
An atmometer or evaporimeter is a scientific instrument used for measuring the rate of water evaporation from a wet surface to the atmosphere. Atmometers are mainly used by farmers and growers to measure
evapotranspiration
Evapotranspiration (ET) refers to the combined processes which move water from the Earth's surface (open water and ice surfaces, bare soil and vegetation) into the Atmosphere of Earth, atmosphere. It covers both water evaporation (movement of w ...
(ET) rates of crops at any field location.
Evapotranspiration is a measure of all of the water that
evaporates from land surfaces plus the water that
transpires from plant surfaces. Based on the amount of water that does evaporate and transpire, the user can water crops correspondingly, which results in less water use and possibly increased crop yields. Companies that currently sell atmometers include C&M Meteorological Supply and Calsense.
Design
An atmometer consists of a porous, ceramic plate connected to a water reservoir by a glass or plastic tube. The device stands around tall with a diameter of . Water is drawn from the water reservoir through the tube to wet the plate. As the water on the plate evaporates, more water is drawn from the reservoir to re-wet the plate. A canvas cover made of
Gore-Tex is placed over the plate to prevent anything from getting in. The canvas cover is important because it simulates the amount of
solar radiation
Sunlight is the portion of the electromagnetic radiation which is emitted by the Sun (i.e. solar radiation) and received by the Earth, in particular the visible light perceptible to the human eye as well as invisible infrared (typically p ...
a plant absorbs under certain weather conditions and controls the rate of evaporation. Different types of canvas covers simulate different amounts of evapotranspiration rates that various plant surfaces would undergo. For example,
alfalfa
Alfalfa () (''Medicago sativa''), also called lucerne, is a perennial plant, perennial flowering plant in the legume family Fabaceae. It is cultivated as an important forage crop in many countries around the world. It is used for grazing, hay, ...
ET rates are estimated using the No. 54 green canvas cover, while grass ET rates are estimated using the No. 30 green canvas cover. A
membrane
A membrane is a selective barrier; it allows some things to pass through but stops others. Such things may be molecules, ions, or other small particles. Membranes can be generally classified into synthetic membranes and biological membranes. Bi ...
between the plate and canvas cover prevents rain water from wetting the gauge yet allows water vapor to escape. A gauge that runs up the side of the atmometer measures the level of water (in inches) in the reservoir, which shows how much water has evaporated.
An electronic model of the atmometer is also available, which includes a
data logger
A data logger (also datalogger or data recorder) is an electronic device that records data over time or about location either with a built-in instrument or sensor or via external instruments and sensors. Increasingly, but not entirely, they a ...
attached to it. A data logger automatically records the level of water every time a change due to evaporation occurs. The data that the data logger records can be downloaded onto a computer to record the results. The electronic model eliminates possible human error that could occur from reading the gauge, but costs approximately $900 while the manual model costs approximately $300.
Use and maintenance
An atmometer is fairly easy to install and use. It is usually mounted on a wooden post about s above the ground in an area representative of the weather and field conditions. The plate of the atmometer should be placed in direct sunlight so the evaporation rates are not affected. It should not be placed near tall trees or buildings, as they can affect the amount of exposure that the atmometer has to environmental factors, which affect evapotranspiration rates. To measure the amount of water that has evaporated, calculate the change in water level on the gauge by subtracting the final water level from the initial water level.
Advantages (compared to weather stations)
In a 2003 study conducted on the
Central Coast of California, the performance of atmometers was compared with that of the more expensive
weather station
A weather station is a facility, either on land or sea, with instruments and equipment for measuring atmosphere of Earth, atmospheric conditions to provide information for weather forecasting, weather forecasts and to study the weather and clima ...
s. Atmometers were placed adjacent to seven weather stations around the area and weekly values were recorded for each method. The results indicated that the atmometers and weather stations yielded very similar results, both producing comparable evapotranspiration (ET) readings. Under conditions of lower evapotranspiration, the atmometers produced slightly lower ET values than those of the weather stations.
Studies conducted by
Colorado State University
Colorado State University (Colorado State or CSU) is a Public university, public Land-grant university, land-grant research university in Fort Collins, Colorado, United States. It is the flagship university of the Colorado State University Syst ...
and the
United States Department of Agriculture
The United States Department of Agriculture (USDA) is an executive department of the United States federal government that aims to meet the needs of commercial farming and livestock food production, promotes agricultural trade and producti ...
in
Fort Collins produced similar results. The atmometers produced evapotranspiration values that were very close to the data calculated from weather stations. These studies show that atmometers are especially useful for areas that do not have access to nearby weather stations and/or evapotranspiration data.
* Low cost
* Easy operation
* Convenience
* No computer or power required
Disadvantages (compared to weather stations)
* Potential weather damage can occur to the device
* Constant need to refill water supply
* Gauge must be read manually (only on manual model)
See also
*
Pan evaporation
Pan evaporation is a measurement that combines or integrates the effects of several climate elements: temperature, humidity, rain fall, drought dispersion, solar radiation, and wind. Evaporation is greatest on hot, windy, dry, sunny days; and is g ...
References
External links
C&M Meteorological SupplyCalsense Atmometer
{{Meteorological equipment
Agronomy
Water and the environment