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11.4 Evaporation and Transpiration 367
temperature and dew-point temperature of the atmosphere, respectively. The vapor
pressure or vapor tension of water is the maximum gaseous pressure exerted at a given
temperature by water vapor in contact with a water surface. The pressure of water vapor
in air not saturated with aqueous vapor equals the vapor pressure of water at the dew-
point temperature of the air, namely, the temperature at which the air would be satu-
rated by the moisture actually in it. In other words, vapor pressure is the partial pressure
exerted by the water vapor in the atmosphere and, in accordance with Dalton’s law,
evaporation is proportional to it.
Thermodynamic formulations relate evaporation to the difference between solar radi-
ation and heat accounted for as (a) back radiation, (b) heat stored in the water, and (c) heat
lost in other ways.

EXAMPLE 11.2 ESTIMATION OF DAILY EVAPORATION
Estimate the evaporation for a day during which the following averages are obtained: water temper-
ature 60 F (15 C); maximum vapor pressure P w 0.52 in. Hg (13.2 mm Hg); air temperature
80 F(27 C); relative humidity 40%; vapor pressure P d 1.03 0.40 0.41 in. Hg (10.4 mm Hg);
wind velocity w 8 mi/h (12.9 km/h); and barometric pressure p a 29.0 in. Hg (737 mmHg).

Solution 1 (U.S. Customary System):
By Eq. 11.9,
E 0.497(1 1.32 10 2 P a )(1 0.268w) (P w P d )
0.497(1 1.32 10 2 29.0)(1 0.268 8)(0.52 0.41)
0.497(1 0.38) (1 2.14) 0.11
0.497 0.62 3.14 0.11
0.10 in./d

Solution 2 (SI System):
By Eq. 11.10,
E 0.497(1 5.2 10 4 P a )(1 0.167w)(P w P d )
0.497(1 5.2 10 4 737)(1 0.167 12.9)(13.2 10.4)
0.497(1 0.38) (1 2.15) 2.8
0.497 0.62 3.15 2.8
2.7 mm/d.

11.4.2 Evaporation from Land Surfaces
Wet soil loses water rapidly to the atmosphere; moist soil does so more slowly.
Immediately after a rainstorm, water, intercepted by vegetation or present in films, pools or
puddles on roofs and pavings and starts to evaporate at about the rate of loss from shallow
water. As the free moisture disappears, the rate of evaporation slows. Interstitial moisture
is evaporated internally or drawn to the surface by capillarity before it evaporates. Soil
characteristics and depth to groundwater are governing factors. Where the water table lies
3 ft (0.90 m) or more down, there is little evaporation after the surface layers have dried
out. Because cultivated soils lose less water than uncultivated soils, the practice of loosen-
ing soils to conserve moisture prevails.