Glossary of Irrigation Terms

When irrigating a field, soil pores fill with water. When this water drains away, smaller pores remain filled with water, while larger (macro) pores end up filled with a mixture of water and air. This state is referred to as field capacity (Cc). Aeration capacity is simply a volume fraction of air-filled pores in soil at field capacity.

An increase in pressure in the downstream piping of an irrigation system that causes it to be greater than the supply pressure is known as back pressure. It can result in many and varied problems including a reversal in the normal direction of water flow.

When backflow (a reversal of flow) happens because of a reduction in system pressure and this in turn causes a negative or sub-atmospheric pressure to exist at any location in a system, back siphonage can occur.

Any water moving the wrong way through an irrigation system is referred to as backflow. Backflow can lead to contaminants in the water - or debris from the surface - infiltrating and damaging your irrigation system. Backflow prevention involves equipping a system with devices such as valves that only allow water to move in one direction.

Many factors influence the amount of water needed by plants. The Blaney-Criddle Method is a formula used to estimate water loss due to evapotranspiration in crops. At its simplest level, the calculation takes into consideration such factors as consumptive water use, mean monthly temperatures, and percentage of total annual daylight hours that were available over the period being measured. There are several other formulae employed to assess the amount of water plants need and it's an important aspect of the science of irrigation - whether in agriculture or for domestic and leisure purposes. Given the growing need to conserve water supplies and the increasing number of restrictions governing how much water we use for irrigation, knowing how much and when to water is a critical aspect of landscape management.

A centrifugal pump consists of a pump with an impeller (rotating vanes) and either an electric motor or engine. The rotational energy the motor creates is converted to the hydrodynamic energy of the water as it flows.

Sometimes called a uniformity coefficient, this is a measure of how evenly irrigation water is applied across a specific area of landscaping. To calculate, take the average depth to which irrigation water infiltrates the soil, subtract the average deviation from this depth, and then divide the result by the average depth to produce a percentage measurement.

A characteristic of a region with an arid climate is low rainfall and a high evaporation potential. As a general rule, a region is considered arid if it normally experiences an annual average rainfall of less than 10-inches.

A characteristic of a region with a humid climate is high rainfall and a low evaporation potential. As a general rule, a region is considered humid if it normally experiences an annual average rainfall of more than 40-inches.

As its name suggests, a region that's considered semiarid is neither completely humid nor arid. As a general rule, a region is considered semiarid if it normally experiences an annual average of between 10 and 20-inches rainfall.

A region that's considered subhumid is not the opposite of subarid! As a general rule, a region is subhumid when it experiences moderate rainfall - more than 20-inches but less than 40-inches annually - and a moderate to high evaporation potential.

Deep percolation occurs when excess irrigation water penetrates the soil and percolates below the root zone of any vegetation planted there. The deep percolation percentage is the ratio of the average depth of water below the root zone to the average depth of irrigation water overall, with a higher percentage generally reflecting greater water wastage.

Emission uniformity  tells you how evenly water is being discharged by all emitters in a specific irrigation or micro-irrigation system. To calculate emission uniformity, take the average discharge of the 25% of emitters with the lowest discharge rate, and divide this by the average volume discharged across all emitters in the system. Higher emission uniformity equates to a more efficient irrigation system.

The means by which nutrients are applied to the landscape via an irrigation system. For example irrigation water can be diverted to a tank of fertilizer where it dilutes the product and then rejoins the pipeline. 

Friction factor is used when calculating the pressure loss in an irrigation system caused by friction. This friction can be between the water and the inner surface of the pipes, and is influenced by factors such as the length and diameter of the pipes, as well as the material used and the inclusion of fittings such as tee or elbow joints