3. EVAPORATION - GitHub Pages

1 EVAPORATION 49 _____ . 3. EVAPORATION EVAPORATION occurs when water is converted into water vapor at the evaporating surface, the contact between water body and overlapping air. At the evaporative surface, there is a continuous exchange of liquid water molecule into water vapor & vice versa. The two main factors influencing EVAPORATION from an open water surface are the supply of energy to provide the latent heat of vaporization, and the ability to transport the vapor away from the evaporative surface.

2 The latent heat of vaporization lv is the amount of heat absorbed by a unit mass of a substance. Solar radiation is the main source of heat energy. The magnitude of annual EVAPORATION is highly dependent on the prevailing climate in and around the water body. EVAPORATION has significant impact on water resources development especially in arid and semi-arid regions. EVAPORATION from Lake Nasser in Egypt (arid region) is about 3000 mm/year, where as EVAPORATION from Lake Koka is about 1500 mm/year, which is half of that from Lake Nasser.

3 EVAPORATION rate is a function of several meteorological and environmental factors such as net radiation, saturation vapor pressure , actual vapor pressure of air, air and water surface temperature, wind velocity and atmospheric pressure . Section discusses definition and measurements of these variables. Definition of some meteorological variables The atmosphere forms a distinctive, protective layer about 100 km thick around the Earth. To the hydrologist, the troposphere (the first 11 km) is the most important layer because it contains 75% of the weight of the atmosphere and virtually all its moisture.

4 On average, the temperature from ground level to the tropopause falls steadily with increasing altitude at the rate of oC /km. This is known as the lapse rate. EVAPORATION 50 _____ . EVAPORATION at high altitudes is promoted due to low atmospheric pressure as expressed in the psychrometric constant. The effect is, however, small and in the calculation procedures, the average value for a location is sufficient.

5 A simplification of the ideal gas law, assuming 20 C for a standard atmosphere, can be employed to calculate atmospheric pressure P: ( ) where: P atmospheric pressure [kPa], z elevation above sea level [m], The psychrometric constant, , is given by: ( ) where = Psychrometric constant [kPa C-1], P = Atmospheric pressure [kPa], = Latent heat of vaporization, [MJ kg-1], cp = Specific heat at constant pressure , 10-3 [MJ kg-1 C-1], and = ratio molecular weight of water vapour / dry air = The specific heat at constant pressure is the amount of energy required to increase the temperature of a unit mass of air by one degree at constant pressure .

6 Its value depends on the composition of the air, , on its humidity. For average atmospheric conditions a value cp = 10-3 [MJ kg-1 C-1] can be used as an average atmospheric pressure is used for each location. Air density: air density of moist air (kg/m3)is estimated by a = (p/(275 + T)) where p is the atmospheric pressure in kPa and T is air temperature in degrees Celsius. EVAPORATION 51 _____.

7 Water vapor: the amount of water vapor in the atmosphere is directly related to the temperature. The water vapor content or humidity of air is usually measured as a vapor pressure , and the units used is millibar (mb). Specific humidity: The mass of water vapor per unit mass of moist air is called specific humidity qv and equals the ratio of the densities of water vapor v and of moist air a Vapor pressure : Dalton's law of partial pressures states that the pressure exerted by a gas (its vapor pressure ) is independent of the pressure of other gases.

8 The vapor pressure e of the water vapor is given by the ideal gas law as where T is the absolute temperature in K and Rv is the gas constant for water vapor. If the total pressure exerted by the moist air is p, then p-e is the partial pressure due to the dry air, and The gas constant for water vapor is avv = q ( ) TR = evv ( ) vda + = ( ) T R = e - pdd ( )

9 = Rdv ( ) EVAPORATION 52 _____ . where is the ratio of the molecular weight of water vapor to the average molecular weight of dry air. Combining Eqs.( ), ( ) and ( ) we get The specific humidity qv is approximated by whereTR = paa The relationship between the gas constants for moist air and dry air is given by Saturation vapor pressure es : For a given air temperature, there is a maximum moisture content the air can hold and the corresponding vapor pressure is called saturation vapor pressure es.

10 At this vapor pressure , the rates of EVAPORATION and condensation are equal. Over a water surface the saturation vapor pressure is related to the air temperature with equation Where es is in Pascal (Pa = N/m2) and T is air temperature in degree Celsius. Due to the non-linearity of the above equation, the mean saturation vapour pressure for a day, week, decade or month should be computed as the mean TR ) + ( = pdvd ( ) = qv ( ) 287 = R ), + (1 R = Rdvda ( ) T + 611 = esexp ( ) EVAPORATION 53 _____.