What do you mean by Evaporation and Factors affecting Evaporation?

What is Evaporation?

                   Evaporation is the phase of the hydrological cycle. Evaporation from water surface and soil is very important in hydrometeorological studies. Evaporation is the transition of water molecules into vapors. Water from the ocean and from the land and from the surface of leaves move toward the atmosphere in the form of vapors. Sun driven it, heat energy used to convert the water molecules into vapor and move toward atmosphere from the earth. It took energy when it evaporates and makes cooling in surrounding from there in evaporate.

 What is Evapotranspiration?

                   Evaporation from the surface of the soil and from the lake is the evaporation while Evaporation of water from the surface of leaves is called transpiration. Evaporation occurs in only days not continuously while Transpiration occurs day and night and continuously action. When evaporation and transpiration are meet then the process which makes is called Evapotranspiration.

Factors affecting the Evaporation:

                          Factors affecting Evaporation are:

Air and Water Temperature

Relative humidity

Wind velocity

Surface area (exposed)

Barometric pressure

The salinity of the water

The rate of evaporation is a function of the difference in vapor pressure at the water surface and in the atmosphere. Daltons law of Evaporation is given here by pressing the word.

                                      E = u ( e_s-e_a ) 

U is the function of wind speed, e_{a is the current vapor pressure,} e_{s is the saturated vapor pressure at the temperature.}

_{The higher the temperature and wind velocity higher the evaporation, while the greater the humidity and dissolved salt, the smaller the evaporation. Higher evaporation is in the summer months of April and May.}

_{Higher evaporation occurs on the ocean than the land while precipitation is lower on the ocean than on land.}

_{Methods of Estimating Evaporation from the lakes:}

        _{Evaporation from the surface can be determined by the following methods.}

1. _{The storage equation}
2.                                  _{S= inflow - outflow}
3.     _{Auxilary pan  like land pan ,float pan,colarado sunken pan etc.}      
4. The water budget[ same as storage equation]
5. Humidity and wind velocity gradient
6. The energy budget equation - this method involves too many hydro-meteorological factors
7. Combination of aerodynamic and energy balance equation Penman's Equation
8. Evaporation formula like Dalton's law                                                                                                                                                                                                                                                                                                                                                                                                                         
   
   
   

Evaporation pan:

                         Float pan of 90cm square and 40cm deep are mounted on a raft floating in the water. The volume of water lost due to evaporation in the pan is determined by finding the volume of water required to bring up the level of the water up to the original mark daily and making allowance for rainfall if there have been any.

                         Land pan:    Evaporation pans are installed in the vicinity of a lake or reservoir to determine lake evaporation. The IMD land pan is 122cm diameter and 25.5cm deep, made of unpainted GI, set on wooden grillage 10cm above the ground to permit circulation of air under the pan. The pan has a stilling well, vernier point gauge, a thermometer with clip, and covered with wire screen.

          Colorado sunken pan:  This is 92 cm square and 42-92cm deep and is sunk in the ground such that only 5-25cm depth above the ground and the water level is maintained at ground level. Evaporation is measured by a pony gauge.

           

Pan-coefficient:

                 Evaporation pan data cannot be applied directly to the free water surface but must in adjusting the difference between physical and climatological factors. For example, the lake is larger and deeper and has a different wind speed as compared to the pan. The small volume of water in a metallic pan is greatly attracted by temperature fluctuation in the air or by solar radiation in contrast with large water bodies with little temperature fluctuation.
                                  Pan coefficient = lake evaporation/pan evaporation

 Soil Evaporation: 

                            Evaporation from the wet soil surface immediately after rain or escape of water molecules with more resistance when the level of water lies one meter from the ground is called soil evaporation.
 The percentage of evaporation from the free water surface is called evaporation opportunity.
 Evaporation opportunity =  Actual evaporation from the land at a given time*100/Evaporation from an 
                                                    equivalent water surface
            Soil evaporation will continue at a high rate for some time after the cessation of rainfall, then decrease at the ground surface starts drying, until a constant rate is reached which is dependent on the depth of the water table and nature of the soil.

Lysimeter: 

               Lysimeter is a  device that is used to measure the evaporation of soil.