Environmental Engineering Reference
In-Depth Information
width of vegetation near surface-water bodies that cannot be
destroyed.
Carolina, called the Bigwoods Experimental Forest, revealed
that the groundwater level in shallow wells in a stand of
loblolly pine declined during the summer and increased dur-
ing the winter following the clear cutting of a 200-ft (61 m)
long stand of pines (Trousdell and Hoover 1955). Before
cutting, the observed groundwater level declined about 9.5 ft
(2.8 m) during the summer. Clear cutting in late July reversed
the decline to the point that the groundwater level rose 8.8 ft
(2.6 m). In comparison, the groundwater level in a nearby
stand of uncut trees remained low.
The relation between plants and fluctuations in ground-
water levels also was noted in a humid area by Meyboom
(1966), who was studying the poorly drained glacial moraine
areas of the Canadian Plains. The Canadian Plains are dotted
with numerous, small, water-filled depressions, called
sloughs (pronounced sloo). Meyboom noted that these
sloughs were surrounded by willow trees, such as basket
willow (
Salix petiolaris
) near and in the water, and aspen
poplar (
Populus tremuloides
) on higher ground (Meyboom
1966). To investigate the relations of surface water, ground-
water, precipitation, and water use by the willows,
Meyboom employed the nested well approach described in
Chap. 4. The observation wells he installed were essentially
1- to 2-in. (2.5-5 cm) pipes with slots on the bottom and
were installed at various depths. Wells with larger diameters
were drilled and equipped with automatic water-level
recorders, similar to those used by USGS hydrogeologists
such as Meinzer, Brown, and White, as described in Chap. 1.
When evapotranspiration was low in the winter, the level of
water in the sloughs was higher than the water table, and
water moved vertically down to the water table beneath the
slough (Fig.
5.4
). As evapotranspiration increased during the
summer month of July, the flow direction in the water table
reversed to one of higher hydraulic head beneath the slough,
and water moved vertically upward (Fig.
5.4
). This change
in flow direction was caused by the seasonal removal of
groundwater by the willows along the banks of the slough
(Meyboom 1966). During the summer months, Meyboom
concluded that one-fifth of the flow beneath the slough was
diverted by willow uptake.
Subsequent explanations of the interactions of riparian
plants, shallow groundwater, and resultant effects on surface
water were given by Winter (1999), and examples are
provided in Winter and Rosenberry (1995). For example,
groundwater transpired by plants results in a decrease in the
discharge to surface water, and surface water acts as a source
of water
5.3
Plants and Groundwater Levels
The effect of fluctuations in the water table on the distri-
bution of plants in a basin can be explained in areas
where natural fluctuations of groundwater occur. For exam-
ple, in marshes and swamps adjacent to tidally influenced
surface-water bodies, the groundwater table can rise and fall
in response to the daily tidal highs and lows, as well as to
changes in barometric pressure. In such areas, the plant
distribution is related closely to the mean depth to the
water table, rather than the surface-water level. This relation
of groundwater to plant distribution is primarily a result of
less variability in the mean groundwater table in tidal
swamps compared to nontidal swamps. Even a small differ-
ence in mean groundwater level results in a noticeable dif-
ference in dominant plant species distribution. For example,
in a tidal stream in coastal Virginia, a groundwater
table elevation difference of only 1.9 in. (5 cm) produced
ash-blackgum dominated areas relative to maple-sweetgum
areas associated with lower water-table conditions
(Rheinhardt and Hershner 1992). Moreover, these
researchers concluded that contrary to conventional thought,
a more appropriate biological measure of wetness in tidal
swamps should be the mean depth to water table, not the
flooding duration, flooding height, or hydroperiod, as are
more commonly cited.
The relation between plants and groundwater levels in
individual wells also has been a focus of study. G.E.P.
Smith (1915) indicated that the water table declined in wells
installed in areas covered with trees during the growing sea-
son except at night or during dormancy. Other examples of a
similar relation between groundwater table fluctuations and
tree uptake of groundwater were shown by tank experiments
by Lee (1912) and White (1932). The effects of phreatophytes
on groundwater, as evidenced by daily fluctuations in ground-
water levels in wells, were recorded in the Safford Valley in
Arizona in 1944. Little groundwater-level fluctuation was
noted before plant growth began in March. A few months
later, however, after the trees, in this case saltcedar, had leafed
out, the cyclical daily fluctuation observed earlier (as reported
in Chap. 1) became evident, with a maximum observed
decrease in groundwater level, or drawdown, of 0.19 ft
(0.06 m). After the growing season ended in early winter,
the fluctuations decreased (Robinson 1958).
A similar daily fluctuation in groundwater levels near a
forested stream occurred in Michigan (Ferris 1949), which
indicates that phreatophytes in the more humid eastern United
States can affect water supplies, though less obviously. In the
mid-1950s, results of a study at an experimental site in North
to meet
later evapotranspiration demands on
groundwater.
Mower et al. (1964) used the method developed by White
(1932) to determine the amount of groundwater taken up by
transpiring plants. Again, this is the amount of water taken
up by the plant from groundwater—a significant advance in
understanding compared to alternative plant physiology
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