Measurement of evaporation

a) Pan measurement method:

• a pan of certain standard dimension is taken and the water is filled in this pan up to a certain range of level. The amount of water evaporated from this pan is measured, from which the rate of evaporation is calculated.
• The rate of evaporation is multiplied by a suitable pan coefficient, so as to obtain the evaporation rate from the given water body. This is necessary because it is found that the evaporation from a large surface source is not the same as that from a small pan. Different shape of pan has been designed by different designers and different values of pan coefficient have been given.

I. US Class A pan evaporimeter:

• consists of a shallow vessel about 1.22m in diameter and 25.5cm deep
• made of unpainted non-corrosive galvanized iron sheet
• is placed on a wooden platform such that its base is 15 cm above the ground surface to allow free circulation of air below the pan
• is filled to a depth of 20cm (reference point) and water surface level is measured daily with a hook gauge installed in the stilling well
• Evaporation is computed as the difference between observed water levels on two consecutive days. Alternatively, it is computed from the water added each day to bring the water level up to a fixed mark in the stilling well. Pan coefficient is about 0.6 to 0.8
• Opening portion of the pan is covered by wire gauge to protect from birds

II. Using empirical formula:

I. Meyer’s formula

E=Km(es-ea)(1+v9/16)

Where; E=Evaporation from water body in mm/day

Km= a coefficient having value of 0.36 for large deep water, & 0.5 for small & shallow water

es= Saturated vapor pressure at the water surface

ea = Actual vapor pressure of the overlying air at specific height

v9=monthly mean wind velocity in km/hr at about 9 m above the ground

II. Rohwer’s formulaa

E=0.771(1.465-0.000732 Pa)(0.44+0.0733 V0.6)(es-ea)

Where E, es, ea have the same meaning as in above equation

Pa= Mean atmospheric pressure

V0.6=Mean wind velocity (km/hr) at ground level, which can be considered at 0.6m above the ground.

III. Storage equation method or Water budget method or mass balance method

• It is based on measurement of continuity of water flow-essentially, the budget comprised by the various items of input, and water storage of hydrologic system.

Continuity equation

E = (S1 – S2) + I + P+G – (O – Os)

Where, E = evaporation

S1 = storage at beginning

S2 = storage at End

I = surface inflow

P = precipitation

G=Groundwater flow

O = surface outflow

Os = subsurface seepage (most difficult to evaluate)

Advantage: This method is simple in concept.

Disadvantage: This method is difficult to do so accurately because of the effects of errors in measuring various items involved in the water balance.

IV. Energy budget method

This method is similar to the Water Balance Method except that it deals with the continuity of energy flow instead of water flow. Use Continuity equation in energy units;

Where, E = Evaporation

Hn=Net heat energy received or net incoming energy

Hg=Heat lost to the ground

Hi=Net heat conducted out of the system by water flow

Hs=Heat stored in the water body

ρ =Density of water

La= Latent heat of vaporization of water

ᵝ=Bowen’s ration=ratio of heat loss by conduction to heat loss by evaporation

V. Water budget method

• Based on measurement of continuity of water flow.
• Equation: E= (S₁– S₂) + I +P-O – O_s

Where S₁ and S₂ are storage time 1 and 2.

I= Surface flow, P = Precipitation, O= Surface outflow , O_s = Subsurface seepage