What is evapotranspiration?

#ClimateDictionary #EcoEducation #Evapotranspiration

You may be out on a walk when your gaze falls on a tremendous plot of weeds. What a waste, you may think, as your thoughts turn to weed whackers or herds of goats.

What you are probably not thinking is that these plants are conducting two of the most important reactions on earth. One is photosynthesis, the ability of plants to make sugars from sunlight and carbon dioxide. Life on earth would be very sparse if the plants did not carry out this reaction.

The other momentous action of plants is evapotranspiration, also known as ET. While this process is less well known than photosynthesis, it is critical for life on earth as we know it. Water that enters the atmosphere from this process is critical – particularly during droughts – and helps to protect against climate change.

1) Transpiration

Evapotranspiration involves two things that release water into the air. One is the transpiration of water that enters plants through their roots, takes part in numerous metabolic and physiological functions, and then is released as vapor through tiny pores in the leaves called stomata.

Plants often go through great lengths to prevent this loss of water from their stomata. They frequently close at night or when the plants are in the shade. Plants that grow in arid regions have modified stomata to minimize their loss of water.

2) Evaporation

You are much more likely to be aware of evaporation. Water evaporates from bodies of water, which many scientists consider to be a part of ET. It also evaporates from the soil around the plant’s roots, which is a critical part of any definition of ET.

Many scientists now have a more encompassing view of ET that includes water lost from oceans, seas, wetlands, rivers, lakes, and streams through evaporation. These bodies of water provide nearly 90% of the water that enters the atmosphere.

Evaporation and transpiration occur simultaneously, and there is no simple way to distinguish between these two properties.

Water cycle of the earth’s water

The water cycle is an indicator of where, when, and how water moves. This is critical knowledge since water is necessary for life on earth. An overview of the water cycle of the earth’s water helps to understand the movement of water due to the process of evapotranspiration. Water falls over the earth as rain, ice, or snow and can take many paths from there:

• Evaporate and return to the atmosphere
• Seep into the ground as groundwater or soil moisture
• Run into streams or rivers
• Evaporate from bodies of water
• Be transpired from vegetation
• Sublime from ice and snow and condense

A histogram from the US Geological Survey shows where the earth’s water is dispersed. Note that only 4.4% of the earth’s surface water and other freshwater is found in living things and in the soil.

Use of evapotranspiration in determining the water cycle

Recent research has shown that the water cycle in the US has been moving more intensively and quickly over the past 70 years. The researchers utilized a new metric for their study – the sum of evapotranspiration and precipitation per year.

They also studied how much moisture was stored in the soil and found that the amount was decreasing in the northwest, southeast, and upper midwestern US. However, the amount of ET increased in the southeast over the same period.

While this trend poses the grave threat of drought to the areas impacted, too much storage of moisture in the soil or rain could lead to increased flooding in Texas or the northeastern part of the US.

The constant monitoring of ET and precipitation should help provide guidance for future water needs.

Volumes of water involved in evapotranspiration

If you are intensely curious, you can see the amount of water that transpires by putting a clear plastic bag over a plant. As time passes, the bag will become thoroughly fogged over from the water that is eliminated. And that is just one plant!

On a much larger scale, an acre of corn can transpire 11,400 to 15,100 litres (3,000 to 5,000 gallons) in one day. Areas that are more intensively covered with forests tend to release a lot more water into the air through evapotranspiration than pastures or grasslands.

The U.S. Geological Survey reported that the lower five miles of the atmosphere in the continental US transports an average of 40,000 billion gallons of water vapor a day. The fate of this water varies. Meteorological conditions can affect the calculation of these numbers, which come from a series of different studies. Thus, these numbers do not add up to 100%.

• More than 10% is precipitated as rain, snow, sleet, or hail
• About 65% returns to the atmosphere through evapotranspiration
• Nearly 30% enters the Pacific and Atlantic Oceans and into Canada and Mexico
• About 2% is lost to ground-water outflow
• Animals, plants, people, and industrial and commercial processes use 2%

Why Adopt An Irrigation System Based on ET?

