Monitoring Plant and Groundwater Interactions - Introduction to Phytoremediation of Contaminated Groundwater - page 230

have humid growing seasons but dry winters. The VPD is
lowest during the period of highest ET P , solar radiation, and
plant growth, but can be highest during the winter when ET P
is the lowest and plants are dormant.
Wind speed can affect the amount of water removed by
evaporation and can affect the VPD . In areas with little wind,
evaporation of water from open surfaces or leaves is con-
trolled by diffusion gradients. As wind speeds increase,
recently evaporated water close to these surfaces is removed,
and evaporation is increased (Kucera 1954).
Eddy covariance was used by Scott et al. (2003) to exam-
ine the energy and water fluxes of riparian vegetation that
used either shallow soil moisture or groundwater at sites
in southern Arizona. They reported that the water-use
characteristics of the deep-rooted plants under open canopy
conditions was decoupled from atmospheric inputs such as
precipitation, which more strongly controlled water use of
shallower rooted plants. These results confirm the applica-
bility of eddy covariance methods to investigate the linkage
between plants and groundwater. Moreover, these results are
particularly applicable to conditions at recently planted
phytoremediation sites, where the deep-rooted trees have
not reached closed canopy, and where interplanting of
annual or perennial grasses will limit recharge.

in question. Remote sensing technologies can be used to
assess the interaction between plants and groundwater. For
example, moisture levels in plants can be detected using
thermal infrared (Hunt et al. 1987) and passive microwave
sensing. These techniques have been shown to be able to
delineate differences in tree types based on the unique spec-
tral signature of trees. This can be used to identify possible
groundwater use by plants. Becker (2006) provides an excel-
lent review of the application of remote sensing to under-
stand hydrogeologic parameters and plant interactions with
soil moisture and groundwater. Remote sensing works on
large tracts of land such as natural riparian systems, but its
application to the smaller areas planted at phytoremediation
sites is not well known, or at least widely documented.
Remote sensing technologies such as visible, microwave,
and gravity sensors can indicate shallow groundwater levels,
as well as fracture lineaments for preferential flow in bed-
rock aquifers. For example, Rodell and Famiglietti (2002)
suggest that the Gravity Recovery and Climate Experiment
(GRACE) satellite can be used to detect changes in ground-
water storage and may indicate the extent of recharge, for
unconfined aquifer systems. Other methods to detect the
depth to water table include those based on heat capacity.
Because the unsaturated zone contains water and air in
pores, there is a lower heat capacity than when the pores
are filled with water, because of the higher heat capacity of
water. This heat can be detected with thermal

infrared

9.3.4 Water-Balance Equation

imagery.
Electrical capacitance was used by Preston et al. (2004) to
determine the root mass of hybrid poplar trees. The advan-
tage to such a method is that it is non-invasive. The authors
were able to strongly correlate root electrical capacitance
measurements with root dry mass and root wet mass.
If plants known to interact with groundwater can be used
as indicators by remote sensing, then remote sensing will
help detect groundwater, especially in discharge areas under
natural conditions (Klijn and Witte 1999). The component of
evapotranspiration derived from groundwater was examined
using the USGS model MODFLOW and simple moisture
transfer models, and York et al. (2002) concluded that in
Kansas, between 5% and 20% of the ET was derived from
groundwater. These techniques will further the collection of
data that will provide more evidence of the interaction
between plants and groundwater.
In another remote sensing technique, Raman Lidar can be
used to measure the latent heat energy of the water flux from
plant-water vapor. Cooper et al. (2000) used this technique
to map ET from riparian trees in Arizona. The Lidar values
from ET were calibrated to sap-flow measurements.
Limitations of this technique include the requirement for
uniform surface cover and flat terrain. Both the Lidar ET
and sap-flow measurements agreed and indicated the ET was
about 6 mm/day for the forest studied.

For water distribution in a forest Spittlehouse and Black
(1981) presented a form of the water-balance equation that
required inputs of only daily solar radiation, precipitation,
and the minimum and maximum air temperatures. Net radi-
ation is determined by the solar radiation and air temperature
fluctuation.
Another form of the water-balance equation that can be
used to estimate the amount of groundwater removed by
plants was provided in Leenhouts et al. (2006). It is based
on measurements of ET , such that the groundwater taken up,
Q , is given as

Q ¼ ET ð P D S Þ

(9.12)

S is the change in soil moisture
in the shallow soil zone. Essentially, when ET is larger than
precipitation and there is little change in soil moisture stor-
age, groundwater uptake by plants will occur.

where P is precipitation and D

9.3.5 Remote Sensing

Remote sensing is the collection of information about an
object without there being any direct contact with the object