What is meant by evaporation?

Evaporation is a slow process that turns a liquid into its vapor phase. Evaporation is a common phenomenon of water, which gets converted into water vapor. Water vapor is the gaseous state of water that has suspended water molecules in it. Liquid water can generally turn into its gaseous phase in two ways, one is the rapid process known as boiling, and the other is the slow process, known as evaporation. During the process of boiling, the molecules absorb the heat energy from the external heat source upon reaching their boiling point and rise to the surface of the water. Upon gaining the latent heat of evaporation, they break the bonds and form a vapor. But in the case of evaporation, upon receiving the thermal energy from the sun, the surface liquid molecules leave the liquid surface upon utilizing the latent heat. The temperature during the phase change of a liquid remains constant.

Mechanism of evaporation

The mechanism of evaporation can be explained in terms of thermal agitation and the rate of diffusion of molecules of the liquid. Under the application of an external temperature source or net heat flux, it propels the molecules of gas and liquid to undergo thermal agitation. The motion thus leads to an increase in the kinetic energy of molecules and induces a net diffusion in all the areas. The rate of diffusion and motion is proportional to the magnitude of temperature and density of the gas and liquid.

A substance in the liquid state shares its boundary with a gaseous phase which is known as an interface. If the rate of diffusion of molecules of the liquid through the interface is higher than the rate of diffusion of molecules from the gaseous phase back into the interface, the liquid is said to be undergoing an evaporation process. The end of the evaporation process completely evaporates the liquid into the vapor phase or gaseous phase. Hence, the evaporation rate is directly proportional to the rate of diffusion of molecules from the liquid interface. However, if the liquid is confined in a cylindrical container and is acted by an external temperature source, a moment will be reached when the molecules of the gas will diffuse back into the liquid through the interface. At this moment, the evaporation rate will eventually cease and both the phases will be in equilibrium.

The rate of evaporation is primarily given by an expression as,

$E_R=m\left[\mu r{\left(\frac{2K{T}_1}{m}\right)}^{\frac{1}{2}}-\frac{\left({\displaystyle \frac{r}{R}}\right){\left({\displaystyle \frac{2K}{m}}\right)}^{{\displaystyle \frac{1}{2}}}}{{T_g}^{{\displaystyle \frac{1}{2}}}}\right]$

Where $E_R$ represents the rate of evaporation

$\mu$ represents the molecular density in the liquid phase

$R$ represents the universal gas constant

$K$ represents the Boltzmann's constant

$m$ represents the molecular mass

$T_g$ represents the temperature of the substance in the gaseous phase

$T_1$ represents the temperature in the liquid phase

$r$ is a constant

The equation signifies that if the temperature of the substance in the gaseous phase increases, the evaporation rate increases, and the liquid tends to evaporate more rapidly. The possibility of backflow of molecules from the gaseous phase to the liquid phase decreases as the liquid density decreases since the liquid has a higher degree of tendency to evaporate into the gaseous phase. Hence, temperature plays a crucial role in controlling the rate and direction of evaporation. 

Transpiration and evapotranspiration

Water is essential for a plant's survival, this necessity of water is absorbed by the roots and via capillary action reaches its vital aerial parts, such as leaves, fruits, and flowers. Only 5% of the water absorbed by the roots of the plants is required for the plant's growth and metabolism, the remaining 95% remains unused. Transpiration is the process of evaporation of the excess amount of water through leaves surfaces via stomata, flowers, and fruits. In other words, transpiration is the process of water movement inside plants and evaporation through aerial parts. Water tends to evaporate more from the surface of the leaves during the daytime due to more absorption of solar radiation.

Evaporation forms an important part of the water cycle on the earth. Evapotranspiration accounts for the overall transportation of water from a specific surface of the soil to the atmosphere. It includes the water movement from the soil, canopy interception, and water sources. Transpiration forms an intermediate part of the evapotranspiration, which involves the major role of plants in transporting the water to the atmosphere. Plants that form the part of evapotranspiration are called evapotranspirators.

Methods to estimate evapotranspiration

Evapotranspiration can be measured generally by indirect and experimental methods. Some of them are discussed below.

Indirect methods

• Catchment water balance: Here, evapotranspiration is determined by evaluating the change in water quantity stored in the drainage basin with its inputs and outputs. The method uses a water balance equation given by, $∆S=P-\varnothing -\partial -D$, where, $∆S$ represents the change in water stored in the basin, $P$ represents the precipitation input, $\varnothing$ represents the missing flux, $\partial$ represents the streamflow, and $D$ represents the groundwater recharge.
• Energy balance equation: The energy balance equation is given by $\lambda E={\varpi }_n-G-\psi$, where, $\lambda E$ represents the energy which should be supplied to change the liquid into gas, ${\varpi }_n$ represents the net solar radiation, $G$ represents the soil heat flux, and $\psi$ represents the sensible heat flux. The components at the right side of the equation can be calculated using a scintillometer, soil heat flux plates, and radiation meters.  

Experimental methods

• Estimation using lysimeter: Lysimeter is a device that is used to measure the amount of evapotranspiration by measuring the amount of precipitation that is received by an area and the amount of water lost by evaporation. This is a very cost-effective process. Here the rate of evapotranspiration is measured as, $T_E=∆W+Rainfall-Percolation$, where, $T_E$ represents the amount of evapotranspiration, and $∆W$ represents the change in weight of soil whose weight is monitored continuously upon the change in water content by lysimeter.  

Context and Applications

This topic is majorly taught in various undergraduate and postgraduate degree courses of:

• Bachelors in Civil Engineering
• Masters in Civil Engineering
• Masters in Earth and Science
• Masters in Geotechnical Engineering

Practice Problems

1. Which of the following is a rapid process to convert gas liquid into vapor?

1. Boiling
2. Transpiration
3. Evapotranspiration
4. Evaporation

Answer: Option a

Explanation: Boiling is a rapid phenomenon to convert liquid into its constituent vapors.

2. Which is the following energy is possessed by molecules to undergo thermal agitation to account for diffusion?

1. Kinetic energy
2. Vapor pressure
3. Internal energy
4. None of these

Answer: Option a

Explanation: Kinetic energy is responsible for the thermal agitation of molecules to account for diffusion.

3. Which of the following is responsible for liquid molecules to break bonds and leaving the liquid interface?

1. Vapor pressure
2. Aqueous solution
3. Latent heat
4. Heat flux

Answer: Option c

Explanation: Latent heat of evaporation is responsible to allow molecules of the liquid to separate from the liquid-gas interface and change into the vapor phase.

4. Which of the following parts of plants is responsible for transpiration?

1. Leaves surface
2. Stem
3. Roots
4. Xylem

Answer: Option a

Explanation: Transpiration occurs through the surface of the leaves.

5. Which of the following tends to make liquids evaporate faster?

1. Decrease in diffusion rate
2. Increase in temperature
3. Decrease in temperature
4. Both a and b

Answer: Option b

Explanation: Liquid tends to evaporate faster with the increase in temperature.