METEORIC WATER SOURCES OF SURFACE WATER AND GROUNDWATER AND
IMPLICATIONS FOR CLIMATE WARMING EFFECTS IN THE SOUTHERN ROCKY MOUNTAINS
Fengjing Liu1, Roger Bales1, Mark Williams2, and Martha Conklin1
1 University of California, Merced, California; 2 University of Colorado, Boulder, Colorado
Groundwater
(n = 61)
Winter (Oct-Jun)
Summer (Jul-Sep)
-200
Surface Water
n = 42)
Groundwater
LMWL
Method 2
(a) Monthly Arithmetic Mean Isotope in Precipitation
-25
Method 3
LDV
Winter
(n = 17)
-50
LMWL
δD = 8.3δ18O + 15.8
R2 = 1.0
-100
LDV
Groundwater
(n = 111)
-150
Winter (Oct-Jun)
-200
Summer (Jul-Sep)
-250
Surface Water
Groundwater
LMWL
Method 1
1971-2004
120
Summer
(n = 60)
2000-2004
LDV
Method 2
100
80
Method 3
60
Method 1
40
20
Surface Water
(n = 38)
δ18O in winter and summer precipitation was distinct but varied
significantly (Figure 3). δ18O in surface water and groundwater was
less variable than in precipitation and confined by δ18O values in
winter and summer precipitation. δ18O was about -20‰ from November
to March at both GLV and LDV and became gradually enriched toward
August at about -7‰ (Figure 4a). δ18O in precipitation at VC was
much more enriched than that at GLV and LDV during winter, but
very close during summer.
(b) GLV
140
Precipitation (mm)
0
Precipitation was dominated by winter snow at GLV, with 85% of
annual precipitation occurring from October to May (Figure 4b).
Precipitation patterns were significantly different at LDV and VC
(Figures 4c and 4d). Precipitation from summer monsoon contributed
35% at LDV (July-September) and 63% at VC (July-November).
Method 2
0
Method 3
60
(c) LDV
50
Winter
(n = 44)
-100
VC
Summer
(n = 48)
LMWL
δD = 7.6δ18O + 7.0
R2 = 0.93
-50
Groundwater
(n = 11)
VC
-250
-25
-20
-15
-10
Method 2
Method 3
140
-25
-20
-15
-10
-5
0
120
δ18O (‰)
-5
0
18
δ O (‰)
Figure 2 Local Meteoric Water Line
(LMWL) determined by winter and summer
precipitation samples; also showing surface
water and groundwater samples.
VC
Method 3
20
0
-300
-30
Method 2
30
Surface Water
(n = 24)
LMWL
-35
2001-2003
Method 1
Figure 3 Boxplots showing median, 25% and
75% quartiles (boxes), and 5% and 95%
quartiles (whiskers) of δ18O values in winter
and summer precipitation, surface water and
groundwater.
Methods
Samples were collected from snow, rainwater, streamflow, groundwater, and springs
from 2000 to 2005 (years vary at each site; see Figure 4a for details). Geothermal
groundwater collected in 1980s at VC from Vuataz and Goff et al. [1986] was also
included in this study.
Contribution of winter and summer precipitation to surface water and groundwater
was quantified using a two-component mixing model that was usually used for
new and old waters following equation 1, where f is fraction and C is isotopic
compostion in the components indicated by subscripts.
C
− Cstream
fwinter = 1 − fsummer= summer
Csummer − Cwinter
(1)
Precipitation (mm)
-200
40
10
-150
Winter (Dec-May)
Summer (Jun-Nov)
Surface Water
Groundwater
Method 1
1949-2005
Precipitation (mm)
-300
δ D (‰)
Method 1
VC (1990-1993)
160
0
Research sites were Green Lakes Valley (GLV)
in Colorado Front Range, Leadville (LDV)
in southern Colorado and Valles Caldera (VC)
in northern New Mexico (Figure 1). Elevation
ranges from 3,250 to 4,000 m in GLV, 2,960
to 3,800 m in LDV and from 2,300 to 3,430
m in VC. GLV is a typical alpine drainage,
while LDV is alpine-subalpine in nature with
many abandoned mines, shafts, and bedrock
wells that allow for bedrock groundwater
sampling. The lower part of LDV is forested,
while the VC drainage is entirely covered by
vegetation with ponderosa pine and meadows
at lower elevations and mixed conifer at higher
elevations.
GLV (2000-2004)
LDV (2001-2003)
-300
Research Site
Method 3
-15
-20
Surface Water
-250
Method 2
-10
18
GLV
-150
GLV
(d) VC
40
1931-2005
100
1990-1993
50
60
70
80
90
100
Composition of Winter Precipitation (%)
To parameterize winter and summer precipitation for δ18O, three methods
were used: arithmetic monthly mean (Method 1) from available samples,
precipitation-weighted mean by event-based precipitation (Methode 2)
and by long-term monthly mean precipitation (Method 3). The δ18O
values in winter and summer precipitation from these methods are
listed in Table 1.
