Monitoring Plant and Groundwater Interactions - Introduction to Phytoremediation of Contaminated Groundwater - page 217

have all experienced LAI ; when we are hot and seeking relief
from the sun, we go to sit in planted areas where the shade is
the most complete (a LAI of 10) and avoid areas where some
patches of sunlight are making it to the ground ( LAI

T ¼ sLAI = L (9.3)

Where T is the transpiration rate (mm/day), s is the sap
flow (kg/day), L is the leaf area of the stem used for sap flow,
and LAI is the Leaf Area Index (Allen and Grime 1995).

<

10).
Typically, more leaves equate to a higher rate of transpi-
ration. More leaves can mean either a higher number of
small leaves or a lower number of large leaves. For example,
a deciduous forest on the eastern part of the United States
may have a leaf surface area up to four times greater than the
ground area the forest grows on. A maple tree's trunk covers
1 square yard, but the leaves, of which there may be as many
as 100,000, have a combined surface area of more than 2,000
square yards, almost 0.5-acre (2,023 m 2 ).
In the mid-1990s, researchers in western Australia
observed a relation over time between LAI , precipitation,
and VPD . They were able to relate the seasonal increase in
soil moisture with an increase in LAI (Rural Industries
Research and Development Corporation 2000; Figs. 9.9
and 9.10 ). At the site they investigated, the climate is
characterized by wet winters and dry summers.
The LAI also can be used to estimate transpiration rates.
This can be done with the assumption that transpiration is
directly related to leaf area by

9.2 Hydrogeologic Monitoring Methods

Although plants interact with groundwater as outlined in
Part I, there is a paucity of data on how this interaction
actually can affect groundwater levels at phytoremediation
sites, and how this information can be used to assist water
managers in making decisions about using plants to achieve
hydrologic-based remedial goals at sites. This lack of infor-
mation may simply reflect the fact that the water-table ele-
vation history at a particular site often consists of one-time
measurements made on the day of the sampling event, rather
than continual measurements made over time. As such,
only gross changes in groundwater levels and flow can be
detected using this conventional way of monitoring ground-
water levels. In fact, Quinn and Johnson (2005) suggest that
the frequency of groundwater-level data collection may be
as important as the total number of wells being monitored.

Fig. 9.9 Relation of LAI to
precipitation for two species of
Eucalyptus (Modified from Rural
Industries Research and
Development Corporation 2000).
One millimeter is equivalent to
0.039 in.

Fig. 9.10 Relation of LAI to
VPD (in kilopascals) for two
species of Eucalyptus (Modified
from Rural Industries Research
and Development Corporation
2000).