of measurements of irrigation water infiltrated to the average depth of water applied. (NRCS, 1997) 
  • best efficiency point [BEP]: Highest efficiency on a pump characteristic curve. (Pumps, 1998) 
  • conveyance efficiency* [Ec]: Ratio of the water delivered, to the total water diverted or pumped into an open channel or pipeline at the upstream end. (NRCS, 1997) 
    irrigation efficiency* [Ei, IE]: Ratio of the average depth of irrigation water that is beneficially used to the average depth of irrigation water applied ... (ASAE, 1998) (On-Farm Committee, 1979) 
  • potential application efficiency of Low Quarter [PELQ] {%): Low quarter application efficiency obtainable with a given irrigation system when the depth of irrigation water infiltrated in the quarter of the area receiving the least water equals some predetermined value of the soil moisture deficit. (On-Farm Committee, 1979) 
  • pumping plant efficiency (overall) [Epp] {%}: is the ratio of the output power into the water to the input power to the driver. (ITRC, 2001) 
  • motor efficiency* [Em]: Ratio of the power delivered to the pump by the power unit to the input power to the motor. 
  • project efficiency: Overall efficiency of irrigation water use in a project setting that accounts for all water uses and losses, such as crop ET, environmental control, salinity control, deep percolation, runoff, ditch and canal leakage, phreatophyte use, wetlands use, operational spills, and open water evaporation. (NRCS. 1997) 
  • pump efficiency* [Ep]: Ratio of the water power produced by the pump, to the power delivered to the pump by the power unit. (ASAE, 1998) 
  • (water) storage efficiency*: Ratio of the amount of water stored in the root zone during irrigation to the amount of water needed to fill the root zone to field capacity. (James, 1988) 
  • water use efficiency [WUE]: Ratio of the yield per unit area to the applied irrigation water per unit area. (Styles, S., 1999. Unpublished)

effluent irrigation: Land application of treated wastewater for irrigation and beneficial use of nutrients. (ASAE, 1998)
electrical conductivity* [EC] {mmho/cm, dS/m}: Measure of the ability of the soil water to transfer an electrical charge. Use as an indicator for the estimation of salt concentration.

  • [ECe] Electrical conductivity of soil water extract. 
  • [ECi] Electrical conductivity of irrigation water.
  • [ECaw] Electrical conductivity of applied water. (NRCS, 1997)

electrical resistance block: Block made up of various materials containing electrical contact wires that is placed in the soil at selected depths to measure soil moisture content (tension). Electrical resistance, as affected by moisture in the block is read with a meter. (NRCS, 1997)
elevation head: See head.
emission point: Location where water is discharged from an emitter. (ASAE, 1998)
emission uniformity* [EU] {-,%}: Index of the uniformity of emitter discharge rates throughout a micro-irrigation system. Takes account of both variations in emitters and variations in the pressure under which the emitters operate. (ASAE, 1998) (Emission uniformity is also used with other types of irrigation systems. Editor)
emitter: Small micro-irrigation dispensing device designed to dissipate pressure and discharge a small uniform flow or trickle of water at a constant discharge, which does not vary significantly because of minor differences in pressure head. Also called a "dripper" or "trickler". (ASAE, 1998)

  • compensating emitter: See pressure compensating emitter.
  • continuous flushing emitter: Micro irrigation system emitter designed to continuously permit passage of large solid particles while operating at a trickle or drip flow, thus reducing filtration requirements. (NRCS, 1997) 
  • flushing emitter: Emitter designed to have flushing flow of water to clear the discharge opening every time the system is turned on. (NRCS, 1997) 
  • line source emitter: Water is discharged from closely spaced perforations, emitters, or a porous wall along the tubing. (NRCS, 1997) 
  • long-path emitter: Emitter which employs a long capillary sized tube or channel to dissipate pressure. (NRCS, 1997) 
  • multi-outlet emitter: Device which supplies water to two or more points through small diameter auxiliary tubing. (NRCS, 1997) 
  • non-pressure compensating emitter: Emitter designed with a fixed orifice or other components and contains no pressure compensating features. 
  • orifice emitter: Emitter which employs a series of orifices to dissipate pressure. (NRCS, 1997) 
  • pressure compensating emitter: Emitter designed to discharge water at a near constant rate over a wide range of lateral line pressures. (NRCS, 1997) 
  • vortex emitter: Emitter which employs a vortex effect to dissipate pressure. (NRCS, 1997)

entrance loss {ft, m}: Energy lost in eddies and friction at the inlet to a conduit or structure. (ASAE, 1998)
evaporation* [E] {in./day, in./wk, mm/wk, mm/month} (part of ET):

  • Water movement from a wet soil or plant surface which does not pass through the plant. (Burt, 1998) 
  • Physical process by which a liquid is transformed to the gaseous state, which in irrigation generally is restricted to the change of water from liquid to vapor. Occurs from plant leaf surface, ground surface, water surface and sprinkler spray. (NRCS, 1997)

evaporation pan:

  • Standard U.S. Weather Bureau Class A pan (48-inch diameter by 10-inch deep) used to estimate the reference crop evapotranspiration rate. Water levels are measured daily in the pan to determine amount of evaporation.
  • Pan or container placed at or about crop canopy height containing water. Water levels are measured daily in the pan to determine the amount of evaporation. (NRCS, 1997)

evapotranspiration* [ET] {in./day, in./week, mm/wk, mm/day } (11/6/99)Combination of water transpired from vegetation and evaporated from the soil and plant surfaces. (ASAE, 1998)

