Question

Provide a brief review, at least 500 words, on evaporation measurements and estimation methods. Acknowledge the used references and provide a list of references.

Answer 1

Evaporation is the process in which liquid changes to gaseous state at the free surface below the boiling point through the tranfer of heat energy. Evaporation is a cooling process in that the latent heat of vapourization must be provided by water body.

Measurement and Estimation of Evaporation

1.Using Evaporimeter data

An evaporation pan is used to hold the rain water for measurement and Estimation of Evaporation or water loss. It has following types

i) Class A evaporation pan or US class A evaporation pan or class A land pan

Pan coefficient Cp =Lake evaporation /Pan evaporation

ii) ISI Pan or Modified class A pan

Diameter =1220mm depth =255mm

This pan is placed over square wooden platform of 1225mm with and 100mm height to enable circulation of air underneath the pan.

iii) Colorado sunken pan

This pan 920mm square and 460mm deep and buried into the ground within 100mm of the pop.

2.Emperical Evaporation equations

General formula EL =kf(u) (ew-ea​​​​​​)

where EL=Lake evaporation in mm/day

ew=saturation vapour pressure at the water surface temperature in mm of Hg.

es= actual vapour pressure of overlaying air at a specified height in mm of Hg.

f(u) =wind speed correction function

k=a coefficient

Commonly used emperical formul is

Mayer's formula EL=KM(ew-ea) (1+U9/16)

U9=monthly mean wind Velocity in km/hai at about 9m above ground

K=coefficient(0.36 for deep and 0.5 for shallow water)

3.Analytical Method

i) Water budget method

ii) Energy Budget equation

ii) Mass transfer method