**2. Concept of evaporation, transpiration, and evapotranspiration and its relation to crop productivity**

Evaporation is the physical process through which liquid water is converted to water vapor. The rate of E depends on the saturated vapor pressure of the liquid and increases with increase in temperature until the atmospheric pressure at the boiling point [12].

**97**

**Figure 1.**

*conditions (adapted from [15, 16]).*

*Concept and Consequence of Evapotranspiration for Sustainable Crop Production in the Era…*

Transpiration is the process by which moisture is carried through plants from roots to small pores on the underside of leaves, where it changes to vapor and is released to the atmosphere. Transpiration is principally E of water to the atmosphere from plants roots to small pores on the underside of leaves. Another type of water loss from the uninjured leaf or stem of the plant, mainly by stomata is called guttation [13]. Nearly 10% of the moisture available in the atmosphere is from the T process [14]. The remaining 90% is mainly from the E process from different water bodies [15, 16]. reported that the T is attained by the movement of water, at the vapor phase through the conductor system from the roots to the leaves of the plant, as a function of a water potential gradient from the soil (ψsoil) to the air (ψair) as

In general, ET is the sum of E and T. It is the simultaneous process of water transfer to the atmosphere both by soil water E and plants T. The study found that during a growing season, a leaf will transpire many times more water than its own weight. An acre of corn gives off about 11,400–15,100 l of water each day, and a

Depending on the vegetation conditions, size of the vegetated area, and soil water supply, different conceptions are to be defined, such as potential, actual,

Potential ET represents the combined loss of water through the plant's process of T and E of water from the Earth's surface. Both the processes are influenced by temperature, humidity, sunlight, and wind as well as Earth vegetation. ETp values indicate the amount of water that has been lost, and thus needs to be replaced,

Actual ET is the amount of water actually utilized by an extensive surface vegetated with grass, at an active growth stage, covering completely the soil surface. ETa is the quantity of water that is actually removed from a surface due to the processes

*Schematic representation of the water motion in the soil-plant-atmosphere system under optimal development* 

large oak tree can transpire about 1,51,000 l per year [13] (USGS, 2016).

oasis, and crop ET [16]. Such particular terms are described as follows.

*DOI: http://dx.doi.org/10.5772/intechopen.83707*

**2.1 Potential evapotranspiration (ETp)**

through irrigation and/or rainfall [16, 17].

**2.2 Actual or real evapotranspiration (ETa)**

shown in **Figure 1**.

*Concept and Consequence of Evapotranspiration for Sustainable Crop Production in the Era… DOI: http://dx.doi.org/10.5772/intechopen.83707*

Transpiration is the process by which moisture is carried through plants from roots to small pores on the underside of leaves, where it changes to vapor and is released to the atmosphere. Transpiration is principally E of water to the atmosphere from plants roots to small pores on the underside of leaves. Another type of water loss from the uninjured leaf or stem of the plant, mainly by stomata is called guttation [13]. Nearly 10% of the moisture available in the atmosphere is from the T process [14]. The remaining 90% is mainly from the E process from different water bodies [15, 16]. reported that the T is attained by the movement of water, at the vapor phase through the conductor system from the roots to the leaves of the plant, as a function of a water potential gradient from the soil (ψsoil) to the air (ψair) as shown in **Figure 1**.

In general, ET is the sum of E and T. It is the simultaneous process of water transfer to the atmosphere both by soil water E and plants T. The study found that during a growing season, a leaf will transpire many times more water than its own weight. An acre of corn gives off about 11,400–15,100 l of water each day, and a large oak tree can transpire about 1,51,000 l per year [13] (USGS, 2016).

Depending on the vegetation conditions, size of the vegetated area, and soil water supply, different conceptions are to be defined, such as potential, actual, oasis, and crop ET [16]. Such particular terms are described as follows.
