Evapotranspiration (ET) is the combined processes of evaporation of water from the soil and plant surfaces and transpiration of water through the plant tissues.
Any of the following with cause a change in the ET:
In other words, a change in any component of the Soil-Plant-Atmosphere Continuum will influence ET.
Water is lost from a crop as the plants pull the water from the soil into their roots, the water is pulled through the plants to their leaves, and the water is vaporized in the leaves and transpired out into the atmosphere. The movement of water from the soil, through the plant, and into the atmosphere, and eventually returned to the soil as rainfall, is called the Soil-Plant-Atmosphere Continuum (SPAC).
A change in any one part of the Soil-Plant-Atmosphere Continuum (SPAC) will also change the requirement of when and how much to irrigate a field. Other tools, like soil moisture sensors, can inform you about only one facet of the SPAC. Evapotranspiration integrates the SPAC – and accomplishes this over a broad area of a crop field.
If there is insufficient water available in the soil, then the ET will decrease as the plants physiologically regulate their water use. If the plants are hedged, then the ET will decrease because there is less leaf area intercepting light and losing water to the atmosphere. If there is a heat spell, the ET will increase because the atmospheric demand has increased and can more strongly pull water through the plants.
The Actual ET (in contrast to Reference ET) is the amount of water that is vaporized and transpired from your field and lost to the atmosphere. In other words, the Actual ET is the evapotranspiration from the field being measured.
Reference ET describes the atmospheric demand. The Reference ET is the ET from a healthy and actively-transpiring grass field with ample water in the soil. Because there is no limitation in water availability in the soil and because the field is completely covered with actively-transpiring plant tissues (i.e., the soil and plant aspects of the SPAC are fixed), then any increase or decrease in Reference ET is due to changes in the atmospheric demand.
Reference ET tells you how hard the atmosphere can pull water through a plant.
If the Reference ET is forecasted to increase (for example, a heat spell is forecasted), then the ability of the atmosphere to pull water through the crop will increase. A forecasted increase in the Reference ET means you should consider applying more water with your next irrigation. Conversely, a forecasted decrease in the atmospheric demand means you may be able to irrigate with less water or wait longer until the next irrigation.
The Plant Response Index (PRI) describes the ability of the plants to keep up with the atmospheric demand for water. It decreases if the plants have access to less water in the soil. A decrease means it is time to consider irrigating or applying more water with the next irrigation. Conversely, the Plant Response Index will increase as the canopies develop. An increase in the PRI means the plants have grown, have more transpiring tissues, and can use more water in response to the atmospheric demand.