Irrigation Home Study Course Glossary

active root zone

that portion of the total root zone that contains the greatest root concentration and resulting water extraction.

advance cycles

Cycles that result in water passing over dry soil. The number of advance cycles is usually from two to eight depending on field conditions. The time for each successive advance cycle is longer than the previous one.

advance time

The on-time required to advance­ water from the upstream to the downstream end of the furrow.

available soil water

water that is retained in the soil and can be extracted by the plant. The available soil water is most important for crop production. It is the water held by the soil between field capacity and permanent wilting point.

bulk density

The weight of soil per unit volume. Symbol: Bd. The ratio of the weight of a volume of soil to the weight of the same volume of water.

change time

The on-time required to advance water to the portion of the field where wetting is desired­ during the first cycle time. This is one-half of the first advance cycle using the measured advance method.


as a soil separate, the mineral soil particles less than 0.02 millimeter in diameter. As a soil textural class, soil material that is 40 percent or more clay, less than 45 percent sand, and less than 40 percent silt.


One on-off sequence for one side of the valve. During this time, both sides of the valve receive­ water. One side is on-off, the other side is off-on.­


the amount of useful work accomplished by a system divided by the amount of work that must be input into the system.


the wearing away of the land surface by wind or running water. 


The combined loss of water from a given area, and during a specific time, by evaporation from the soil surface and by transpiration from plants. (1997. Soil Science Society of America. p. 36)

excess soil water or gravitational water

water that drains or readily percolates below the active root zone by the force of gravity. Since drainage takes time, part of the excess water may be used by plants before it moves out of the root zone.

field capacity

the water content of a soil at the upper limit of the available soil water range. It is the amount of water remaining in a soil after the soil has been saturated and allowed to drain for approximately 24 hours.

furrow irrigation

water is directed down sloping channels cut into the soil where it infiltrates into the soil both laterally and vertically; one method of surface irrigation.


the process by which water enters into the soil.

infiltration rate

the speed at which water can be taken into soil during an irrigation or rainfall event.


the speed at which water can be taken into soil during an irrigation or rainfall event.

irrigation efficiency

refers to the amount of water­ removed­ from the water source that is used by the crop.

irrigation scheduling

The process of applying the right amount of water at the right time

minimum allowable balance

the soil water content at which crops begin to experience water stress. Plants can use approximately 50 percent of the available soil water without experiencing water stress (a shortage of water). Normally, the minimum allowable balance is 50 percent of the available soil water. For example, if your soil is a uniform loam with available soil water of 2.0 inches/foot, and the crop’s active root zone is 3 feet, then the available soil water in the active root zone is 2.0 inches/foot times 3 feet or 6.0 inches. The minimum allowable balance in that three-foot active root zone would therefore be 6.0 inches times 50% or 3.0 inches.


The total amount of time that water is actually applied (flowing into) a given furrow.

particle density

the weight of a soil particle or soil separate divided by the volume of that soil particle or separate; measured in gm/cm3. Many Nebraska soils have particle densities of 2.6 to 2.7 g/cm3. Symbol: Pd.


The downward movement of water through the soil, made possible by pore space in the soil.

permanent wiltinng point

the lower limit of the available soil water range. When plants have removed all of the available water from a given soil, they wilt and will not recover. Figure 3.1 illustrates the concepts of field capacity and permanent wilting point.

pore space

spaces in soil filled with water or air; between the mineral and organic matter.


The portion of precipitation or irrigation on an area that does not infiltrate but, instead, is discharged from the area. That which is lost without entering the soil is called "surface runoff."


individual rock or mineral fragments in a soil that range in diameter from 0.05 to 2.0 millimeters. Most sand grains consist of quartz, but they may be of any mineral composition. The textural class name of any soil that contains 85 percent or more sand and not more than 10 percent clay.


The total number of furrows with gates open at any time. Note that this is gates open, not flowing. Thus the total is for both sides of the valve.


The total time that water runs between­ gate changes.


individual mineral particles in a soil that range in diameter from the upper limit of clay (0.002 millimeter) to the lower limit of very fine sand (0.05 millimeter). Soil of the silt textural class is 80 percent or more silt and less than 12 percent clay.

soak or cutback cycles

Cycles that result in water passing over dry soil. The number of advance cycles is usually from two to eight depending on field conditions. The time for each successive advance cycle is longer than the previous one.


weathered and fragmented outer layer of the earth’s terrestrial surface. A given volume of soil consists of mineral matter, organic matter, water and air.

soil aggregates

groupings of individual soil particles to give structure to soil; principal types of aggregates are platy, prismatic, columnar, blocky and granular.

soil bulk density

the weight of a given soil per unit volume of soil; including solids and pore spaces; reflects the soil’s porosity; indicative of how easily water will infiltrate an soil and how it will hold it. Bulk density of a soil increases with compaction. Symbol: Bd, units: gm/cm3.

soil horizons

soils are classified according to the number and properties of the soil layers (horizons) in the soil profile. Soil horizons differ in color, texture, structure, and organic matter content. A Horizon is the upper surface or topsoil and usually has the highest organic matter content; the B Horizon is the subsoil; and the C Horizon is the parent material. A given soil may have one or all three horizons. 

soil moisture blocks

used to measure soil water. Two gypsum blocks, each with two wire mesh rings, are strategically placed in the soil; wire leads from the rings go to the soil surface where the electrical resistance between the two wire mesh rings can be measured with a portable gauge. As soil water content increases, the gypsum absorbs more water and the electrical resistance between the wire mesh rings decreases.

soil separates

the three components of soil — sand, silt and clay; the proportion of each in a given soil determines its texture.

soil structure

the arrangement and organization of soil separates or individual soil particles into units called soil aggregates; includes three broad categories: single grained, massive, and aggregated.

soil texture

determined by the relative amounts of three groups of soil particles or soil separates: sand, silt and clay.

soil water

Water contained within or flowing through the soil profile. Surface water must infiltrate the soil profile to become soil water. Classified into three categories: excess soil water or gravitational water; available soil water, and unavailable soil water.

soil water balance

The status of the soil water content.

soil water content

The amount of water held in the soil.

textural triangle

triangular soil identification guide with each side representing one of the soil separates (clay, silt or sand) and indicating the percentage of each in the soil. The intersection points of three lines from each side of the triangle determine how the soil texture will be classified. 

uavailable water

soil water held so firmly to soil particles by adsorptive soil forces that it cannot be extracted by plants. Unavailable water is still present when soil is drier than permanent wilting point.