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Irrigation Glossary


pan coefficient: See coefficient, pan.
peak use rate {in./day, in./week}: Maximum rate at which a crop uses water. (NRCS, 1997)
percentage fines {%}: Percentage of water volume falling in fine (< 1 mm in diameter) drops. (Contractor, 1999) Term also used relative to soil particle size.
percolation: beneficial deep percolation-leaching. [] {in.} is a beneficial use when it leaches salts from the root zone to a level required for acceptable crop production.  (Burt et al 1997) 
nonbeneficial (excess) deep percolation [] {in.}: If the actual depth of deep percolation at a given location is more than the required beneficial leaching depth, that which is in excess of the requirement is nonbeneficial.  (Burt et al 1997)
percolation rate {in./h, mm/h}: Rate at which water moves through porous media, such as soil. (ASAE, 1998)
performance curve: Graph showing the capability of a product with varying inputs.  i.e. the dynamic head of a pump as it varies with discharge.
permanent irrigation: Irrigation having underground piping with risers and sprinklers. (Soil, 1996)  Preferred term is stationary sprinklers.
(permanent) wilting point* [PWP ] {%, in./in., in./ft , mm/m  (approved via RM):

  • Moisture content, on a dry weight basis, at which plants can no longer obtain sufficient moisture from the soil to satisfy water requirements.  Plants will not fully recover when water is added to the crop root zone once permanent wilting point has been experienced.  Classically, 15 atmospheres (15 bars), soil moisture tension is used to estimate PWP. (NRCS, 1997) 
  • Moisture content of the soil after the plant can no longer extract moisture at a sufficient rate for wilted leaves to recover overnight or when placed in a saturated environment. (On-Farm Committee, 1978) 
  • Also known as wilting percentage, wilting coefficient or wilting point. (Reference Manual)

permeability {in./h, mm/h}:

  • Qualitatively, the ease with which gases, liquids, or plant roots penetrate or pass through a layer of soil. (NRCS, 1997) 
  • Quantitatively, the specific soil property designating the rate at which gases and liquids can flow through the soil or porous media. (NRCS, 1997)

pesticide: Chemical agent used to control specific organisms. Includes insecticides, herbicides, and fungicides. (NRCS, 1997)
pipe dope: Common name for commercial products used to apply to pipe fittings to assist in the appropriate fit of the threaded joints.
pitot tube: Small ell shaped tube which can be attached to a pressure gauge or other measuring device to measure the velocity head of water discharging from a nozzle or flowing in a pipe.
pH* {-}: Measure of acidity or alkalinity. (Burt, 1998)
plant available water*[PAW] {in., mm} (11/6/99)Available water located in the root zone. Same as root zone available water.   See Fig. 1 at end of document.
plant water requirement* [PWR] {in./day, in/week, mm/day}: Same as evapotranspiration.
point of connection [POC]: Location where irrigation system is connected to a (potable) water supply.
polyethylene [PE]: Flexible (usually black) plastic material used to make irrigation pipe and other items.
polyvinyl-chloride [PVC]: Semi-rigid plastic material used to make irrigation pipe and other items.
porosity {%,-}:

  • Volume of pores in a soil sample relative to the total volume of the sample. (Hess, 1999) 
  • Percentage of soil volume which is pore space. (Drip, 1997)

portable (hand move) irrigation: See irrigation system.
positive displacement pump: Pump that moves a fixed quantity of fluid with each stroke or rotation, such as a piston or gear pump. (ASAE, 1998)
potable water:

  • Domestic or drinking water. It can be used as a source of irrigation water, but once water enters an irrigation system (and passes through the backflow device) it is no longer considered potable. 
  • Treated water which is meant for human consumption. It can be used for irrigation systems as long as protection is provided to prevent contamination of the domestic supply. (Texas Natural Resource Conservation Commission)


  • soil water potential [] {ft, bar, kPa}: Amount of work that must be done per unit quantity of pure water in order to transport reversibly and isothermally an infinitesimal quantity of water from a pool of pure water at a specified elevation at atmospheric pressure to the soil water at the point under consideration. (ASAE, 1998) 
  • total potential [Qt] {ft, bar, kPa}: Sum of matric, pressure, solute and gravitational potentials. (Burt, 1998) 
  • matric potential [Qm] {ft, bar, kPa}: Attraction of the solid soil matrix for water. (Burt, 1998) 
  • pressure potential [Qp] {ft, bar, kPa}: Potential caused by water pressure. (Burt, 1998) 
  • solute or osmotic potential [Qs] {ft, bar,kPa}: Potential caused by salinity. (Burt, 1998) 
  • gravitational potential  [Qz] {ft, bar, kPa}: Relative height of a point above or below a reference elevation. (Burt, 1998)

potential evapotranspiration: See evapotranspiration.
power*[P] {kw, hp}: Rate of doing work. (Derryberry, 1994)  See also horsepower
precipitation {in., mm}: Total of all atmospheric water deposited on the surface.  That is rain, snow, hail, dew and condensation. (Hess, 1999)
precipitation rate* [PR] {in./h, mm/h} (11/6/99): Rate at which a sprinkler system applies water to a given area.  (NRCS, 1997) 

