Integration of Plant and Groundwater Interactions - Introduction to Phytoremediation of Contaminated Groundwater - page 38

atmosphere for water in a given area at a given time under
given climatic conditions. Hence, ET p cannot be considered
a constant or a maximum for a given area if the area is
characterized by significant fluctuations in climate. It is
simply a concept to describe ET if water quantity is not
constrained.
For the calculation of ET p to be useful, plant transpiration
is treated as one large leaf with no water stress that
completely shades the soil and is growing maximally. In
this manner, ET p can be expressed in the following equation:

Chap. 4. If certain meteorological conditions are known,
methods to use include the Thornthwaite method (Thornthwaite
and Holzman 1939), the Penman method (Penman 1948), the
Van Bavel method (Van Bavel 1966), and the Penman-
Monteith method (Monteith 1965). These methods all are
based primarily on an energy-budget concept, in which inflows
and outflows of energy are balanced. The direct determination
of evapotranspiration from large groups of trees is a difficult
task because of the variability inherent to individual trees and
the physical variables that affect ET . On the other hand,
although estimates of ET on individual trees can be made fairly
accurately, scaling these values up to the stand level is often
problematic.
A fundamental approach to estimating ET is to solve the
energy-budget equation. Under steady-state conditions,
energy added to a system must be balanced by energy
leaving the system. Mathematically, this can be expressed as

ET p ¼ C int C air = R int þ R air ; (2.9)

where ET p is the rate of potential evapotranspiration (g/m 2 /s),
C int and R int are the vapor concentration of water at the surface
of a moist leaf or soil, respectively, C air is the vapor concen-
tration in the air above the leaf surface (g/m 3 ), and R air is the
resistance to the diffusion of water vapor (s/m). Because
both processes of evaporation and transpiration require
energy from the sun to turn water into a vapor, varying the
temperature of air by 10 C increments under equal atmo-
spheric conditions will increase evaporation and transpira-
tion by a factor of 2.
Because plants have access to subsurface sources of water
along with exposed surface water, it is possible that transpi-
ration can exceed free-surface evaporation, as was men-
tioned previously. This has been termed the oasis effect.
The oasis effect was demonstrated by using water hyacinth
[ Eichhornia crassipes (Mart.) Solms] in reservoirs in Texas
(Benton et al. 1978). The authors looked at lakes that had
about 20% surface coverage by water hyacinth. This cover-
age was calculated to result in a transpiration loss of over
2,000,000 acre-ft (2.46

R n ¼ l E þ H þ G ; (2.10)

where R n is the net radiation, E is the latent heat flux or the

energy absorbed when the water evaporates or released
when it condenses, H is the sensible heat flux or convective
energy initiated by temperature gradients in the air, and G is
the ground heat flux or heat that conducts into the soil, all in
watts per meter squared (w/m 2 ). R n can further be defined as

l

R n ¼ ð radiant energy in Þ ð radiant energy out Þ; (2.11)

where radiant energy consists of long-wave, thermal and
short wave, solar radiation, which can go into and out of
the canopy.
The Penman method of estimating ET ironically requires
no direct measurements of plant characteristics but instead
relies on physical conditions, such as meteorological data,
that are more routinely measured and, therefore, available
to those interested in assessing the application of phytore-
mediation at a particular location. The Penman method is
shown in Eq. 2.12 as

10 6 m 3 ) of water per year, about
20% of the annual yield of the lake. Moreover, when plants
grow in isolated areas, either arid or humid, and receive
warm, dry air from prevailing winds, the additional heat
causes additional evaporation. Under these conditions, ET
can exceed pan evaporation rates.

E ¼ sR nw þ yE a = s þ y ; (2.12)

2.3.8 Measurement of Evapotranspiration

where E is the evaporation rate, s is the slope of the satura-
tion-vapor-pressure curve at the wet-bulb temperature, R nw
is the net radiation over water, y is the psychrometric con-
stant, and E a is a function of the wind-speed and vapor-
pressure deficits.
The Penman-Montieth method is a modification of the
Penman method. It also is a physics-based model that links
the energy budget with aerodynamic conditions, but, unlike
the Penman method, also includes conditions designed to
account for plant feedback on the rate of ET , such as terms
that relate to the resistance of water movement through plant

A variety of methods have been developed to estimate ET for
individual or groups of plants. In general, these methods
include quantification of (1) the mass flux of water, (2) mass
balance methods in basins, and (3) energy balance methods.
Weighing lysimeters, mentioned earlier, also can be used.
Estimation of evapotranspiration also can follow the water-
budget approach described for field studies. Evapotranspira-
tion can be estimated from the rise and fall of the water-table
surface if sufficient groundwater is drawn up to induce water
release from storage; this method is described in detail in