Water managers, who plan and adjudicate the distribution of water, tend to be exceptionally knowledgeable about ET. Droughts do not stop during these processes. ET will deplete the water that remains in the soil, lakes, and streams. This could have grave consequences for farmers, ranchers, and homeowners among others.

Ironically, many plant diseases are more active when plants and trees are overwatered. Many people immediately water their plants when they show symptoms of stress, which could damage the plants even further. In addition to problems with plants, overwatering can damage masonry, retaining walls, and wooden fences.

An understanding of ET can greatly increase the chances of success of watering plants properly. This knowledge can provide accurate calculations of the amount of water needed for specific weather, enabling agriculturalists and horticulturalists to plan an irrigation system that reduces runoff. This enables the soil to absorb the water, which plants can then use for their growth.

What Factors Affect Evapotranspiration?

Climatological Factors

The amount of water on earth remains the same. However, this water is constantly in flux, continuously circulating between the ocean, atmosphere, and land. Without the movement between the skies and the continuous movement of ET, water would be much less invaluable to life on earth. This process of evapotranspiration and water that evaporates from the ground is responsible for 15% of the water that falls, and therefore, it provides the necessary water to plants and the atmosphere. Evaporation from the oceans provides the rest of the water that reaches land.

1) Solar radiation:

A key climatological factor that affects the amount of ET is solar radiation. Areas with more intense solar radiation have much higher levels of ET.

A prime example is the US. The Southwest has the greatest evaporation from lakes and the maximum solar radiation. In contrast, The Northeast and Northwest receive the lowest amounts of solar radiation in the continental US. Not surprisingly, they also have the lowest amount of evaporation from lakes.

2) Relative humidity (RH):

If the air around a plant is more saturated with water, i.e., has a higher relative humidity, the rate of its transpiration decreases. It is much easier for the water to evaporate into drier air than in air that is already saturated with water.

3) Temperature:

As the temperatures increase, so does the amount of transpiration. In hot weather, the plants open the little structures on their leaves called stomata. Because of this, the pores release more water when their surroundings get hotter.

4) Wind speed:

Greater amounts of wind will increase the transpiration rates of plants. Winds move the air around. This results in the replacement of the saturated air next to the leaf with drier air, so that more ET takes place.

Soil Characteristics

Since much of the land’s moisture remains in the ground, it is essential to consider the amount of this moisture when trying to calculate the amount of groundwater.

Much of the groundwater requires highly specific measures to determine its utility.

1) Texture:

An analysis of a desired planting by Wilcox (1967) shed light on evaluating the degree of soil texture in a type of agriculture. The soil can range from sandy to finely draining loamy water.

Highly significant correlations were identified between the requirements for annual highly significant irrigation, soil texture, and the net ET.

Another finding of this study was that the two factors of soil texture, along with the number of days of irrigation, comprised 91% to 94% of the variation in the annual applications of irrigation. Wilcox concluded that soil that had a coarser texture required greater amounts of the annual irrigation requirements.

2) Structure:

With the exception of roots that grow near lakes and oceans, most plant roots grow above the water table, the layer of water in the ground. Therefore, this soil is not constantly moist.

However, the soil can become wetter following precipitation, which infiltrates lower levels in the soil. Since the roots of plants are generally above the water table and cannot access this water, precipitation provides much of the water needed by plants.

The precipitation can add water to the soil that plants can use to draw down their water. This degree of transpiration is comparable to the pumping of water from a well. This practice leaves visible levels of depression in the soil where the plants have taken up the groundwater.

3) Density:

Crops that grow under standard conditions with optimal soil water and excellent environmental conditions tend to grow densely. However, this is not the case when the conditions for growth are less than ideal. Soil factors that can affect the density of a crop include pests and diseases, water shortage or waterlogging, low soil fertility, and soil salinity.

Plants in these conditions frequently conduct less evapotranspiration.

