Evaporation, Transpiration, and Evapotranspiration

Spread the love
  • 6

Last Updated on April 8, 2020 by Naeem Javid Muhammad Hassani

Evaporation, Transpiration, and Evapotranspiration


It is the loss of free water from soil, lakes, stream oceans etc to the atmosphere.


The transfer of water in the vapor phase from the surface of the soil, open water surfaces, snow and the surface of the vegetation to the atmosphere.


The loss of water from the cuticle or the stomata opening in the leaves of plants, into the atmosphere is called transpiration.

Evaporation and Transpiration 1 - Forestrypedia.jpg


(consumptive use of water by vegetation)

The combined process of loss of water through evaporation and transpiration is called Evapotranspiration. Evapotranspiration is also called consumptive use or total evaporation from land areas.

It has two types:

Potential Evapotranspiration:-

            It represents the maximum rate of evaporation possible from the land area when the available supply of moisture is equal to or greater than the demand.

Actual Evapotranspiration:-

            It there is the deficiency of water in or on the land areas and the evapotranspiration takes please will be called actual Evapotranspiration.

Moisture Deficiency:-

The difference between potential evapotranspiration and actual evapotranspiration is known as moisture deficiency.


  1. Solar Radiation:-

For evaporation of 1cc water about 600 cals, of heat is required and the only source of heat is radiation. Free evaporation in nature is continuous as long as the heat of vaporization (600 calories per gram of water) is supplied Therefore evaporation not possible without solar radiation.

  1. Air Temperature:-

            Hot air expands, hence its vapor concentration decrease and with this, its vapor pressure decreases. A pressure gradient builds-up and evaporation proceeds. Therefore evaporation is directly proportional to air and temperature. I-e, increase in air temp-will increase the process of evaporation.

  1. Relative humidity of the air:-

Relative humidity means moisture percentage in the air e.g. – RH 60% means that the air can hold 100gm of water. Its relative humidity decrease, it increases the vapor pressure gradient and hence the rate of evaporation increases.

  1. Wind velocity: –

Wind velocity has considerable influence on evaporation. Wind causes continual disturbance and exchange of moist air above the evaporating surface with that of drier air. It can be shown that an increase of wind velocity by 10% results in an increase of evaporation rate by 1 to 3%.

  1. Atmospheric Pressure:-

            Vapor concentration in the air decreases with decreasing atmospheric pressure. While net evaporation, by consequence increases as condensation from atmosphere decreases.

  1. Properties of water:-

            Evaporation is also influenced by the properties of the water itself, namely its turbidity and salt concentration.

Evaporation from oceans is 2 to 3% lower under the same conditions than that of fresh water. So properties of water also affect the evaporation rate.

  1. Evaporation from soil:-

Evaporation potential of the soil is influenced by moisture content in the soil presence of groundwater, and depth of that water. To some extent, it is influenced by soil color. Dark soils absorb more radiation and evaporate more water.

  1. Aspect:-

            Exposure of the mountainside towards the sun is called aspect. The southern aspect which is hotter evaporates more water the northern aspect which is the cooler one.

Evaporation from snow surface: – (Ablation)

            Evaporation from snowpack or ice is termed as ablation. As in free evaporation, there must be a vapor pressure gradient from the snow surface to the adjacent atmosphere, Ablation occurs only when the dew point of the air is lower than 0o C.


Conversion of water from liquid to vapor is an important step in the never-ending hydrologic cycle. This process, called evaporation, occurs almost continuously from all water surfaces and moist soil. It is even done by plants, which absorb water from the soil and, in a process known as transpiration, give off water vapor through their leaves. Hydrologists usually measure evaporation by indirect means. They maintain records of evaporation from special pans of a specified size that are exposed to the elements. The data so obtained must be adjusted for each individual case to estimate the evaporation from a particular surface.



  • Transpiration is defined as the loss of water vapor from plants.
  • Transpiration is a process similar to evaporation. It is the loss of water from parts of plants, especially leaves but also stems, flowers and roots.
  • Water vapor can be lost from lenticels (pores on plant stems) in the bark, and through the cuticle of the leaf, but most vapor passes through leaf stomata.
  • Leaf surfaces are dotted with openings called stomata, and in most plants they are more numerous on the undersides of the foliage. The stomata are bordered by guard cells that open and close the pore. Collectively the structures are called stomata.
  • Leaf transpiration occurs through stomata, and can be thought of as a necessary “cost” associated with the opening of the stomata to allow the diffusion of carbon dioxide gas from the air for photosynthesis. Transpiration also cools plants and enables mass flow of mineral nutrients and water from roots to shoots.
  • Stomatal transpiration accounts for most of the water loss by a plant, but some direct evaporation also takes place through the surfaces of the epidermal cells of the leaves.
  • The amount of water lost by a plant depends on its size, along with the surrounding light intensity, temperature, humidity, and wind speed (all of which influence evaporative demand). Soil water supply and soil temperature can influence stomatal opening, and thus transpiration rate.
  • A fully grown tree may lose several hundred gallons (a few cubic meters) of water through its leaves on a hot, dry day. About 90% of the water that enters a plant’s roots is used for this process. The transpiration ratio is the ratio of the mass of water transpired to the mass of dry matter produced; the transpiration ratio of crops tends to fall between 200 and 1000 (i.e., crop plants transpire 200 to 1000 kg of water for every kg of dry matter produced).
  • Through transpiration, an average-sized maple tree can lose more than 200 L (200 kg) of water per hour on a summer day.
  • Transpiration rate of plants can be measured by a number of techniques, including potometers, lysimeters, porometers, and heat balance sap flow gauges.


