Irrigation systems are generally rated with respect to application efficiency (Ea), which is the fraction of the water that has been applied by the irrigation system and that is available to the plant for use. Applied water that is not available may have been lost from the crop root zone through evaporation or wind drifts of spray droplets, leaks in the pipe system, surface runoff, sub-surface runoff or deep percolation.
Irrigation requirements are determined by dividing the desired amount of water to provide to the plant by Ea as a decimal fraction. For example, if it is desired to apply 0.5 inches to the crop with a 75 percent efficient system, the system should apply 0.5/0.75=0.67 inches.
Irrigation systems are often used for delivery of chemicals such as fertilizers, soil fumigants or insecticides.
Fertigation should not begin until the system is pressurized. It is recommended to always end a fertigation/chemigation event with a short irrigation with clear water to avoid the accumulation of fertilizer or chemical deposits in the irrigation system and/or rinse crop foliage. The length of the flushing cycle should be 10 minutes longer than the travel time of the fertilizer from the irrigation point to the farthest point on the system.
Irrigation systems require periodic maintenance throughout the growing season. Drip irrigation systems may require high levels of maintenance to prevent clogging and system failure. Typically, cleaning agents are injected weekly, but in some instances more frequent injections are needed.
For some crops, irrigation is used for frost protection during the winter. For strawberries, sprinkler irrigation is primarily used with application rates of about .025 inches per hour during freeze events.
Overhead freeze protection is efficient for air temperatures as low as 26 to 28 F, but seldom below. For vegetable fields with subirrigation systems, the heat properties of groundwater can be used for cold protection.
Irrigation scheduling is used to timely apply the proper amount of water to a crop. The irrigation system, crop needs, soil properties and atmospheric conditions must all be considered.
Poor timing or insufficient water application can result in crop stress and reduced yields from inappropriate amounts of available water and/or nutrients. Excessive water applications may reduce yield and quality, are a waste of water and increase the risk of nutrient leaching.