Integration of Plant and Groundwater Interactions - Introduction to Phytoremediation of Contaminated Groundwater - page 35

seconds per meter (s/m) (Kramer and Boyer 1995). As with
the equation by Dalton, the net evaporation, E , is propor-
tional to the difference between C water and C air . A similar
relation between evaporation and water vapor is given by the
equation of Thornthwaite and Holzman (1939).
The removal of water molecules entering and leaving the
atmosphere above the water surface is controlled by many
physical factors. Transfer of water from a liquid to a vapor
will continue if a vertical gradient is established from the
water to the atmosphere in which water vapor is removed
and prevented from accumulating, as described in Eqs. 2.6
and 2.7 . This vertical gradient is affected by the amount of
moisture in the air above the water body. If the relative
humidity, or ratio of absolute humidity to the saturation
humidity of the air at a given temperature, is high, evapora-
tion will be low, and the converse also is true. Evaporation
also is affected by the amount of solar radiation, which
warms both the water and air directly above it; cloud
cover; air temperature; wind speed and, to a lesser extent,
direction; and atmospheric pressure changes in relation to
elevation that affect water vapor pressure.
Under conditions in which the water table is close to land
surface, evaporation of groundwater can occur if the soil and
sediment are coarsely textured, such as sands or gravels
(Bouwer 1978), and the hydraulic conductivity of the soil
and sediment, or the relative ease in which water moves
through sediment pores, is conducive to water flow in the
zone above the water table. Evidence of the evaporation of
shallow groundwater is provided by the accumulation of
salts at land surface where this process occurs. Areas
associated with low agricultural output tend to have saline
surface soils caused by the evaporation of shallow soil mois-
ture and groundwater. Evaporation of water from soil when
the water table is deep, however, also can result in soil that
gradually dries out (Hillel 1998). More discussion on the
potential for groundwater evaporation is discussed in Chap.
4. Moreover, because water continually is cycled between a
liquid to vapor and back again, the amount of water in the
atmosphere constitutes only a small portion of the total
global supply of water at any given time, or about 0.04%
of non-ocean water. The process of ice water being
converted directly to water vapor is a special form of evapo-
ration called sublimation.

Fig. 2.6 The pressure exerted by water vapor, shown as the arrows
above liquid water ( gray area ), is measured by inverting a tube filled
with the vapor being measured and mercury in a dish. The difference in
mercury level, Hg (t o ) and Hg (t l ) is equivalent to the vapor pressure.
A higher vapor pressure indicates a greater tendency to become a gas.
This figure also demonstrates the results of surface tension for two
liquids of different properties. The top of the water has a meniscus
shaped concave up, as the water molecules are more attracted to the
walls of the glass tubing than to themselves. Conversely, the mercury
meniscus is shaped concave down, since the mercury is more attracted
to itself.

conditions, energy added to a closed system will be
dissipated to the same magnitude somewhere else. If energy
is added to a body of water, for example, the energy also is
removed when liquid water changes state to a vapor.
The relation between evaporation and water vapor pres-
sure was recognized as early as 1802 by John Dalton, known
perhaps most commonly for his discovery of the partial
pressures of gases, or Dalton's Law of Partial Pressures, as
shown in Eq. 2.6

E ¼ be o ð e a Þ; (2.6)

where E is the evaporation rate, b is a coefficient that refers
to the resistance of water vapor transfer to the atmosphere, e o
is the vapor pressure of the water surface, or supply, under
saturated conditions, and e a is the vapor pressure of water in
the air at any given time, or demand. Hence, the rate of
evaporation, E , is proportional to the difference between e o
and e a , called the vapor pressure gradient. If e a reaches
conditions of e o , evaporation stops. An alternative way to
look at evaporation is in Eq. 2.7

2.3.4 Measurement of Evaporation

E ¼ C water C air = R air ; (2.7)

Measurement of evaporation can be made using indirect and
direct methods. Evaporation can be estimated indirectly by
using Eq. 2.3 if the other components of water inflow and
outflow in a particular basin are known. Because the
measurement of these factors can contain errors of between
5% and 10%, such estimates generally are not accurate.

where E is evaporation rate, in grams per square meter per
second (g/m 2 /s), C water is the vapor concentration of water at
the surface, C air is the vapor concentration in the air above
the surface, in grams per cubic meter (g/m 3 ), and R air is the
resistance to the diffusion of water from liquid to vapor, in