4) Chemical composition:

Modeling and monitoring vegetation processes and land surface are essential to assess the amount of water and CO2. These measurements help to assess ET and soil moisture content (SMC) in addition to other critical factors, such as large management systems, the detection of pollution and wildfires, food security research, and the modeling of desert locusts and carbon balances.

Variations in the SMC entail a strong impact on the dynamics of land surface energy, crop yields and regional runoff dynamics.

Plant Factors

1) Plantation type:

The types of plant can immensely affect the amount of ET that takes place. One extreme are plants that grow in arid regions. They modify their stomata to minimize ET.

On the other hand are forest denizens that have enormous root systems and transpire a substantial amount of water. The amount of water in play here can be affected by the following:

• Age
• Soils and subsoils
• Forest type
• Depth of the water-table

2) Root depth:

Root depth is a key factor that affects whether soil moisture is lost by evaporation or transpiration. Water is predominately lost by soil evaporation when a crop is small. However, as the plants grow, cover the soil surface, and develop enormous root systems, most of the water is lost by transpiration.

It is highly difficult for most species of plants to obtain their moisture from groundwater. The depth of the water in the ground changes throughout the season.

One plant that is responsible for an enormous loss of water is salt-cedar (Tamara chinensis), which has roots so long that they can frequently reach the ground water table. This enables salt-cedar to outcompete other types of plants.

3) Foliar density:

Transpiration and evaporation often occur at the same time. The density of the leaves that cover the plants can have a very strong effect on evapotranspiration, especially on small plants that primarily release water through evaporation.

4) Plant height:

The ET of short plants, such as pastures, release water from evaporation and transpiration.

The ET of trees behaves in a radically different manner. Short trees contribute water to the atmosphere by both evaporation and transpiration. However, as they grow larger, the trees develop a deep and elaborate network of roots, and their leaves overshadow much of the growth below.

Therefore, their release of water is primarily due to transpiration.

5) Growth status:

Over time, the crop canopy grows increasingly high and in the end, covers the canopy. As this happens, transpiration becomes the main way that plants use to transport water vapor to the environment.

How can you measure Evapotranspiration?

It should not come as a surprise that measuring the combination of transpiration and evaporation would be a complex matter. It is possible to directly determine the ET, but it is a difficult and expensive endeavor. There are also equations that can be used to estimate the ET.

Lysimetry

The most obvious and direct method is by transplanting plants into large containers called lysimeters that can be as large as several meters across.

Use of equations

Newer methods have taken care of some of the problems of calculating the literal evapotranspiration by using previous knowledge of climatic characteristics to determine the ET of the crop in question.

Several types of equations can be used. This article focuses on one of the most common ones that was developed by the UN Food and Agricultural Organization (FAO) and is known as the Penman-Monteith method.

Defining the parameters used to calculate ET

ETo = A climatic parameter that expresses the power of the atmosphere to evaporate. It factors in the solar radiation, vapor pressure, relative humidity, air temperature, and wind speed.

ETc = Crop evapotranspiration under standard conditions

E c adj = Crop evapotranspiration under non-standard conditions

Kc = the single crop coefficient - the difference in ET between a cropped and a reference (well-watered) crop surface. This parameter can vary depending on factors, such as the characteristics of the stomata, leaf anatomy, and aerodynamic properties. In addition, the Kc for a given crop changes from sowing to harvesting.

Calculating the ET for a crop

The first step is to identify the growth stages of a crop, determine their lengths, and select the appropriate Kc. There are charts for many crops, such as alfalfa. Adjust the selected coefficient for climatic conditions or the frequency of wetting. The final step is to calculate the ETc

Determine the ETo Atmospheric conditions

Determine the ETc Plants

Multiply ETo by Kc

Determine the E c adj Plants

Multiply ETo by Kc

Evapotranspiration in Forests

Since 30% of the surface of the earth is covered by trees, they play a significant role in the global process of evapotranspiration. Just one large oak tree can release 151,000 liters (40,000 gallons) of water a year.

