Environmental Engineering Reference
In-Depth Information
studies, attempts to use the recharge estimates
of Dugan and Peckenpaugh (
1985
) resulted in
excessively high computed water levels.
groundwater-flow modeling produced an esti-
mate of 48 mm/yr. The discrepancy was attrib-
uted to uncertainty in values of unsaturated
and saturated hydraulic conductivity.
Streamflow hydrograph analysis methods
are commonly applied in this region because
most streams are gaining streams. Holtschlag
(
1997
) used streamflow hydrograph separation
in conjunction with regression analysis to esti-
mate average base flow across the lower penin-
sula of Michigan for the period 1951 to 1980 at
about 211 mm, or about 28% of precipitation.
Similar approaches were used to estimate base
flow across the states of Ohio (Dumouchelle and
Schiefer,
2002
), where estimates ranged from
100 to about 300 mm/yr, and Minnesota (Lorenz
and Delin,
2007
), where estimates ranged from
less than 50 to 290 mm/yr (
Figure 3.17
).
Szilagy
et al
. (2005) combined base-flow esti-
mates derived from hydrograph-separation
analysis with water-budget estimates of eva-
potranspiration of groundwater to derive esti-
mates of total recharge for the state of Nebraska;
recharge estimates averaged 48 mm/yr and
increased across Nebraska from west, where
rates were less than 30 mm/yr, to east, where
rates exceeded 140 mm/yr. Recharge accounted
for 3 to 26% of annual precipitation. The meth-
ods used by Szilagy
et al
. (2005) were applied
statewide across multiple groundwater regions
as defined in
Figure 9.1
. Methods are not lim-
ited to specific regions, and divisions between
regions should not interfere with application of
methods.
Models are commonly used to estimate
recharge in this region, as is true in all regions
discussed in this chapter. Groundwater-flow
models (Mandle and Kontis,
1992
; Yager,
1996
;
Juckem and Hunt,
2007
) are used more than
other models for this purpose, but recharge esti-
mates have also been obtained from application
of watershed models. Steuer and Hunt (
2001
)
used a watershed model to examine the pos-
sible impacts of urbanization on the Pheasant
Branch watershed near Madison, Wisconsin.
Natural recharge rates, prior to development,
were estimated to be about 220 mm/yr (25% of
precipitation). Model simulations indicated that
with urbanization of the watershed, the average
9.4.7 Glaciated Central Region
The Glaciated Central region, which overlies the
north-central United States, is covered by drift
deposited by continental glaciers. The drift can
be as much as 300 m thick, and although it con-
sists mostly of fine-grained rock debris, the area
has many sand and gravel aquifers (principally,
outwash plains and buried bedrock channels)
that can yield substantial quantities of water
(Heath,
1984
). Groundwater also exists in the
underlying limestone, dolomite, and sedimen-
tary bedrock aquifers in many areas. Climate
is subhumid to humid, except for the westward
extent, which is categorized as semiarid.
Recharge to the glacial deposits is mostly
diffuse, occurring over large areas in response
to precipitation. Recharge can occur through-
out the year, but most occurs in the spring, as
soils thaw, and in the fall, after plant senes-
cence and before soils freeze. Natural ground-
water discharge from the glacial deposits is to
streams, to plants, and to underlying bedrock
aquifers.
Most methods described in this text have
been applied at one time or another in some
part of this region, although the use of unsatu-
rated-zone tracer methods is generally lim-
ited to the more arid western areas. Rehm
et
al
. (
1982
) applied the water-table fluctuation,
Darcy, and flow-net methods (
Section 6.3.3
) to
estimate recharge in a 150 km
2
area of central
North Dakota on the basis of groundwater lev-
els monitored in 175 wells and piezometers;
recharge estimates varied in space (higher rates
occurred where soils were more permeable and
where runoff accumulated in surface depres-
sions) and in time (most recharge occurred in
spring). Average recharge for the entire area
was estimated at between 10 and 40 mm/yr (2%
to 9% of precipitation). Healy (
1989
) and Healy
et al
. (
1989
) applied multiple methods to esti-
mate recharge at an 8 ha site in northwestern
Illinois (
Section 2.3
); the unsaturated zone
Darcy method produced an average estimate
of 208 mm/yr (22% of precipitation), whereas
