Environmental Engineering Reference
In-Depth Information
The effect on surface-water flow by plant uptake of
groundwater in humid areas becomes more apparent during
times of drought. At the Coweeta Experimental Forest in the
Appalachian Mountains of western North Carolina, for
example, the effect of trees on streamflow was investigated
as early as 1947 (Dunford and Fletcher 1947). Similarly, the
effect of transpiration on the daily fluctuations of groundwa-
ter adjacent to the North River near Annapolis, Maryland,
during the summer of 1954 was observed as daily fluctuations
in surface-water discharge. During a study period of 5 days
in July 1954, the discharge of surface water varied from about
2.74 to 3.29 ft
3
/s each day, for a change of about 0.55 ft
3
/s/
day. If this difference in surface-water discharge was
attributed primarily to water lost daily to transpiration, it
would be about 0.56 acre-ft/day (690 m
3
/day). In a study of
two small watersheds in the Coastal Plain of Georgia, Bosch
et al. (2003) reported that increased evapotranspiration and
plant uptake of groundwater affected the discharge of
groundwater to streams by affecting the hydraulic gradient
between the shallow aquifer and the surface water. During
winter months and reduced evapotranspiration, the
ET
p
ranged from 48% to 74%; during the summer growing sea-
son,
ET
p
exceeded monthly average precipitation (Bosch
et al. 2003). As a result, the water table was higher during
winter months and lower during summer months. Gradients
of the water table toward the surface water approached 3%, or
the slope of the topography, during winter months. During
drier periods, the lack of recharge or uptake of groundwater
by plants dropped the hydraulic gradient to 1% or lower.
Fluctuations in the water table caused by phreatophytes
were used to determine the effect of various approaches to
control use of groundwater by phreatophytes in the south-
western United States (Butler et al. 2005). Phreatophyte
consumption of groundwater prior to discharge is the cause
of decreased surface-water flows in the Cimarron basin
in Kansas. Many different control measures to increase
surface-water flow by reducing the phreatophyte population
have been done, and the metric of effectiveness for increas-
ing streamflow is to monitor groundwater fluctuations. In
simple terms, the presence of phreatophytes causes a diurnal
fluctuation in monitoring wells; as these plants are removed,
the diurnal fluctuations decrease.
In order to affect surface-water flows, the flow rate of
water through the plant, as transpiration, has to be substantial,
there has to be many plants, or both. Van der Leeden et al.
(1990a, 1990b) published a table of the consumptive use of
common phreatophytes in the western United States. The
values are reported as use of groundwater in acre-ft per
acre. Rates range from 3.3 (
Prosopis velutina
) to 7.8 (
Juncus
balticus
) acre-ft per acre (4,068-9,617 m
3
/acre). Measure-
ments of sap flow, a surrogate for transpiration, in riparian
trees were correlated to fluctuations in surface-water flows in
a basin in Oregon (Bond et al. 2002). The daily variations in
surface-water flows were recorded during summer drought
periods, when flows are sustained by groundwater discharge
(base flow) and are a function of the daily fluctuations in sap
flow of streamside trees, such as red alder (
Alnus rubra
Bong.) and the evergreen, Douglas Fir (
Pseudotsuga
menziesii
(Mirb.) Franco). The measured surface-water
flows during summer were lower daily peak flows, and
Bond et al. (2002) used this difference to estimate the amount
of water that the riparian plants would have to transpire from
the groundwater to account for this flow difference; essen-
tially, the tree uptake of groundwater truncates that water
which would have become surface water. Refer to Table
5.1
and
5.2
for additional details from the above studies.
Table 5.1
Groundwater use by phreatophytes growing along riparian systems.
River or State
Acre-ft/acre
a
/year (m
3
/m
2
/year)
Date
Study area (acres, mi or km)
10
8
)
Green river
1950s
552 (168)
40,000 acres (1.6
10
9
)
Utah
1958
175 (53)
440,000 acres (1.7
10
7
)
Idaho
1957
7.5 (2.2)
13,000 acres (5.2
AZ
1950s
9 (2.7)
NA
MD
1954
204 (62)
NA
CA
1996
600 (182)
2 mi (3.2 km)
CA
1995
6,000 (1,828)
NA
means no data available
a
The term acre-feet per acre (
acre-ft/acre
) is equivalent to the depth of water in feet (
ft
), such that 3 ft refers to 3 acre-ft/acre
Table 5.2
Percentage of surface-flow reduction by phreatophyte uptake of groundwater.
Study area
Groundwater removed, in acre-ft/year (m
3
/year)
Total of surface-water flow, in percent
Cottonwood wash, AZ
(with plants)
80 (along 4 river mi)
18
(after plants removed
a
)
42
12
a
Bowie and Kam (1968)
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