Irrigation scheduling is commonly based on estimating crop evapotranspiration from atmospheric measurements (temperature, solar radiation, humidity, and wind speed) or from soil moisture monitoring.
While one or a combination of the above two methods can be used to successfully determine when to irrigate and how much water to apply, plant-based monitoring could be used to schedule irrigation.
Infrared thermometers can be used to measure canopy temperatures for irrigation scheduling in arid regions. This method is based on the fact that canopy temperatures increase due to stomatal closure that is associated with water stress. When plants become stressed due to a decrease in soil-water availability, the cooling associated with transpiration decreases and plant temperature increases.
Infrared sensors could be used to measure canopy temperature without physically contacting the plant. Measurements could be taken from a distance and a few measurements could be used to measure the average temperature of a large area of the field.
This practice is effective in arid or semi-arid areas and may not be practical in humid regions.
A crop water stress index (CWSI) can be developed and used to schedule irrigation. The index is based on the difference between canopy temperature and air temperature normalized for the vapor pressure deficit of the air. The index could be used to determine when to irrigate based on the stress level of the plant.
Another method must be used to determine the amount of water needed for the irrigation event. This method could be used along with other irrigation scheduling methods to conserve water and increase water use efficiency.