Monitoring Plant and Groundwater Interactions - Introduction to Phytoremediation of Contaminated Groundwater - page 228

water balance, and direct measurement, as discussed in
Chaps. 2 and 7. The classic method for estimating potential
transpiration from a grove of plants using solely meteoro-
logical data is provided by the Penman-Monteith equation
(Monteith 1965). As was stated previously, the Penman-
Monteith equation is a physics-based model that couples
the energy budget with aerodynamic parameters and essen-
tially considers that the vegetation in a grove of trees
behaves as one large, well-watered leaf with no moisture
stress and no canopy or stomatal resistance to vapor trans-
port. The equation is based on wind speed, relative humidity,
PAR, precipitation, and air temperature, all of which affect
sap flow and, therefore, transpiration. Water stress occurs
during periods of low humidity and high air temperature and
wind speed (Denmead and Shaw 1960). A modification to
the Penman-Monteith equation was prepared by van Bavel
and van Bavel (1988) into software that computes ET P from
weather station data.
In many areas of the northern hemisphere, the drier atmo-
spheric conditions during the winter seasons, when plants
are dormant, are actually more conducive to transpiration
and, therefore, the uptake of groundwater than during
the more humid and higher precipitation periods during
summer, when plants are growing. During the winter,
water loss can occur through bark, or green tissue such as
stems, where photosynthesis can occur in deciduous plants
that have lost their leaves.
In Texas, the effect of climatic variables on the water use
of poplar trees was measured at the Air Force Plant 4 site. At
a given air temperature, a higher VPD indicates a higher
tendency for the air to accept water vapor and therefore be
conducive to transpiration. As such, sap flow increased for
both whips and 1-year-old trees as the VPD increased (Vose
et al. 2000). For a given VPD , sap flow was lowest during
August but highest during June. Sap flow did not increase
with increasing VPD but reached a plateau after about
1.5 kPa (Vose et al. 2000). At that site, sap flow was linearly
related to solar radiation for all the months observed (Vose
et al. 2000). The highest sap flows were in June and the
lowest in August for both whips and 1-year-old trees
(Fig. 9.20 ).
The lower readings in August at the Texas study site
would be considered contrary to what would be expected,
save for the fact that August was considered to be under
drought conditions, when little precipitation occurred and
low soil moistures of less than 30% were recorded. The
authors stated that severe water stress was avoided because
of two reasons (1) the roots had reached the water table and
(2) leaf fall of upwards of one-half the total leaf area
occurred when VPD and air temperature increased as precip-
itation and soil moisture decreased. It is more likely, how-
ever, that the primary explanation is leaf drop, because no
measurements of groundwater uptake were provided to

Fig. 9.20 Sap flow and VPD for ( ) 1-year-old poplar

trees (Modified from Vose et al. 2000) . One centimeter is equivalent to
0.39 in.

a ) whips and ( b

assess the first hypothesis. If the plantation were older, and
groundwater being used as a source of water, then perhaps
the effect of the drought would have been less apparent, and
water use measurements (sap flow, etc.) would have been
higher.
The VPD is an important parameter for water flow through
plants and can be determined readily at a potential phyto-
remediation site, as indicated in Chap. 3. To review, the VPD
is not a single parameter measured directly, but is calculated
from measurements of air temperature and relative humidity
at the site, or looked up on a table that shows the relation
between air temperature and VPD over a range of relative
humidity. Basically, the VPD is the difference between the
amounts of moisture in the air at a given time relative to how
much moisture the air can hold at saturation conditions, or
100% relative humidity. In equation form, VPD is

VPD ¼ VP ðsaturationÞ VP ðairÞ (9.11)

The VP , or vapor pressure, is the amount of water vapor
present, and a higher VP means more water vapor is present.