Transpiration and Evapotranspiration

Transpiration :

Transpiration is the process in which water leaves the body of a living plant and reaches the atmosphere as water vapour. The water leaves the plant from its leaves and stomata (Stomata are cell structures in the epidermis of tree leaves and needles that are involved in the exchange of carbon dioxide and water between plants and the atmosphere.) The transpiration occur when the process of photosynthesis is run. Maximum transpiration around 90% occurs in a day time only because the process of photosynthesis is runs in day.  

Factors affects on Transpiration :

• Temperature
• Wind 
• Intensity of Light
• Plant Characteristics 
• Humidity

Temperature :

Increases molecular movement so that more water molecules evaporate from cell surfaces. The rate of diffusion of water molecules from the leaf is increased, as the temperature rate increases the amount of transpiration is also increases. 

Wind :

Removes water vapour from leaf surfaces. More water diffuses from the leaf, as the wind rate increases the transpiration rate is also increases. 

Intensity of Light :

Increases the rate of photosynthesis. Stomata open so that water diffuses out of the leaf, as the intensity of light is increases the rate of transpiration is increases.

Plant Characteristics:

The amount of transpiration is also depends on the characteristics of plant. same plant have different amount of transpiration at different location.

Humidity:

Reduces the concentration of water molecules outside the leaf. Diffusion of water from the leaf increases, as the humidity increases the rate of transpiration is decreases.  

Different plants will evidently transpire different amount of water and their water consuming characteristics are compared by the transpiration ratio,

= Total mass of water transpired by the plant during its full growth 

Mass of dry matter produced 

Evapotranspiration : 

Evapotranspiration is main part of hydrology cycle, when the transpiration takes place, the land area in which plants stand also lose moisture by the evaporation of water. Since in the process of vegetation growth, it is generally not possible to separate the transpiration and connected evaporation from the plants surrounding. So evaporation and transpiration are considered under one head as evapotranspiration.

When sufficient moisture is freely available to completely meet the needs of the vegetation fully covering the area, the resulting evapotranspiration is called potential evapotranspiration (PET).

The real evapotranspiration occurring in a specific situation in the field is called actual evapotranspiration (AET).

PET is mainly depends upon climatological factors, rather than on characteristics of plants and soil, while AET is largely affected by the characteristics of soil and vegetation.

Estimation of Evapotranspiration :

Indirect methods :

Pan evaporation data can be used to estimate lake evaporation, but transpiration and evaporation of intercepted rain on vegetation are unknown. There are three general approaches to estimate evapotranspiration indirectly.

Catchment water balance :

Evapotranspiration may be estimated by creating an equation of the water balance of a drainage basin. The equation balances the change in water stored within the basin (S) with inputs and outgoes:

Where,

P = Precipitation,

ET = Evapotranspiration (which is to be estimated), 

Q = Stream Flow,

D = Groundwater Recharge. 

If the change in storage, precipitation, stream flow, and groundwater recharge are all estimated, the missing flux, ET, can be estimated by rearranging the above equation as follows:

Energy balance :

A third methodology to estimate the actual evapotranspiration is the use of the energy balance.

where,

λE = Energy needed to change the phase of water from liquid to gas, 

Rn = Net radiation, 

G = Soil heat flux, 

H = Sensible heat flux. 

Using instruments like a scintillometer, soil heat flux plates or radiation meters, the components of the energy balance can be calculated and the energy available for actual evapotranspiration can be solved.

Experimental methods for measuring evapotranspiration :

One method for measuring evapotranspiration is with a weighing lysimeter. The weight of a soil column is measured continuously and the change in storage of water in the soil is modeled by the change in weight. The change in weight is converted to units of length using the surface area of the weighing lysimeter and the unit weight of water, evapotranspiration is computed as the change in weight plus rainfall minus percolation. Lysimeter studies are time consuming and expensive.