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Water and crops: Evapotranspiration important process to consider in water management

by Wyoming Livestock Roundup

According to University of Wyoming Irrigation Specialist Vivek Sharma, increasing changing climatic conditions and climatic variability make water conservation a top priority for agricultural producers.

An important concept that Sharma works to educate Wyoming growers on is crop water use or evapotranspiration.

“Evapotranspiration the combined transfer of water from the land surface to the atmosphere in the form of water vapor either by evaporation from the soil or transpiration from plants,” he explains.


Many climatological factors influence evapotranspiration, explains Sharma.

“If we look at climatological factors, such as temperature, relative humidity, wind speed and solar radiation, all affect the amount of evapotranspiration happening,” he says.

At the same time, other factors including crop characteristics, such as crop type and crop growth stage, also impact the rate of evapotranspiration.

“For example, sugarbeets have a different evapotranspiration rate than dry beans, and dry beans have a different rate than alfalfa. The crop and what growth stage over the growing season governs how much water we use,” continues Sharma.

Management practices, such as irrigation methods, tillage management and soil conditions also influence evapotranspiration, he says.

“If we adopt no-till practices, there are a lot of residues available over the soil surface, which reduces evaporation and increases transpiration of evapotranspiration,” he comments.


Sharma explains that there are several different methods for measuring evapotranspiration, ranging from soil water balance to the plant monitoring method.

“Three common methods to measure crop evapotranspiration include soil water balance approach, lysimetry and the use of flux towers, such as eddy covariance systems or Bowen Ratio-Energy Balance,” says Sharma.

Another method is computing evapotranspiration from weather data. A large number of models have been developed for assessing crop evapotranspiration using climate data.

“The soil-water balance approach which is also called checkbook method, takes into account water inputs, such as rainfall or irrigation, and water losses from the field by runoff, deep percolation and evapotranspiration to quantify the change in soil water storage,” he comments.

“By balancing all of those components, we can also calculate evapotranspiration and schedule irrigation events,” continues Sharma. “Improved techniques are needed to accurately quantify evapotranspiration to enhance the efficient use of agricultural water resources.”

At the start of this growing season, Sharma will begin utilizing a Bowen Ratio-Energy Balance System at University of Wyoming Powell Research and Extension Center, which is extremely precise method, to quantify short- and long-term crop evaporation rates.

Sharma explains, “We will apply this knowledge and experience to improve our understanding on growing season and non-growing season evaporative losses in Wyoming.”


Evapotranspiration has a large impact on water resources both at the regional scale and at the farm operation level.

Sharma notes that evapotranspiration accounts for the largest flux in the hydrological cycle, and the collection of long-term data is important for evaluating land-use management and estimating vegetation response to environmental changes.

At the farm level, Sharma comments, “Reliable estimates of evapotranspiration are vital to develop criteria for in-season water management, particularly, in the context of irrigated agriculture, such as scheduling of irrigation, predicting the status of the soil water supplies, long-term estimates of water supply and design and management of water management infrastructures.”

He continues, “Evapotranspiration data will tell us how much irrigation we need to apply and at what time we need to apply it.”

Future use

According to Sharma, the use of evapotranspiration data will be increasingly important in coming years.

As such, he states, “One of my objectives in the state of Wyoming is to familiarize users with crop water use or evapotranspiration and its the importance in managing agricultural water resources.”

Sharma notes that evapotranspiration is already a common discussion topic in midwestern states where groundwater is used.

Sharma continues, “In our case, if we have either excess water or lack of water, in both cases, evapotranspiration plays an important role in managing the state’s water resources.”

Emilee Gibb is editor of Wyoming Livestock Roundup and can be reached at

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