Environmental Engineering Reference
In-Depth Information
NHP rangeland
NHP irrigated corn
NHP irrigated corn
0
10
20
30
40
early 1970s
late 1950s
early 1970s
2002
2002
2002
50
60
0
10
20
30
40
50
0
10
20
30
40
50
0
10
20
30
40
50
Tritium concentration (TU)
Core
Monitoring well
Irrigation well
systems from several watersheds in south-cen-
tral Texas by Ockerman (
2002
,
2005
,
2007
). For
the Upper Cibolo Creek watershed, average
annual recharge between 1992 and 2004 was
estimated to be 98 million m
3
or about 139 mm
when averaged across the watershed, equiva-
lent to 15% of precipitation (Ockerman,
2007
).
Focused recharge from streams accounted for
74% of all recharge; diffuse recharge accounted
for the remaining 26%. A large year-to-year
variability in streamflow and recharge was
attributed to variable amounts of precipitation.
Recharge in 1999 was estimated at 25 mm (6% of
precipitation), whereas the recharge estimate for
1992 was about 340 mm (25% of precipitation).
The average recharge estimate of 128 mm/yr
obtained with HSPF for the Hondo Creek water-
shed (Ockerman,
2005
) was about 22% less than
that obtained by application of a stream water-
budget method proposed by Puente (
1978
).
A soil-water budget model was used to esti-
mate recharge over a large part of the cen-
tral United States by Dugan and Peckenpaugh
(
1985
); estimates of recharge in the Ozark area
ranged between 178 and 380 mm/yr. Imes and
Emmett (
1994
) and later Czarnecki
et al
. (
2009
)
derived much lower estimates of recharge to
the Ozark Plateaus aquifer system by using
groundwater-flow models that were calibrated
with measured water levels and base flows
estimated from streamflow duration curves.
Estimates of recharge determined by Imes and
Emmett (
1994
) were between 34 and 41 mm/
yr; those determined by Czarnecki
et al
. (2009)
ranged from 2 to 89 mm/yr. In both modeling
Figure 9.5
Profiles of tritium extracted from unsaturated
zone sediment cores and tritium concentrations in
groundwater from a rangeland and two irrigated sites in
the northern High Plains (NHP). Vertical dashed lines
represent the estimated maximum concentration of
prebomb tritium in soil water in 2000. Horizontal dashed
lines represent water-table depths at indicated times
(McMahon
et al
.,
2006
).
K a r s t s y s t e m s m a y o c c u r i in l i m e s t o in e , d o l o -
mite, and other carbonate rocks where solu-
tion cavities can act as preferential flow paths
that focus recharge to and discharge from
aquifers. Estimation of recharge rates in karst
systems is particularly challenging; the flow
paths are difficult to trace, and direct meas-
urement of flow through a solution cavity is
seldom possible (Bakalowicz,
2005
). Recharge
processes in karst systems have been stud-
ied worldwide. Natural and applied tracers
are often used to obtain qualitative informa-
tion on flow paths in karst systems (Greene,
1997
; Katz
et al
.,
1997
). Streamflow and spring
hydrograph analysis (Padilla
e t a l
.,
1994
; Pérez,
1997
; Rimmer and Salingar,
2006
) and chem-
ical hydrograph analysis (Nativ
et al
.,
1999
;
Lee and Krothe,
2001
) have been used to deter-
mine base flow. Various modeling approaches
have been used to estimate recharge in karst
systems, including soil water-budget models
(Hughes
et al
.,
2008
), groundwater flow mod-
els (Weiss and Gvirtzman,
2007
), and water-
shed models (Ockerman,
2002
,
2005
,
2007
).
The HSPF watershed model was used to esti-
mate recharge to the Edwards and Trinity aquifer
