Irrigation Glossary

Radial flow pump Centrifugal pump that uses diffuser vanes to transform the velocity head into pressure head. Commonly called a "turbine pump"

Radius of throw Distance from the sprinkler head to the furthest point of water application and is used only for heads with circular wetting patterns.

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.

Rainfall intensity 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.)

Rational equation Equation used to predict the runoff from a watershed.

Readily available water Portion of available water that is more readily available for plant usage. It varies with plant type.

Reclaimed water Water that has been collected after an original use for reuse.

Rectangular spacing Sprinklers are spaced in a rectangular pattern.

Reduced pressure 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.

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.

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.

Resistance Hindrance to current flow in a given circuit.

Reynolds Number Dimensionless number used to characterize flow of water.

Riparian Area of flowing streams that lies between the normal water line and some defined high water line.

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.

R.P.A. Abbreviation for reduced pressure assembly. 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.

Root zone Area of the soil from which the crop roots extract water and nutrients.

Rotational speed Number of revolutions made per unit time (usually minutes) by a shaft or other object.

Rotor sprinkler Sprinkler that rotates, but may more specifically refer to a gear driven sprinkler

Row spacing Distance between adjacent rows of sprinklers.

Runoff Portion of precipitation, snow melt or irrigation, that flows over the soil, eventually making its way to surface water supplies.

Runoff rate 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.

Run time Length of time to operate an irrigation system or an individual zone.

Run time multiplier Multiplier used to compensate for the lack of perfect uniformity in a sprinkler system.