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Irrigation Glossary


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.

Peak use rate Maximum rate at which a crop uses water.

Percentage fines Percentage of water volume falling in fine (< 1 mm in diameter) drops.)  Term also used relative to soil particle size.


  • 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. 
  • 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. 

Percolation rate Rate at which water moves through porous media, such as soil.

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. Preferred term is stationary sprinklers.

Permanent wilting point

  • 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.
  • 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.
  • Also known as wilting percentage, wilting coefficient or wilting point.


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

Pesticide Chemical agent used to control specific organisms. Includes insecticides, herbicides, and fungicides.

Pipe dope Common name for commercial products used to apply to pipe fittings to assist in the appropriate fit of the threaded joints.

Pipe joint compond A material applied to steel pipe threads to ensure a watertight or airtight seal. Also called pipe dope.

Pipe thread tape A synthetic pipe thread wrapping that seals a joint.

Pitot tube Small ell shaped tube which can be attached to a pressure gage 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.

Plant available water Available water located in the root zone.

Point of connection  Location where irrigation system is connected to a (potable) water supply.

Polyethylene  Flexible (usually black) plastic material used to make irrigation pipe and other items.

Polyvinyl-chloride Semi-rigid plastic material used to make irrigation pipe and other items.

Pop-up Spray Head A sprinkler head that is generally set flush with the soil. These heads contain a stem that rises when the water is turned on.


  • Volume of pores in a soil sample relative to the total volume of the sample.
  • Percentage of soil volume which is pore space.

Positive displacement pump Pump that moves a fixed quantity of fluid with each stroke or rotation, such as a piston or gear pump.

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.


  • soil water potential 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.
  • total potential Sum of matric, pressure, solute and gravitational potentials.
  • matric potential Attraction of the solid soil matrix for water.
  • pressure potential Potential caused by water pressure.
  • solute or osmotic potential Potential caused by salinity.
  • gravitational potential Relative height of a point above or below a reference elevation.

Precipitation Total of all atmospheric water deposited on the surface.  That is rain, snow, hail, dew and condensation.

Precipitation rate

  • Rate at which a sprinkler system applies water to a given area.
  • precipitation rate, lowest Lowest precipitation rate in a defined contiguous area.
  • instantaneous precipitation (application) rate Maximum rate, usually localized, that a sprinkler application device applies water to the soil.
  • net precipitation rate 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. 
  • 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.
  • system precipitation rate 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.

Pressure 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 Water pressure caused due to changes in water velocity in a pipe system.  Also referred to as surge pressure.

Pressure Gauge A device that can be attached to your home's water system to measure the psi (pounds per square inch) of static water pressure.

Pressure loss Amount of pressure lost as water flows through a system.

Pressure rating 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.

Pressure regulator Device which maintains constant downstream operating pressure (immediately downstream of the device) which is lower than the upstream pressure.

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 Backflow device configured with a spring loaded float and an independent spring loaded check valve.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. 

Profile (soil) Vertical section of the soil through all its horizons and extending into the C horizon.

Profile (sprinkler) Chart showing the application rates vs. distance of throw for a sprinkler head. See also single leg profile.

Propeller pump Pump which develops most of its head by the lifting action of vanes on the water.

PSI Acronym for pounds per square inch.  See also pressure. Water pressure is measured in pounds per square inch.

Pump Mechanical device that converts mechanical forms of energy into hydraulic energy.

  • centrifugal Pump consisting of rotating vanes (impeller) enclosed in a housing and used to impart energy to a fluid through centrifugal force.
  • 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.
  • multi-stage Pump having more than one impeller mounted on a single shaft.
  • radial flow pump   Centrifugal pump that uses diffuser vanes to transform the velocity head into pressure head.  Commonly called a "turbine pump".
  • 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.

Pump column Pipe through which water from well pumps (vertical turbine impellers) is conveyed to the ground surface (pump discharge head).

Pump efficiency Ratio of the water power produced by the pump, to the power delivered to the pump by the power unit.

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.

Pump start relay Low-amperage or electric switch designed for use with pump start circuits.

PVC Polyvinylchloride (PVC) pipe is the most common accepted type of plastic drain pipe. PVC is sometimes also used for supply pipes in irrigation installations.

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