Environmental Engineering Reference
In-Depth Information
In his study, Nichols (1993) estimated that greasewood
(
Sacrobatus vermiculatis
) could remove up to 9 in. (24 cm)
of groundwater per year that otherwise would have discharged
to the river.
Other studies of consumptive use of groundwater by
plants and the effects of reduced surface-water availability
can be found in Mower and Nace (1957), Mower et al.
(1964), and Mower and Feltis (1968). Mower and Nace
(1957) reported that phreatophytes use from 2 to 7.5 acre-
ft/year (from 24,670 to 92,475 m
3
/year) of groundwater
within a 13,000 acre (5.2
with tamarisk (
Tamarix ramosissima
) as groundwater
withdrawals have increased to support agricultural, urban,
and industrial needs (Lite and Stromberg 2005). As the depth
to water table increased beyond 11.4 ft (3.5 m) below land
surface, for example, Goodding willows had the highest rate
of mortality, relative to areas where the depth to water table
was more constant. This resulted in a reduced quality
of wildlife habitat and increased potential for flood peaks
and erosion. Results of a study on riparian cottonwoods
along the Mojave River in California also showed higher
tree mortality rates, between 60% and 95%, when
groundwater-level declines were greater than 4.5 ft (1.4 m)
(Scott et al. 1999).
Similar effects of groundwater withdrawals and riparian
plants have been observed by using repeat photography.
Some of the best examples come from the southwestern
United States where mesquite and cottonwood trees were
growing in the riparian area of the Mojave River in 1917 but
after almost 80 years of groundwater development, all native
species are gone, and tamarisk has replaced the native
cottonwoods.
Lines and Bilhorn (1996) reported a decrease in surface-
water flow in the Mojave River in southern California as a
result of groundwater uptake by riparian vegetation. Based
on the annual depletion of surface-water base flow, the
riparian vegetation was estimated to have removed about
600 acre-ft (739,800 m
3
) of water per year along a 2-mi
(3.2 km) long stretch of the Mojave River.
As was outlined in Chap. 1, most of the original studies
of phreatophytes occurred in the arid western United States
where the distinction between phreatophyte and non-
phreatophyte is clear and unambiguous. However, phreatophytes
also are present in more humid, eastern areas of the United
States. In more humid conditions, phreatophytes often are
ignored, because their effect on water supplies is less drastic,
especially if surface water is readily available. Phreatophytes
in humid areas are predominately found where the depth to
water table is shallow, but also can exist where the water
table is more than 100 ft (30 m) below land surface. Along
most flood plains in the eastern United States, phreatophytes
can occupy large areas of point bars. The Congaree National
Park near Columbia, South Carolina, for example, has the
largest stand of bottomland hardwood trees in the United
States, including willow (
Salix nigra
) and poplar (
Populus
deltoides
) along the banks of the meandering Congaree
River. In other low lying spots that receive groundwater
discharge massive loblolly pines (
Pinus taeda
) exist that
are 3 times older than most long-lived loblollies elsewhere,
supporting the usage of the common name loblolly, which
means moist depressions. In the Sand Hills regions of the
eastern United States Coastal Plain, longleaf pine (
Pinus
palustris
) have tap roots that can reach the water table deeper
than 80 ft below land surface.
10
7
m
2
) part of the Malad
Valley, Idaho. In Mower et al. (1964) different methods
were used to determine consumptive use of water by
phreatophytes in the Recos River basin in New Mexico. In
this area of New Mexico, plant roots reach no deeper than
20 ft (6 m) below land surface. Mower and Feltis (1968)
reported that between 135,000 and 175,000 acre-ft
(1.6
10
8
m
3
) of groundwater was consumed
by phreatophytes in a 440,000-acre (1.78
10
8
-2.1
10
9
m
2
) portion
of the Sevier Desert in Utah, more than four times the
amount withdrawn by wells!
Bond et al. (2002) reported that a daily pattern in
streamflow at an Oregon study site could be explained by
the direct transpiration of groundwater, especially during the
summer months when transpiration rates were higher.
Although the decrease in stream base flow was small
(between 1% and 6% of maximum measured base flow), it
was an order of magnitude larger than water losses from
evaporation. The importance of this study in terms of previ-
ously recognized seasonal relations between plants, ground-
water use, and streamflow is that it demonstrates that
this relation is based on the continuous, daily competition
between plants and surface water for groundwater.
Another example of the well-studied interaction between
phreatophytes and groundwater uptake and surface-water
flow patterns is provided in the upper San Pedro basin in
Arizona (Leenhouts et al. 2006). This river changes from an
interrupted perennial stream to a continuous perennial
stream along its course and has a dense riparian community
along most of its length. Cottonwood and willows are the
most abundant tree species in the flood plain. The presence
of cottonwood was correlated to the median annual maxi-
mum depth to groundwater of 6.5 ft (2 m), and for willows
5.9 ft (1.8 m). The researchers showed that the continually
decreasing groundwater levels from 30.8 to 31.8 ft
(9.4-9.7 m) below land surface were caused by increased
evapotranspiration.
In addition to the removal of groundwater by plants
adjacent to rivers, groundwater withdrawals by man also
affect riparian environments. In the San Pedro River valley
in Arizona, the numbers of native riparian trees, such as
the cottonwood (
Populus fremontii
) and Goodding willow
(
Salix gooddingii
), have decreased and been replaced
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