EVAPOTRANSPIRATION
• wind — When there is no breeze, the air surrounding a leaf becomes increasingly humid, which reduces the rate of transpiration. When a breeze is present, the humid air is carried away and replaced by drier air, thereby increasing the water potential gradient between the leaf and the atmosphere.
• air temperature — Plants transpire more rapidly at higher temperatures because water evaporates more rapidly as the temperature rises. For example, at 86 degrees Fahrenheit a leaf may transpire three times as fast as it does at 68 degrees Fahrenheit.
ET is
important to a grower because it represents the amount of irrigation water that must be supplied to ensure that there is sufficient water to meet the needs of the plant.
• humidity — Humidity is a measure of the water vapor content of the air. Lower humidity (drier air) provides a greater differential between the moist leaf surface and the air, which increases the rate of water loss from the plant. Humidity is typically quantified in terms of relative humidity (relative to completely saturated air).
• soil water — A plant cannot continue to transpire rapidly if its water loss is not made up by replacement from the soil. When absorption of water by the roots fails to keep up with the rate of transpiration, loss of turgor occurs, and the stomata close. This immediately reduces the rate of transpiration (as well as of photosynthesis). If the loss of turgor extends to the rest of the leaf and stem, the plant wilts.
When a crop becomes stressed because of the lack of moisture at critical times in the growth cycle, it can have a negative impact on the potential yield or quality of the yield.
Stilling well for measuring evaporation
An atmometer is used to measure ET and is easier to use and maintain than an evaporative pan.
Photo credit: University of Nebraska MEASURING ET
Over the years, there have been many attempts to measure and quantify evapotranspiration rates. One that was used for many years was the use of an evaporative pan. The rate of evaporation out of the pan would be correlated to the rate of plant water use. The challenge is taking readings each day and making sure the pan was clean and not growing scum or algae, which would interfere with water evaporating.
Another instrument used to measure ET is an atmometer, which is easier to use and maintain than an evaporative pan. It is essentially a plastic pipe with a sight tube attached to measure the amount of water that is evaporated or “wicked” through the ceramic disc on top. The disc is covered with a fabric that controls the rate of water that is evaporated.
WEATHER STATIONS
Evaporative pans have been used to measure and quantify evapotranspiration rates.
20 Irrigation TODAY | Winter 2020
Currently, the most common method for estimating ET is with weather stations that are measuring sunlight (solar radiation), wind, temperature and
irrigationtoday.org
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