Crop water use, also known as evapotranspiration (ET), is the water used
by a crop for growth and cooling purposes. This water is extracted from the soil root zone by the root system, which represents transpiration and is no longer available as stored water in the soil. Consequently, the term "ET" is used interchangeably with crop water use. All these terms refer to the same process, ET, is which the plant extracts water from the soil for tissue building and cooling purposes, as well as soil evaporation.

The evapotranspiration process is composed of two separate processes: transpiration (T) and evaporation (E). Transpiration is the water transpired or "lost" to the atmosphere from small openings on the leaf surfaces, called stomata.
Evaporation is the water evaporated or "lost" from the wet soil and plant surface. Significant evaporation can take place only when the soil's top layer (1 to 2 inches) or plant canopy is wet. Once the soil surface is dried out, evaporation decreases sharply. Thus significant evaporation occurs after rain or irrigation. Furthermore, as the growing season progresses and canopy cover increases, evaporation from the wet soil surface gradually decreases. When the crop reaches full cover, approximately 95 percent of the ET is due to transpiration and evaporation from the crop canopy where most of the solar radiation is intercepted.Prevailing weather conditions, available water in the soil, crop species and growth stage influence crop water use (ET). At full cover, a crop will have the maximum ET rate if soil water is not limited; namely, if the soil root zone is at field capacity. Full cover is a growth stage at which most of the soil is shaded by the crop canopy. In a more technical term, the crop is at full cover when the leaf area is three times the soil surface area under the canopy. At this growth stage, the crop canopy intercepts most of the incoming solar radiation, thereby reducing the amount of energy reaching the soil surface.
Different crops reach full cover at different growth stages and times after planting
In order to standardise ET measurements and calculations, a reference crop ET (ET ratio) is used to estimate actual ET for other crops. In humid and semi humid areas where water usually is not a limiting factor, grass is used as a reference ET crop. In arid or semi-arid areas, alfalfa is more suitable as a reference ET crop because it has a deep root system, which reduces its susceptibility to water stress resulting from dry weather.

The actual soil water content also influences crop water use. As soil dries, it becomes more difficult for a plant to extract water from the soil. At field capacity (maximum water content), plants use water at the maximum rate. When the soil water content drops below field capacity, plants use less water.

Managing Irrigation According to Growth Stage
Crops are different in their response to water stress at a given growth stage. Crops summarized according to their sensitivity to water stress at various growth stages reveal the importance of these stages in making the irrigation decision.
Crops that are in the sensitive stage of growth should be irrigated at a lower soil water depletion level than those that can withstand water stress. If a crop is in the last irrigation rotation and is at a sensitive stage of growth, the recommended strategy may be to apply partial or lighter irrigations in order to reach the end of the field before the sensitive crop is subjected to water stress.

Crop appearance is considered one of many field indicators that can be used in irrigation scheduling. A crop suffering from water stress tends to have a darker color and exhibits curling or wilting. This is a physiological defense mechanism of the crop that is evident on hot, windy afternoons when the crop cannot transpire fast enough, even if the water is readily available in the soil. If the crop does not recover from these symptoms overnight, the crop is suffering from water stress. Any changes in crop appearance due to water stress may mean a reduction in yield. However, using this indicator alone for irrigation scheduling is not recommended if a maximum yield is desired. This indicator is inferior for modern agriculture due to the inability to determine the actual crop water use. However, ignoring it at the critical growth stages may lead to yield reduction. Using the growth stage as a field indicator in irrigation scheduling should be coupled with more sensitive and accurate methods of determining the crop water use, such as soil moisture measurements and ET data. The main advantage of this indicator is to provide direct and visual feedback from the crop. Different crops have different water requirements and respond differently to water stress. Crop sensitivity to water stress varies from one growth stage to another. A good irrigation-scheduling scheme should consider sensitivity of the crop to water stress at different growth stages.

Surface Irrigation

This is accomplished by impounding water on the soil surface or causing water to flow over the surface. Water is turned in to the field at the high point of the land and it flows to lower areas. The important methods of surface irrigation are:
· Border strip method (Graded border method)
· Check basin method (Check method or level border method)
· Basin or ring method
· Furrow method
· Corrugation method
· Contour ditch method

In this method water is applied beneath the ground surface. Though this method has the advantage of reducing evaporation, it is often applicable only under special situation.

In the sprinkler method of irrigation water is applied above the ground surface as a spray, developed by the flow of water under pressure through a nozzle.

Trickle or drip method
Water is applied through small tubes in drops continuously which can be regulated based on the crop requirement.