evaporation (2)
DESCRIPTION
fTRANSCRIPT
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Water Resources (CE 310)
Factors affecting evaporation
Pyschometric chart
Measurement of evaporation
Evapotranspiration
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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.
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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).
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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.
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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.
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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.
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Pyschrometric chart
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Pyschrometric chart (Illustration)
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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
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Measurement of evaporation
(i) using evaporimeters
(ii) empirical evaporation equations
(iii) analytical methods.
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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
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Water budget
Accurate water budget with evaporation the only unknown
= + + + +
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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)
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Evapotranspiration
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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.