Environmental Engineering Reference
In-Depth Information
many perennial streams in the northern
High Plains is important (Weeks
et al
.,
1988
).
Drainage rates within the unsaturated zone
were estimated by using the tritium profile
method (
Figure 9.5
) to be 70 mm/yr beneath
undisturbed rangeland and 102 to 110 mm/yr
beneath irrigated fields (McMahon
et al
.,
2006
).
At one of the irrigated sites, bomb-pulse trit-
ium was not detected in the unsaturated
zone at depths of 30 to 40 m but was detected
in groundwater, at a depth of about 48 m.
McMahon
et al
. (
2006
) concluded that recharge
to the aquifer was occurring locally as focused
recharge through nearby playas that collect
surface runoff. Under natural conditions,
drainage past the bottom of the root zone typ-
ically occurs from late fall into spring; how-
ever, excess irrigation can cause drainage to
occur during the growing season.
Predevelopment Conditions
Recharge
from
precipitation
277
Discharge
to streams
and
springs
277
Water in storage,
predevelopment
592 billion m
3
Development Conditions in 1997
Recharge from
precipitation
and irrigation
return flow
735
9.4.6 Unglaciated Central Region
This expansive area extends from the Rocky
Mountains eastward to the Appalachian Plateau
and Valley and Ridge physiographic province
and consists of rolling plains and plateaus
underlain by sedimentary rocks (Heath,
1984
).
Dolomitic limestone and sandstone aquifers of
moderate yield are common, and alluvial aqui-
fers exist along major streams, but the region
also contains some very productive aquifers,
including the Edwards Limestone and the
Trinity Group in Texas and the Ozark Plateaus
aquifer system that lies mostly in Arkansas and
Missouri. Large cave systems are a prominent
feature of this region.
Recharge to the bedrock aquifers commonly
occurs in upland areas where bedrock outcrops
and can be diffuse or focused from streams. It
is difficult to make broad statements as to the
applicability of different methods for estimat-
ing recharge in this region because of the varia-
bility in climate (precipitation ranges from 400
to over 1200 mm/yr) and aquifer characteris-
tics. Many approaches have been taken, includ-
ing water-budget studies (Puente,
1978
), tracer
methods (Busenberg and Plummer,
1992
), water-
shed and groundwater flow models (Czarnecki
et al
.,
2009
), and streamflow hydrograph analy-
sis (Hoos,
1990
).
Discharge
to wells
3230
Discharge
to streams
and springs
186
Decrease in water
in storage,
predevelopment to 1997
107 billion m
3
Water in storage,
1997
485 billion m
3
Figure 9.4
Groundwater budget in the southern part
of the central High Plains aquifer during predevelopment
and in 1997. Values without units are in million m
3
/yr (after
Luckey and Becker,
1999
).
In the northern High Plains, groundwater
levels in many areas rose between 1950 and
2000 (McGuire
et al
.,
2003
). Surface water is
the primary source of irrigation water in these
areas; irrigation return flow supplements nat-
ural recharge and has led to a rise in ground-
water levels. Diffuse recharge is the dominant
mechanism, but focused recharge from the