  • crop [ETc] (approved via RM): Crop evapotranspiration is the quantitative amount of ET within the cropped area of a field, and which is associated with growing of a crop. (Burt, et al. 1997) Same as plant water requirement. Amount of water used by the crop in transpiration and building of plant tissue, and that evaporated from adjacent soil or intercepted by plant foliage. It is sometimes referred to as consumptive use. (NRCS, 1997) Rate at which water, if available, would be removed from soil and plant surfaces. (ASAE, 1998) Rate at which water, if available, would be removed from the soil and plant surface expressed as the latent heat transfer per unit area or its equivalent depth of water per unit area. (Jensen, 1980).
  • reference (approved via RM): [ETo] * Rate of evapotranspiration from an extensive surface cool-season green grass cover of uniform height of 12 cm, actively growing, completely shading the ground, and not short of water. (Mecham, Brent, 1999. Unpublished; also see ASCE, 1990) [ETr]* Upper limit or maximum evapotranspiration that occurs under given climate conditions with a field having a well-watered agricultural crop with an aerodynamically rough surface, such as alfalfa with 50 cm. of top growth. (Mecham, Brent, 1999. Unpublished; also see ASCE, 1990)
  • potential [ETp, Etp, ](approved via RM)

exchange capacity: See cation exchange capacity.
exchangeable sodium percentage* [ESP] {%}: 

  • Fraction of cation exchange capacity of a soil occupied by sodium ions. Exchangeable sodium, (meq/100 gram, spoil) divided by CEC (meq/100 gram soil) times 100. It is unreliable in soil containing soluble sodium silicate minerals or large amounts of sodium chloride. (NRCS, 1997)
  • Percentage of the cation exchange capacity (meq.) of a soil which is occupied by sodium. (Burt, 1998)
  • Index of the saturation of the soil exchange complex with sodium ions. ... (Hess, 1999)

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fertigation: Application of nutrients through an irrigation system.
field capacity* [FC] {in./in., in./ft, %, bars, kPa, mm/m } (approved via RM):

  • Moisture remaining in a soil following wetting and natural drainage until free drainage has practically ceased. (On-Farm Irrigation Committee, 1978)
  • Amount of water remaining in a soil when the downward water flow due to gravity becomes negligible. (ASAE, 1998)

final infiltration rate: See basic intake rate.
fine sand: Soil textural class. (Soil, 1996)
fine sandy loam: Soil textural class. See also soil texture. (Soil, 1996)
FIPT: Acronym for female iron pipe thread. (Smith, 1997)
flood irrigation: See irrigation systems.
flow rate* [Q or q] {gpm, gph, gal/min, ft3/s, cfs, Litres/s, Litres/min, m3/h} (11/6/99)Rate of flow or volume per unit period of time.
foot valve: See valve, foot.
free drainage: Movement of water by gravitational forces through and below the plant root zone. This water is unavailable for plant use except while passing through the soil. (NRCS, 1997)
frequency distribution:

  • Values in a sample are grouped into a limited number of classes. A table is made showing the class boundaries and the frequencies (number of members of the sample) in each class. The purpose is to show a compact summary of the data. (Snedecor and Cochran, 1967)
  • Measurement and presentation of various fractions of total water applied for selected depth ranges referenced to average depth applied.

friable: Soil consistency term referring to the ease with which the soil aggregates may be crumbled (in the hand), i.e. a friable soil is easily crumbled in the hand. (Hess, 1999)
friction factor, Christiansen* [F] {-}: Friction factor or coefficient used in the Christiansen Procedure to determine pressure loss in a multiple outlet piping system.
friction factor, Darcy Weisbach [f] {-} (approved via RM): Friction factor used with Darcy Weisbach equation. (Reference Manual, ch 2)
friction factor (lateral)* [Ff] {psi/100 ft, m /100 m}: Factor used to size pipe. (Head Layout, 1998.)
friction loss [hf] {psi, ft, kPa, m}: Also, referred to as pressure loss.

  • Amount of pressure lost through pipes due to water movement and turbulence. (Carruthers, B. 2001, unpublished) 
  • Amount of pressure lost as water flows through an irrigation system (due to friction against the pipe walls). (Rain Bird. 1997)
  • As water moves through an irrigation system, pressure is lost because of turbulence created by the moving water. This turbulence can be created in pipes, valves or fittings. These losses are referred to as friction losses. (Monroe, 1993)

frost protection: Applying irrigation water to affect air temperature, humidity, and dew point to protect plant tissue from freezing. The primary source of heat (called heat of fusion) occurs when water turns to ice, thus protecting sensitive plant tissue. (NRCS, 1997)
full irrigation: Management of water applications to fully replace water used by plants over an entire field. (NRCS, 1997)
furrow irrigation: See irrigation system, furrow.
fungicide: Chemical pesticide that kills fungi or prevents them from causing diseases on plants. (NRCS, 1997)

  • Small channel for conveying irrigation water down slope across the field. Sometimes referred to as a rill or corrugation. (NRCS, 1997) 
  • Trench or channel in the soil made by a tillage tool.

furrow dike: Small earth dike formed in a furrow to prevent water translocation. Typically used with LEPA and LPIC systems. Also used in non-irrigated fields to capture and infiltrate precipitation. Sometimes called reservoir tillage. (NRCS, 1997)
furrow stream: Stream flow in a furrow, corrugation or rill. (NRCS, 1997)