Crop water use, known as evapotranspiration, is the water used by a crop for growth and cooling. This water is extracted from the soil root zone by the root system, which represents transpiration and is then no longer available as stored water in the soil. As the water content of the soil decreases, the soil water becomes more difficult to extract. 99.9% of the water used by an irrigated crop is drawn through the roots and transpires through the leaves.
What is Evapotranspiration?
The evapotranspiration process is composed of two separate processes: transpiration (T) and evaporation (E). Often the term “ET” is used interchangeably with crop water use.
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.
Transpiration and direct evaporation are both affected by
- solar radiation
- air temperature
- air humidity
- wind speed
The soil water content and the ability of the soil to conduct water to the roots influences the transpiration rate, as do waterlogging and soil water salinity. The transpiration rate is also influenced by crop characteristics, environmental aspects and cultivation practices. Different kinds of plants may have different transpiration rates. Not only the type of crop, but also the crop development, environment and management should be considered when assessing transpiration.
The amount of water required to compensate for evapotranspiration loss from the cropped field is defined as crop water requirement. Although the values for crop evapotranspiration and crop water requirement are identical, crop water requirement refers to the amount of water that needs to be supplied, while crop evapotranspiration refers to the amount of water that is lost through evapotranspiration. The irrigation water requirement basically represents the difference between the crop water requirement and effective precipitation. The irrigation water requirement also includes additional water for leaching of salts and to compensate for non-uniformity of water application.
Maximizing Irrigation Efficiency through ET Management
Irrigation efficiency is ET divided by the amount of irrigation water applied. High irrigation efficiency means minimum waste.
Where irrigation water goes:
- stored in the root zone (to supply ET)
- lost by surface runoff
- lost by deep percolation
- lost by spray evaporation from sprinklers
Maximum irrigation efficiency for surface irrigation systems using furrows or borders occurs when ET empties the root zone to a lower limit called the allowable depletion, beyond which crop yield will fall.
Irrigation systems with greater control over water application (center pivots, sprinklers or
drip systems) can irrigate at maximum irrigation efficiency for root zones that are not emptied to the lower limit. When irrigating, the goal is to fill the root zone with the amount of ET since the last irrigation.
Minimizing waste saves water, fertilizer, energy, and labor.
Crop Water Needs at Critical Growth Stages
As most farmers know, crops will show visible symptoms of water stress, and signs such as darkening, curling or wilting are sure signs that irrigation is needed. However, studies have found that by the time a plant is exhibiting visual signs stress, especially at critical growth stages, this can lead to yield reduction.
Irrigation scheduling alone doesn’t solve the problem. In addition to the different weather factors mentioned above, different crop have different water requirements and respond differently to water stress. In addition, crop sensitivity to water stress varies from one growth stage to another.
Studies have shown that most crops have one or two critical growth stages where having the right amount of water available for consumption will have a positive yield impact. For example:
Alfalfa – early spring and immediately after cuttings
Corn – tasseling and silk stage
Sugar beets – post-thinning
Potatoes – tuber formation to harvest
Soybeans – pod development and seed fill
Stone fruit – last two weeks prior to harvest
Identifying these critical growth phases and specific water needs may be achieved with intelligent-crop specific sensors. The AquaSpy system of sensors combined with an intelligent algorithm accurately determines relative soil moisture distinct from electrical conductivity (EC), which allows for the calculation of field capacity at every depth of the AquaSpy sensors which are spaced 4 inches apart.
AquaSpy Insights Reports
AquaSpy has spent over a decade in the field learning about crop-specific needs. Easy-to-read reports show the “fill” from rain or irrigation in addition to the crop “drinking” or consuming the available water. This lets farmers and growers precisely manage irrigation throughout critical growth phases in the season. Each report shows the level of the active root zone for precise application management.
Contact your local agronomy dealer or an AquaSpy regional sales representative to find out how to better manage evapotranspiration and optimize your crop yields through improved irrigation timing and application.