Irrigation Glossary

Deep percolation Movement of water downward through the soil profile below the root zone that cannot be used by plants.

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.

Deep percolation percentage Ratio of the average depth of irrigation water infiltrated and drained out of the root zone to the average depth of irrigation water applied.

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.

Delivery box (irrigation) Structure diverting water from a canal to a farm unit often including measuring devices. Also called delivery site, delivery facility, and turnout.

Delivery loss 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. Preferred term is conveyance loss.

Demand irrigation Procedure where each irrigator may request irrigation water in the amount needed and at the time desired.

Density (of water) Mass of water per unit volume. Same as bulk density of water.

Densogram Graphical representation of precipitation rates. Pattern of dots that shows the expected coverage from a particular combination of sprinklers, nozzles, pressure and spacing.

Depth General term relating to depth of soil, water, or similar.

Depth of irrigation

Depth of water applied
Depth of soil affected by an irrigation event.

Diameter

Dimension / size of a circular pipe, usually but not always the inside diameter
Iinside diameter
Outside diameter
Nominal diameter

Diameter of throw Average diameter of the area wetted by an irrigation sprinkler operating in still air.

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.

Dimension ratio Ratio of the average pipe diameter to the minimum wall thickness. The pipe diameter may be either outside or inside diameter. See related term standard dimension ratio.

Direct current [DC] Current in which electrons flow constantly in one direction.

Distribution system System of ditches, or conduits and their controls, which conveys water from the supply canal to the farm points of delivery.

Distribution uniformity Measure of the uniformity of irrigation water over an area.

Distribution uniformity of lower quarter 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).

Disturbed soils Soils with soil profiles that have been altered because of earth-moving activities

Diversion box Structure built into a canal or ditch for dividing the water into predetermined portions and diverting it to other canals or ditches.

Diversion dam Barrier built in a stream for the purpose of diverting part or all of the water from the stream into a canal.

Double check valve assembly 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.

Drip irrigation also known as microirrigation or trickle irrigation is an irrigation method which minimizes the use of water and fertilizer by allowing water to drip slowly to the roots of plants, either onto the soil surface or directly onto the root zone, through a network of valves, pipes, tubing, and emitters.

Drought Period of dryness especially when prolonged that causes extensive damage to crops or prevents their successful growth.

Dry weight (of soil sample) Oven dry weight of a soil sample.

Ductile iron Form of iron used to make pipe and fittings.

Dynamic head Specific energy in a flow system.

Dynamic pressure Measure of water pressure with the water in motion (also known as working pressure).