Not surprisingly, a number of factors affect the amount of ET from forests. Conifers and deciduous forests differ in the amount of water they release. For example, during the period from May to October, deciduous forests transpire and evaporate 95% of the water that they release during a whole year. In contrast, the comparable proportion for conifers is 85% to 90%.

The amount of ET is significantly lower during the dry summer periods - 90% compared with that of deciduous trees when the trees receive steady rain. The comparable value for conifers was 75%. This reduction is thought to occur because the crowns intercept the water that is evaporated from the trees. This is such a major factor for the ET that there is a term for it - Ep (canopy resistance).

The maturity of the forests is another factor. Mature deciduous trees transpire 25% more than mature conifers. In contrast, conifers and deciduous trees that are young and middle-aged (40 to 60 years) transpire the same amount of water. Older forests that are 140 to 160 years old have ET levels that are 10% less than those of mature forests.

Geography is another critical factor. Studies from the USSR showed that northern taiga forests evaporated 10% less water than the surrounding wilds and grasslands. The leves of ET were the same between forests and the surrounding open land in the central part of the European USSR. In contrast, forests in the south evaporated about 5-10% more than the wilds and grasslands.

What is the role of evapotranspiration in the albedo effect?

The albedo effect is a critical way of measuring the effects of solar radiation that reflects away from an object compared with the radiation that gets absorbed. In short, it is a key part of determining the influence that an object will have on global warming.

For example, light-colored areas like sand or ice reflect much of their radiation back into space, helping to cool the planet. They are considered to have a high albedo. In contrast, Arctic sea waters with a low albedo are so dark that they absorb a lot of radiation, which further subjects the earth to increasing climate change.

The role of the albedo effect on forests can vary a great deal. For example, trees in areas with a lot of ice increase the amount of warming because they displace the ice, which strongly reflects heat away from it – increasing global warming.

However, in much of the world, the evapotranspiration from trees produces clouds, which help to cool the earth. This is one of the reasons why clear-cutting contributes to global warming. It greatly reduces the amount of ET that takes place.

In contrast, our forests help to mitigate global warming in several ways - not the least of which is their evaporation and transpiration.

References

• Earth Observatory. Water cycle is speeding up over much of the US. NASA. Accessed in October 2021.

• FAO. Chapter 1 – Introduction to evapotranspiration. Accessed in September 2021.

• Glenn, E.R., P.L. Nagler, K. Morino, and K. Hultine. Phreatophytes under stress transpiration and stomatal conductance of saltcedar (Tamarix spp.) in a high quality environment. Plant and Soil. Accessed in September 2021.

• Hydropoint. What is evapotranspiration? Accessed in September 2021.

• Law, B. “Albedo effect” in forests can cause added warming, bonus cooling. Oregon State University Newsroom. Accessed October 2021.

• Schlesinger, B. Nicholas School of the Environment. Cary Institute of Ecosystem Studies. The fate of rainfall. Accessed in October 2021.

• Shiklomanov, L.A. and O.I. Krestosky. Forests, climate, and hydrology: regional aspects. United Nations Digital Library. Accessed in September 2021.

• US Geological Survey. Evapotranspiration and the water cycle. Accessed in September 2021.

• US Geological Survey. The fundamentals of the water cycle. Accessed in October 2021.

• Verstraeten, W.W., F. Veroustraete, and J. Feyen. 2008. Assessment of evapotranspiration and soil moisture content across different scales of observation. Sensors. Accessed in September 2021.

• Wilcox, J.C. 1967. Effect of soil texture, net evapotranspiration and other factors on irrigation requirements of orchards as determined by a scheduling producer. Canadian Journal of Soil Science. Accessed in September 2021.

• World Bank. Earth’s water. Accessed October, 2021

• Ziemer, R.R. 1979. Reviews of Geophysics and Space Physics – Evaporation and Transpiration. Accessed in September 2021.