Water Resources (CE 310)

Factors affecting evaporation

Pyschometric chart

Measurement of evaporation

Evapotranspiration

Factors affecting evaporation

Evaporation is dependent on

(i) Solar radiation

(ii) temperature

(iii) wind speed

(iv) relative humidity

(v) soil cover

(vi) crop resistance

(vii) heat storage capacity.

Solar radiation

A higher temperature for both the water body and surrounding air result in increased evaporation.

Temperature

The transfer of water in the liquid state into water vapour requires energy, which is known as the latent heat of

evaporation.

depends on the latitude, atmospheric conditions, reflection from the earth surface known as the albedo, absorption by

the atmosphere and storage in the ground (or water body).

Typical albedo values are: free water surface = 0.06, grass = 0.22-0.25, bare soil 0.10-0.30 and fresh snow = 0.90).

Wind speed

As the airs humidity rises, its ability to absorb more water vapour decreases and the rate of evaporation slows

Relative humidity

Wind speed facilitates removal of saturated air to allow evaporation to proceed.

Soil cover Reduces amount of solar energy reaching the earth and

can cause localised condensation if conditions allow.

Crop resistance

Deep-water bodies have more heat storage than shallow ones.

A deep free-water storage system may store radiation energy received in summer and release it in winter causing less evaporation in summer and more evaporation in winter.

Heat storage capacity

The transpiration of a cropped surface is usually less than the evaporation of an open water surface due to the

additional resistance to the flow of water in the plant and

the transfer of water vapour through the stomata

For tall crops, however, the increased turbulence lowers the aerodynamic resistance leading to increased

evaporation.

Pyschrometric chart

Air conditions can be quickly characterized by using a special graph called a pyschrometric chart

Properties on the chart include dry-bulb and wet-bulb temperatures, relative humidity, humidity ratio, specific

volume, dewpoint temperature, and enthalpy

Two properties are needed to characterize air

The point of intersection of any two property lines defines the state-point of air on a psychrometric chart

Therefore if data is available on any two properties of the chart then all other properties can be established.

Pyschrometric chart

Pyschrometric chart (Illustration)

Pyschrometric chart Example

Given air at 73 degrees F (Tdb) and 20% RH.

wet-bulb temperature is 52 degrees F; enthalpy is 21.3 Btu/lbda; humidity ratio is 0.0035 lb/lbda; dewpoint

temperature is 30 degrees F; and specific volume is 13.5

ft3/lbda

Measurement of evaporation

(i) using evaporimeters

(ii) empirical evaporation equations

(iii) analytical methods.

Class A Evaporation Pan Used by the US Weather Bureau.

1210 mm diameter and 255 mm depth

Water maintained between 18 cm and 20 cm depth

=

Water budget

Accurate water budget with evaporation the only unknown

= + + + +

Penman Method

1

'

0

ER

E

n

E is the estimated evaporation from a free-water surface (mm/day) is the rate of the change of the saturation vapour pressure with respect to the air temperature

Rn is the net radiation is the constant in the wet- and dry-bulb psychrometric equation (approximately equal to 0.49 mm of mercury/ OC)

Evapotranspiration

Evaporation Station Network

WMO recommends the minimum network of evaporimeter stations as below:

Arid zones - One station for every 30,000 km2,

humid temperate climates - one station for every 50,000

km2, and

cold regions - One station for every 100,000 km2.