Environmental Engineering Reference
In-Depth Information
5.2.2 Increase in Surface-Water Flow
estimated that phreatophytes consumed 80 acre-ft/year of
groundwater that, therefore, was not available to supply
the adjacent surface-water body. This represents about
18% of the total surface-water flow (Table
5.2
). After the
phreatophytes were eradicated, a 50% reduction in ground-
water removal by plants was observed, but stream flows
increased by only 6%. In a different study, the water-budget
method was used to determine evapotranspiration for
saltcedar that grew along the Gila River flood plain in
Arizona (Hanson and Dawdy 1976). The study area was
about 5,500 acres (2.2
Plants can decrease surface-water flow, but can their
removal also increase flow? The effect of clear-cutting hard-
wood forests on stream hydrology and ecology was studied
at the Hubbard Brook Experimental Station in the 1970s.
Overall, removing trees, whether by logging, infestation, or
fire, increases the amount of water available for recharge,
and the discharge to local streams increases. This increase in
discharge is assumed to be a result of the increase in ground-
water discharge, or base flow (Bosch et al. 2003; Verry
2003).
Riparian plants in the western United States, which is
characterized by little annual precipitation, can affect the
direction of flow between surface water and groundwater.
For example, the Gila River in Arizona flows into the San
Carlos Reservoir. The area is arid averaging less than 4 in.
(10 cm) of precipitation per year. Melt water from adjacent
mountains is the major source of water to streams in
Arizona, including the Gila River. Because of the coarse
characteristics of the surficial deposits across the lowlands
of Arizona, segments of streams commonly act as sources of
recharge to groundwater, because the total head is higher in
the streams than in the shallow aquifers. Many phreato-
phytes along the banks of these losing-stream segments
send roots to the shallow water table.
A 10-year study, conducted by the USGS and called the
Gila River Phreatophyte Project, began in 1962. The study
area was along a 15-mi (24 km) reach of the Gila River. Up
to 6,000 acres (2.4
10
7
m
2
). Measurements were
made of the various components of water inflow and outflow
both before and after the phreatophytes were removed. Fol-
lowing plant removal, the measured evapotranspiration
decreased as much as 45% along one reach of the river; the
effect on surface water was not given.
The relation between tree removal and increased surface-
water flow is not always directly related. In a study area in
Australia, a setting with eucalyptus trees, which are deep-
rooted phreatophytes, produced little stormwater runoff to a
nearby stream relative to a comparable area cleared of the
same trees (Le Maitre et al. 1999). Other studies also indi-
cate that eradication of phreatophytes does not always lead
to increased surface-water flows (Collings and Myrick
1966). Researchers concluded following modification of
the basin vegetation, that no statistically significant differ-
ence occurred in streamflow before or after plant removal.
However, this may be because the coniferous plants studied
were not directly linked in the first place to discharging
groundwater and would have affected water flowing to the
surface-water body only by changes in runoff.
A lesson could be learned about the importance of these
naturally occurring, plant and groundwater interactions, and
their affect on water resources from the experiences of
ancient Greece. A then prosperous city called Ephesus was
located near the mouth of the Cayster River that emptied into
the Aegean Sea. This location provided easy access by way
of Ephesus' port, to the trade and commerce that could occur
with the then known world. The city enjoyed prosperous
times even after becoming part of the Roman Empire and
even after the sackings of the city by the Goths in 260
AD
and
by Arabic tribes in the early 700s. But the city still pros-
pered. The city finally fell, however, to a seemingly incon-
sequential factor; the port had filled in with sediment carried
to it by the Cayster River, and the city had been effectively
cut off from the Aegean Sea and its link to trade (Freely
2004). What caused this siltation after so many years of
prosperity? In one word-growth. As the city population
increased, more land came under cultivation and large tracts
of native forests were cut down. Not only did this loosen up
the soil and increased erosion, the removal of the forests
caused the water table to rise, as it was no longer being used
for transpiration. In such a low-lying area,
10
7
m
2
) were covered by phreatophytes,
mostly saltcedar and mesquite. The major component of
flow to the shallow aquifer was from the Gila River which
was monitored by using wells and streamflow
measurements. To test the hypothesis that phreatophytes
were causing the surface water to leak into the shallow
aquifer because the plants were depleting local groundwater
levels through transpiration, the plants were removed from
the study area during 1966-1967. This resulted in the losing
segments of the stream to becoming gaining segments
(Culler et al. 1982; Winter et al. 1998).
As would be expected, the greatest effects of phrea-
tophytes on surface-water resources are in small rivers or
lakes where the amount of surface water is small relative to
the amount of groundwater discharge. Bouwer (1975)
presented a procedure for determining the effects of
increased surface-water flow that resulted from the removal
of phreatophytes that grow in flood-plain sediments. The
removal of the phreatophytes permitted more groundwater
to discharge to the surface-water body.
Bowie and Kam (1968) investigated the change in water
use that would occur after riparian plants were removed
along a section of the appropriately named Cottonwood
Wash, Arizona. Along a 4-mi (6.4 km) reach,
it was
this caused
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