  • precipitation rate, lowest [PRL] {in./h, mm/h}: Lowest precipitation rate in a defined contiguous area. (Landscape, 2000) 
  • instantaneous precipitation (application) rate [] {in./h, mm/h}: Maximum rate, usually localized, that a sprinkler application device applies water to the soil. (NRCS, 1997) 
  • net precipitation rate * [PRnet] {in./h, mm/h}: Measure of the amount of water that actually reaches the landscape.  The net precipitation rate is the gross precipitation rate minus the losses that occur between the sprinkler and the landscape surface.  (Landscape, 1996). 
  • sprinkler precipitation rate [] {in./h, mm/h}: Precipitation rate of a group of heads used together and all having the same arc, spacing and flow. (Monroe, 1993) 
    system precipitation rate [PR] {in./h, mm/h} Precipitation rate for a system is the average precipitation rate of all sprinklers in a given area regardless of the arc, spacing, or flow rate of each head. (Monroe, 1993)

pressure* [P, p] {psi , lb/in.2, kPa} (11/6/99)The force exerted over a surface divided by its area (Webster) In irrigation, pressure usually describes the amount of energy available to move water through a pipe, sprinkler or emitter.
pressure due to surge* [ps] {psi, kPa}: Water pressure caused due to changes in water velocity in a pipe system.  Also referred to as surge pressure.
pressure loss {psi, kPa, m}: Amount of pressure lost as water flows through a system.  (Rain Bird, 1997)
pressure rating [PR] {psi, kPa, m}: Estimated maximum internal pressure that can be continuously exerted in a pipe or container with a high degree of certainty that it will not fail. (ASAE, 1998; ANSI/ASAE S376.2 JAN98)
pressure regulator: Device which maintains constant downstream operating pressure (immediately downstream of the device) which is lower than the upstream pressure. (Rain Bird, 1997)
pressure tank: Enclosed container attached to a water system usually containing an air pocket so that it behaves as a temporary water supply.
pressure vacuum breaker: See backflow prevention device.
profile (soil): Vertical section of the soil through all its horizons and extending into the C horizon. (Soil, 1966)
profile (sprinkler): Chart showing the application rates vs. distance of throw for a sprinkler head. (Center for Irrigation Technology. SpacePro Glossary).  See also single leg profile.
program: See scheduling program.
project efficiency: See efficiency.
propeller pump: Pump which develops most of its head by the lifting action of vanes on the water. (ASAE, 1998)
psi* {psi, kPa}: Acronym for pounds per square inch.  See pressure.
pump: Mechanical device that converts mechanical forms of energy into hydraulic energy. (Pumps, 1996)
centrifugal: Pump consisting of rotating vanes (impeller) enclosed in a housing and used to impart energy to a fluid through centrifugal force. (ASAE, 1998)
jockey pump: Usually a small pump used to provide pressure and flow in a multi-pump system.
mixed flow pump: Centrifugal pump in which the pressure is developed partly by centrifugal force and partly by the lifting action of the impellers on the water. (ASAE, 1998)
multi-stage: Pump having more than one impeller mounted on a single shaft. (NRCS, 1997) 
radial flow pump: Centrifugal pump that uses diffuser vanes to transform the velocity head into pressure head.  Commonly called a "turbine pump". (ASAE, 1998)
submersible pump: Pump where the motor and pump are submersed below the water surface.
trash pump: Pump designed to pump large sized particulate matter in addition to liquid.
pto pump: Pump driven by a separate power supply  connected to the pump by a power takeoff (pto) drive.
vertical turbine: Pump having one or more stages, each consisting of an impeller on a vertical shaft, surrounded by stationary and usually symmetrical guide vanes. Combines the energy-imparting characteristics of axial-flow and propeller pumps. (ASAE, 1998)
pump column: Pipe through which water from well pumps (vertical turbine impellers) is conveyed to the ground surface (pump discharge head). (ASAE, 1998)
pump efficiency: See efficiency.
pump start circuit: Feature on automatic controllers which supplies 24 VAC, which can be used to activate a pump through an external pump start relay. (Rain Bird, 1997)
pump start relay: Low-amperage or electric switch designed for use with pump start circuits. (Rain Bird, 1997)
PVC: See polyvinyl-chloride.
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quick coupling valve: See valve.
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radial flow pump: See pump.
radius of throw*[Rt] {ft, m} (11/6/99)Distance from the sprinkler head to the furthest point of water application and is used only for heads with circular wetting patterns. (Rochester, 1995)
rain gauge: Device such as a graduated tube used to measure the amount of rainfall.
rain sensor: Device to measure rainfall.
rain shut-off device: Device which prevents voltage from the controller from activating the valves when a preset amount of rain falls. (Rain Bird, 1997)
rain switch: See rain shut-off device.
rainfall intensity* [i] {in./h, mm/h}: Rate of rainfall for the design storm frequency and for the time of concentration of the drainage area. (Used in rational method to compute runoff.) (LD, 1997)
rational equation: Equation used to predict the runoff from a watershed.
readily available water* [RAW] {in./ft, mm/m } (11/6/99)Portion of available water that is more readily available for plant usage.  It varies with plant type. Also see related term maximum allowable deficiency.  See Fig. 1 at end of document.
reasonable uses: See uses.
reclaimed water: Water that has been collected after an original use for reuse.
rectangular spacing: See spacing.
reduced pressure (backflow preventer) device: See backflow prevention device.
reference crop evapotranspiration: See evapotranspiration.
relative humidity {%}: Ratio of the amount of water vapor present in the atmosphere to the amount required for saturation at the same dry bulb temperature. (NRCS, 1997)
relay: Electrical or electronic device which uses a signal current to actuate a separate electrical circuit.
retraction: Operation when the pop-up riser of a sprinkler such as a spray head or rotor returns to the case in the ground.  Also called pop-down. (Rain Bird, 1997)
resistance*[R, Rw] {ohms} (11/6/99)Hindrance to current flow in a given circuit (Reference Manual, ch. 5)
Reynolds Number*[R] {-}: Dimensionless number used to characterize flow of water.

