Environmental Engineering Reference
In-Depth Information
trees with agricultural crops; likewise, Leduc
et al . ( 2001 ) traced the continuous rise in water
tables since the 1960s in southwest Niger to
removal of native vegetation. Groundwater
levels in Memphis, Tennessee declined from
1935 to 1975 ( Figure 6.2a ) as rates of pump-
ing increased; after 1975, pumping rates and
water levels stabilized (Taylor and Alley, 2001 ).
Seasonal fluctuations in groundwater levels are
common in many areas due to the seasonality of
evapotranspiration, precipitation, groundwater
pumping, and irrigation ( Figure 6.2b ). Short-
term water-table fluctuations occur in response
to rainfall, pumping, barometric pressure fluc-
tuations, evapotranspiration, and other phe-
nomena ( Figure 6.2c ).
The WTF method is most often applied
for short-term water-level rises that occur in
response to individual storms, conditions that
typically exist in humid regions with shallow
water tables. Wetting fronts moving downward
within the unsaturated zone tend to disperse
with increasing depth, so water tables in deep
aquifers may not react to individual storms but
will instead display seasonal rises and falls.
The WTF method, however, can also be applied
using these seasonal or long-term water-table
fluctuations.
Groundwater levels rise and fall in response
to several phenomena. Application of the WTF
method for estimating recharge requires iden-
tification of the water-level rises that are attrib-
utable to recharge from precipitation or a
surface-water body. It can be a difficult task. The
following sections give some details on mecha-
nisms other than recharge that can induce
short-term fluctuations in the water table.
(a)
15.2
Well Sh:P-76 at Memphis
Land-surface altitude: 87.5 m
Well depth: 148.7 m
21.3
Missing
record
27.4
33.5
39.6
45.7
1925
1935
1945
1955
1965 1975
1985
1995
(b)
5.5
6.1
6.7
7.3
7.9
Oct
1986
Oct
1988
Oct
1990
Oct
1992
Oct
1994
Oct
1996
(c)
2.85
2.90
2.95
3.00
25 26 27 28 29 30 31 12345678
Aug
Sept
Figure 6.2 (a) Water level trends in an observation
well in Memphis, Tennessee between 1928 and 1995
(Taylor and Alley, 2001 ); (b) Hydrograph of daily water-
level measurements for a well in Vanderburgh County,
Indiana (Taylor and Alley, 2001 ); (c) Diurnal fluctuations in
response to evapotranspiration by alfalfa in the Escalante
Valley, Utah, August 25 to September 8, 1926. Alfalfa was
cut on August 3 (White, 1932 ).
Evapotranspiration
Shallow water tables may exhibit diurnal
fluctuations, declining during daylight hours
in response to evapotranspiration and rising
through the night when ET gw is essentially
zero. Figure 6.2c shows diurnal fluctuations
in depth to the water table beneath a field of
alfalfa in the Escalante Valley of Utah before
and after cutting (White, 1932 ). White ( 1932 )
developed a formula similar to Equation
( 6.2 ) for estimating ET gw based on such
fluctuations. He assumed that ET gw was zero
between midnight and 4:00 a.m. and defined
h' as the hourly rate of water-table rise during
those hours ( Figure 6.3 ). The total amount of
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