Environmental Engineering Reference
In-Depth Information
land surface. The distinction between obligate and faculta-
tive phreatophytes defined previously often is dependent on
the initial root distribution with depth, the depth to water
table, and the degree of water-table fluctuation; even non-
phreatophytic plants can use groundwater if the water table
rises into a shallow root zone.
In retrospect, the desert ecosystem turned out to be the
best area for O.E. Meinzer to suggest that an interaction
between plants and groundwater occurs. This is because
the plants he believed to rely on groundwater often were
restricted to discrete groups and were physically separated
from the plants that relied on precipitation. To draw such
conclusions about the interaction between plants and
groundwater would have been more difficult to make, for
instance, in humid regions, because humid areas offer more
sources of water to plants.
As we will see in Chap. 9, phreatophytes can use alternative
sources of water, such as precipitation or artificial irrigation,
either simultaneous to using the water table or not. For exam-
ple, although wild alfalfa (
Medicago sativa
) is classified as a
phreatophyte, it can be irrigated for commercial purposes and
apparently has not suffered from being removed from its
native groundwater source (Robinson 1958).
Making the generalized observation between plant occur-
rence and distribution and the relation to groundwater appar-
ently was not enough to satisfy O.E. Meinzer's curiosity about
the effect of phreatophytes on groundwater resources.
Meinzer realized the potential economic implications of his
observations of phreatophytes as being (1) potential indicators
of the presence of sustainable groundwater resources and (2)
as sources of competition with man for limited groundwater
supplies. Meinzer wanted to use phreatophytes to help deter-
mine the potential water yield of aquifers in arid areas and to
produce maps showing the location of watering holes for the
growing population in desert areas, one of the many tasks
assigned to the USGS at that time.
As would be expected at the turn of the twentieth century
with such economic issues at stake, O.E. Meinzer and a few
other hydrogeologists sought to more fully develop the clas-
sification of phreatophytes in arid regions by relating the
presence of phreatophytes to the known hydrogeologic
properties of the desert. Although the distribution of desert
plants with respect to water availability had received attention
from plant physiologists before O.E. Meinzer's investiga-
tions, the discriminating factors they used to ascribe
differences in plant distribution were changes in soil chemis-
try, such as alkalinity, not the depth to the water table.
A modern example of the linkage between plants,
groundwater distribution, and groundwater supplies in the
desert areas of the United States is provided by the aptly
named resort town of Palm Springs. Located in the desert
region of southeastern California, Palm Springs receives an
average of 3-4 in. (7.6-10.1 cm) of precipitation annually in
the valleys, with a little more in the adjacent mountains.
Palm Springs lies within the Coachella Valley, formed
between the Little San Bernardino and San Jacinto
Mountains. The valley was formed when these mountains
were uplifted during geologic activity that occurred along
normal faults. These faults, or breaks in the rocks, created
zones of impermeable, broken fragments of rock and clay.
Groundwater recharged in the mountains tended to accumu-
late in these lower permeability zones at the faults and was
discharged to land surface as springs. The water from these
springs typically is hot, being geothermally heated deep
underground. These springs were used by Native Americans
for many centuries before the arrival of European explorers
and then American pioneers. In 1920, J. Smeaton Chase, a
resident of the then small town of Palm Springs, perhaps
unknowingly wrote of this relation between geology, plants,
and groundwater when he stated that Palm Springs was
the child of the mountain, for it lives in the mountain's protection
and is nourished out of its veins
Chase (1920)
Geologically controlled springs in arid areas that offered
a constant supply of groundwater also offered refuge from
the heat underneath the large fan palms (
Washington filifera
,
America's only native palm) that typically grow there. These
palms have shallow root systems that are less than 20 ft deep.
At many oases, palm distribution is aligned in the direction
of the fault. This relation among geology, plants, and
groundwater led to the rapid development of Palm Springs,
and also brought unintended hydrologic changes. Prior to the
1960s, for example, people flocked to Palm Springs because
the relative humidity of this area was a comfortable 3-5%.
However, the increased numbers of people, landscape
plants, gardens, irrigated golf courses, and swimming pools
have increased the relative humidity to 20-30%!
Knowledge of the location of these fault-induced springs
and subsequent oases was essential in order to survive in this
arid area. The USGS recognized this need and was tasked to
prepare maps of the springs, or water holes, in these areas.
As an added benefit, the locations of springs also indicated
the locations of major faults in the area—information needed
for hazard assessment. The interactions among deep under-
ground faults, upwelling groundwater, and phreatophytes are
referred to as vegetated scarps (Figs.
1.6
and
1.7
).
The relation among geology, plants, and groundwater is
not unique to the deserts of the United States. The Negev
Desert in the southern part of Israel, which is situated in the
desert zone that extends from northern Africa, or the Sahara,
to the Rub' Al Khali in Saudi Arabia, is characterized by rift
valleys that occur between the Sinai Peninsula in Egypt and
the Negev Desert, caused by the Syrian-African rift. Here lie
the Gulfs of Suez and Elat and the Dead Sea. Along the
many fault
lines in these areas, upwelling groundwater
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