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.

Depression Storage and Evaporation

Depression Storage :

When the any form of precipitation reaches to the ground firstly it must fill all the depression before is can flow over the surface. The volume of water trapped in these depression does not contribute to the runoff so these are called depression storage.

Factors effect of Depression Storage:

• Type of Soil
• Nature of Depression 
• Condition of Surface
• Slope of Catchment 
• Past Precipitation. 
• Time

Experimentally evaluated values of depression storage on different type of soil, for Sand, Loam and Clay the values are 0.5 cm, 0.4 cm, 0.25 cm respectively. if we talk about the area depression storage is 0.15 cm for impervious area and 0.65 for pervious area.

Evaporation:

It is the process in which a liquid (water) changes to the gaseous state (vapour) at the free surface, below the boiling point through the transfer of heat energy. Evaporation is a cooling process such that the latent heat of vaporization is taken away from the water body by the escaping water molecule. 

If heat is given to water molecule it increases kinetic energy of the water molecule, As the kinetic energy of the water particle increases, they tends to leave the surface of water body and some water particle tends to come in water body from air. If the net particle is going outward of body then it is called evaporation and if the net particle is going inward of water body then it is called condensation.

Factors affects on Evaporation:

• Temperature 
• wind Speed
• Atmospheric Pressure
• Quality of Water
• Size of Water Body
• Humidity

Temperature :

The rate of evaporation increases with an increase in water temperature. Regarding the air temperature, increase in evaporation rate with increase temperature, but high correlation between evaporation rate and air does not exist. It is possible that evaporation will be different in a different month if the mean monthly temperature is same in all month.

Wind Speed : 

If the wind velocity is large enough to remove all the evaporated water vapour, any further increases in wind velocity does not influence the evaporation. Thus, the of evaporation increases with the speed up to a critical speed beyond which any further increases in the wind speed has no influence on the evaporation rate. For large water bodies high speed turbulent wind are needed to cause maximum rate of evaporation.

Atmospheric Pressure :

A decrease in the atmospheric pressure increase the evaporation as seen in high altitude where the atmospheric pressure reduces.

Quality of Water :

If a salt is dissolved in water, the vapour pressure of the solution is less that that of pure water and hence causes the reduction in the rate of evaporation. Sea water is more saline so they have 2-3 % less evaporation.

Size of Water Body :

For a large water body surface the evaporation rate is highly influenced by the depth of water.

The increase in depth of water body reduces the rate of evaporation from the surface of water body in summer season and in winter season evaporation will increases with the increase of the depth of water body.

Humidity :

If the humidity of the atmosphere is more, the evaporation will be less and vise versa.

Factors affects the evaporation of soil moisture :

• Soil Moisture Content
• Soil Capillary Characteristics 
• Depth of Water Table
• Colour of Soil  

Soil Moisture Content :

The evaporation from saturated soil is more that the evaporation of same surface water body. 

Soil Capillary Characteristics :

Capillary supply the water content to the surface so they increases the evaporation.

Depth of Water Table :

Soil evaporation is maximum when the water table is at the surface and decreases quiet rapidly as the water table goes down.

Colour of Soil :

Soil colour also affects the evaporation rate, A darker soil absorbs more heat and increases the temperature, so increases the evaporation rate.

Abstraction of Precipitation

Abstraction is the term used to describe the amount of rainfall that doesn’t turn into runoff. There are numerous processes that intercept rainfall. Some rainfall never even reaches the ground, instead being intercepted by leaves on trees, by buildings and some rainfall is evaporated back into the atmosphere, of the rainfall that reaches the ground, some is infiltrated and some just ponds with no outlet. 

So processes like evaporation, transpiration, evapotranspiration, interception, percolation, infiltration so on are included in this abstraction of precipitation.

Interception :

When a rain falls it is firstly intercepted by buildings, plants, trees, towers etc., when they become completely wet, then the rain comes down to the earth surface. This initial amount of water used by buildings, plants, trees, towers etc., for getting wet is equal to the evaporation rate. so this complete amount of water is called as interception loss. 

Initially while Raining

After Complete Wetting

It is calculated by,  x = a + bt 

Where, 

x = total interception

a = water required for wetting 

b = evaporation rate from the intercepting surface

t = duration of rainfall 

Interception Loss : water which is retained on surface, and which is later evaporated away is nothing but interception loss.

Interception mainly depends upon the duration of rainfall frequency but very less upon the intensity of rainfall. If the total rainfall is more, the interception may be little more. The interception is expressed as percentage of total rainfall.