How to Measure Evaporation

Evaporation 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.

👉 Factors affects on Evaporation

Measurement of evaporation is of very much important in many hydrological problem like planing and operation of reservoirs and irrigation system. 

Methods to calculate Evaporation : 

• Class A Evaporation Pan
• ISI Standard Pan
• Colorado Sunken Pan
• US Geological Survey Floating Pan

Class A Evaporation Pan :

This method is developed by and mainly used by US weather bureau. The pan is made up of unpainted galvanized iron sheet and monal metal is used where corrosion posses a problem. 

US Class A Evaporation Pan

ISI Standard Pan :

It is also known as modified class A pan. This pan is made up of copper sheet., tinned inside and painted white outside. The top pf the pan is covered fully with a hexagonal wire netting of galvanized iron to protect the water in the pan from birds. the presence of a wire mesh makes the water temperature more uniform during day and night. The evaporation from this pan is 14 % less than US class A pan evaporation. 

  

ISI Evaporation Pan

US Class A Evaporation Pan (Source - Wikipedia)

Pan Coefficient (Cp) :

The evaporation observation observed from a pan has to be corrected to get the evaporation from a lake under similar climatic and exposure conditions.

Lake evaporation calculated from pan evaporation,

Lake Evaporation = Cp X Pan Evaporation

  

Sr. No.	Type of  Evaporation Pan	Pan Coefficient (Cp)
1	US Class A Pan	0.70
2	ISI Pan	0.80
3	Colorado Sunken Pan	0.78
4	US Geological Floating Pan	0.80

Recommendation of minimum number of evaporimeter as per World Meteorological Organization (WMO),

1. Cold Region - 1 station for every 1.00.000 sq km

2. Humid Temperature Region - 1 station for every 50,000 sq km 

3. Arid Region - 1 station for every 30,000 sq km.