  • Area of flowing streams that lies between the normal water line and some defined high water line. (NRCS, 1997) 
  • Pertaining to the banks of a body of water; riparian owner is the one who owns the banks. (NRCS, 1997) 
  • Riparian water right is the right to use and control water by virtue of ownership of the banks. (NRCS, 1997)

riser: Length of pipe which has male nominal pipe threads on each end and is usually affixed to a lateral or sub-main to support a sprinkler or anti-siphon valve. (Rain Bird, 1997)
R.P.A.: Abbreviation for reduced pressure assembly. See backflow prevention devices, reduced pressure.
root depth {in., ft, mm}: Preferred term root zone.
effective {in., ft, mm}: Depth from which roots extract water. The effective rooting depth is generally the depth from which the crop is currently capable of extracting soil water. However, it may also be expressed as the depth from which the crop can extract water when mature or the depth from which a future crop can extract soil water. Maximum effective root depth depends on the rooting capability of the plant, soil profile characteristics, and moisture levels in the soil profile.  (NRCS, 1997)
root zone* [RZ, d or drz,] {in., ft, mm}  (11/6/99):

  • Depth of soil that plants roots readily penetrate and in which the predominant root activity occurs. (ASAE, 1998) 
  • Area of the soil from which the crop roots extract water and nutrients.  (USDA, 1993) 
  • Crop rooting depth is typically taken as the soil depth containing 80 percent of plant roots. (NRCS, 1997) 
  • (May also be used as a portion of the root zone in equations where soil characteristics change within the root zone.)

root zone available water [RZAW] {in., mm}: See preferred term: plant available water.
root zone depth: See root zone and depth.
rotational speed*[N] {rpm} (approved via RM): Number of revolutions made per unit time (usually minutes) by a shaft or other object.
rotor sprinkler: See sprinkler head.
row spacing: See spacing, row
runoff [RO ] {in., mm}:

  • Portion of precipitation, snow melt or irrigation, that flows over the soil, eventually making its way to surface water supplies. (ASAE, 1998) 
  • Surface water that leaves the subject region in liquid form.  (Burt et al, 1997)

runoff rate {in./h, mm/h}:

  • Rate at which water flows above ground from a watershed or field. 
  • Water which is not absorbed by the soil and drains to another location. Runoff occurs when water is applied in excessive amounts or too quickly. (Rain Bird, 1997)

run time*[RT] {minutes, h} (approved via RM): Length of time to operate an irrigation system or an individual zone. (Reference Manual , ch 6)
run time multiplier [RTM] {-}: Multiplier used to compensate for the lack of perfect uniformity in a sprinkler system.  (Scheduling, 1999). Also, see scheduling coefficient.