Monitoring Plant and Groundwater Interactions - Introduction to Phytoremediation of Contaminated Groundwater - page 227

Fig. 9.19 The concept of
hydraulic lift, in which water is
distributed vertically from wetter
sediments to drier shallower
sediments, as shown using stable
hydrogen isotopes. This is not
direct groundwater uptake, as the
water potentials are negative at
the starting point of the root hairs
(Modified from Richards and
Caldwell 1987).

This upward flow of water from deeper zones to a drier
unsaturated zone is a principle mechanism of unsaturated
flow in arid regions (Andraski et al. 2005). Of course, this
process will continue only as long as there are deeper
sources of water in the capillary zone with less negative
water potentials or groundwater at atmospheric pressures.
If this source of water is beyond the root growth, or dries up,
the plants will wilt and will not recover the following day.
Hultline et al. (2003, 2006) and Leenhouts et al. (2006)
present evidence that the reverse of hydraulic lift can occur,
that is, where water in roots in the shallow soils can move
downward to roots in drier, deeper soils. This process may
explain how phreatophytes whose roots, following the fluc-
tuation of the water table, can be established initially after
germination, and how roots can reach declining water tables
over time.
Other studies that used stable isotopes support the notion
of plant and groundwater interaction. Busch et al. (1992)
reported that Populus and Salix used groundwater, whereas
Tamarix used water from the unsaturated zone, based on
differences in stable H and O isotopes of the different
water sources. Snyder and Williams (2000) used the stable
isotopes of water to determine the source of water to a
riparian forest of poplar and willow, along with a mesquite
understory. The stable H and O isotopes of the soil water,
groundwater, and xylem water were compared. Willows had
xylem stable isotope values similar to groundwater present

at 12 ft (3.6 m), even when frequent precipitation of a
different isotopic signature was available. In contrast, the
source of water to poplars was between 26% and 33% from
the soil layers after precipitation, in addition to groundwater.
Cramer et al. (1999) used stable isotopes of water to investi-
gate the interactions between deep-rooted phreatophytes and
groundwater in Australia.

9.3.3 Meteorology and Plant Characteristics
for Groundwater Uptake

The presence of phreatophytes provides a direct link
between subsurface sources of water and the atmospheric
demand for water. This is significant, because unlike surface
water in lakes, ponds, or streams, groundwater is isolated
from the atmosphere. While some groundwater can move
upward from the water table toward the atmosphere by the
physical process of capillary action, in most cases it has no
direct connection with atmospheric processes. Groundwater
does interact indirectly with the atmosphere with regards to
recharge from precipitation and perhaps changes in water
pressures due to changes in atmospheric pressures, but these
are indirect interactions.
Various mathematical models have been created to esti-
mate the relation between atmospheric conditions and plant
transpiration of groundwater. They include energy balance,