  1. Temperature: It has been estimated that transpiration occurs nearly twice as fast at 30 degrees than at 20 degrees.
  2. Stomatal Opening: The rate of transpiration is directly related to the degree of stomatal opening, and to the evaporative demand of the atmosphere surrounding the leaf. The number, size, position, and degree of opening control most transpiration.
  3. Humidity: The presence of humidity decreases the rate of transpiration.
  4. Wind Velocities: Greater is the blowing of wind; greater will be the rate of transpiration. Wind velocities of 1 – 5 km/ hr have the greatest effect on transpiration because at these relatively low velocities the humid zone above the leaf is removed, making the boundary layer thinner, and steepening the diffusion gradient. However, the relationship between wind and transpiration in sunflowers may not occur in other species because wind may cause stomatal closure.
  5. Leaf anatomy, size and shape: Leaf size and shape has also affects. Large leaves retain a thicker boundary layer than small leaves thus having more transpiration. Leaves may also change shape to maintain a boundary layer, such as the curling or rolling of grass blades during drought. All such result in lower transpiration.
  • Similarly, orientation also determines energy absorbed by the leaf, and subsequently leaf temperature, the availability of energy available for vaporization, thus affecting the rate of transpiration.
  • For example, the leaves of some desert species are orientated vertically, minimizing absorption of solar radiation, and thus solar heating of the leaf. Many desert species have compound leaves dissected into small leaflets (e.g. Acacia species). Small leaves or leaflets can dissipate heat by convection better than large leaves, thus maintaining the leaf at ambient temperature instead of above air temperature from solar absorption.
  1. Root- shoot ratio: Root/Shoot ratios can also affect transpiration. Decreased root growth increases resistance of water absorption which results in partial stomatal closure.


            Taking the idea from the above factors which affect evaporation, we can easily explain the factors which affect transpiration.

  • Solar radiation;-Transpiration increase at a certain limit but at more radiation stomata closed.
  • Wind velocity: – Same as above.
  • Water deficit: – water deficit in roots in leaves or in soil decrease the rate of transpiration.
  • Leaves and root surface area: – More surface area, more will be the transpiration.
  • Distribution & No of stomata: – Greater the no. of stomata more will be the transpiration.
  • Water properties: – same as above.
  • Air temperature effects: -Same as above.
  • Day and night effect; –
  • Relative humidity of the air: – same effect as on evaporation.
  • Aspect
  • Evergreen and deciduous trees (the type of vegetation)
  • Number, size, shape, and duration of leaves. Etc.

Factors affection Evapotranspiration or consumptive use of water by vegetation:-

All the above factors affecting evaporation and transpiration are included in this.



Transpiration may be measured by weight loss, including monitoring the weight loss of a potted plant or in the more complex lysimeters.


The volume of water transpired can be measured from cut shoots, or whole plants in a container in which the input and output of liquid water can be measured.

Water Vapor:

Plants can be enclosed in a chamber, and the water vapor content of air going into the chamber and leaving it measured.  The increased water vapor in the exhaust is from transpiration. Methods of measuring water vapor range from absorbing water vapor in a desiccant, e.g. CaCl2, to using electronic humidity sensors.

Determination of measuring leaf resistance:

If a cup containing a humidity sensor is clamped to a leaf, the increase in water vapor content of the cup over time can be determined, and this value converted to leaf in sec   cm-2. Assuming the water on the surface of leaf cells is at saturation and C or e of the air is measured, transpiration can be computed from the above equation.  If leaf area is measured the result can be expressed in mg H2O cm-2/ hr.


A mercury barometer is an accurate and relatively simple way to measure changes in atmospheric pressure. At sea level, the weight of the atmosphere forces mercury 760 mm (29.9 in) up a calibrated glass tube. Higher elevations yield lower readings because the atmosphere is less dense there, and the thinner air exerts less pressure on the mercury.

Measurement of Atmospheric Pressure - Barometer - Forestrypedia


It is a physiological function in which food is prepared.


It is a food formation process.

6CO2 + 6H2O + light à C6H12O6 + O2 + E

It is a light dependant process.

C02 is consumed and O2 is released.

It is also called ‘light reaction’.

It takes place only in day time.

It takes place in chloroplast.

It is constructive or anabolic process.

It takes place only in green cells.

By it plant gains weight.

The volume of CO2 consumed always equals to O2 produced.

It does not take place in the absence of CO2.

Here, CO2 is limiting factor.

It is a dependent reaction.

Photosynthesis uses the products of respiration.

It is a physiological function in which break-down of food takes place.


It is an energy formation process.

C6H12O6 + O2 à6CO2 + H2O + Energy

It is light independent process.

O2 is consumed and C02 is released.

It is also called ‘dark reaction’.

It takes place both in day and night.

It takes place in mitochondria.

It is destructive or catabolic process.

It takes place in all the cells.

By it plant losses weight.

The volume of O2 consumed may not be equal to CO2produced.

It may take place in the absence of O2 (anaerobic respiration).

Here O2 is not limiting factor.

It is a self-sustained reaction.

Respiration uses the products of photosynthesis.

For correction and improvements please use the comments section below.

Spread the love
  • 6

Naeem Javid Muhammad Hassani

NJMH is working as Deputy Conservator of Forests in Balochistan Forest & Wildlife Department (BFWD). He is the CEO of Tech Urdu (techurdu.net) Forestrypedia (forestrypedia.com), Majestic Pakistan (majesticpakistan.pk), All Pak Notifications (allpaknotifications.com), Essayspedia, etc & their YouTube Channels). He is an Environmentalist, Blogger, YouTuber, Developer & Vlogger.

Leave a Reply

Your email address will not be published. Required fields are marked *