2004-2005
80
Figure 5 Percent of winter and summer Surprisingly, the results indicated that groundwater was composed of
precipitation composition in surface water and
80-90% of winter precipitation at all three sites despite the different
groundwater; median with 25% and 75% quartiles
precipitation patterns (Figure 5). Surface water consisted of slightly
(boxes), and 5% and 95% quartiles (whiskers).
60
40
20
0
Jan Feb Mar Apr May Jun
Jul
Aug Sep Oct Nov Dec
Figure 4 Arithmetic monthly mean δ18O in precipitation (a), long-term monthly
mean precipitation, monthly mean precipitation during sampling period at GLV
(b), LDV (b) and VC (c). Isotopic data in precipitation at VC was from Adams et
al. [1995].
Table 1 Arithmetic and precipitation-weighted mean δ18O in winter and
summer precipitation calculated from Figure 4
more summer precipitation than groundwater at all sites except at VC.
Winter precipitation contributed more than 70% to surface water at
LDV and VC. Though summer precipitation was significant at LDV
and dominated at VC, plant use of soil water during growing seasons
may significantly reduce groundwater recharge during summer and
thus groundwater was primarily from winter precipitation. Groundwater
in forested, seasonally snow-covered catchments is much more vulnerable
than that in alpine catchments where ET is very low.
Conclusions
Method 1
Method 2
Method 3
Arithmetic
Event PPT-Weighted
Long-Term Monthly PPT-Weighted
Annual
Winter
Summer
Annual
Winter
Summer
Annual
Winter
Summer
GLV
-15.58
-17.81
-8.49
-18.51
-19.51
-8.87
-17.06
-18.71
-8.62
LDV
-11.60
-20.20
-9.20
-19.44
-19.96
-9.41
-15.22
-18.66
-8.60
VC
-11.24
-13.70
-8.83
-11.23
-13.48
-8.95
-10.32
-13.50
-8.44
References
Figure 1 Research sites in southern
Rocky Mountains
Results and Discussions
The Local Meteoric Water line (LMWL) indicates that surface water
and groundwater did not experience significant evaporation (Figure 2).
Stable isotopic composition in surface water and groundwater is a
result of mixing of winter and summer precipitation.
Groundwater
Summer
(n = 112)
δ O (‰)
-100
Method 1
-5
Surface Water
Winter
(n = 121)
LMWL
δD = 7.9δ18O + 9.4
R2 = 0.99
-50
Partitioning of Water Sources
0
GLV
GLV
0
δ D (‰)
A first step in estimating the potential effects
of climate change on groundwater is to
understand how seasonal precipitation patterns
contribute to groundwater recharge. The shift
to earlier runoff due to less winter precipitation
falling as snow and the melting of winter
snow
earlier in spring [e.g., Mote et al., 2005;
Dettinger and Cayan, 1995; Cayan et al.,
2001] is of particular concern. At 3 sites in
the southern Rocky Mountains, surface water
and groundwater were separated into winter
and summer precipitation sources using stable
isotopes (δ18O and δD).
Variability of Precipitation and δ18O
Local Meteoric Water Line
δ D (‰)
Introduction
Adams, A. I., F. Goff, and D. Counce (1995), Chemical and isotopic variations of precipitation in the Los Alamos region, New Mexico, Los Alamos National Laboratory Report, LA-12895-MS.
Cayan, D. R., S. A., Kammerdiener, M. D. Dettinger, J. M. Caprio, and D. H. Peterson, Changes in the onset of spring in the western United States, Bulletin of American Meteorological Society,
82, 399-415, 2001.
Dettinger, M. D. and D. R. Cayan, Large-scale atmospheric forcing of recent trends toward early snowmelt runoff in California, Journal of Climate, 8, 606-623, 1995.
Mote, P. W., A. F. Hamlet, M. P. Clark, and D. P. Lettenmaier, Declining mountain snowpack in western north America, Bulletin of American Meteorological Society, 86, 39-49, 2005.
Vuataz, F. D. and F. Goff (1986), Isotope geochemistry of thermal and nonthermal waters in the Valles Caldera, New Mexico, Journal of Geophysical Research, 91(B2), 1835-1853.
A decrease in winter precipitation is much more important than a
decrease in summer precipitation. That is, in terms of groundwater
recharge, a decrease in winter precipitation could not be offset by an
increase in summer precipitation. The effect of a change in winter
precipitation on groundwater will depend on the response of ecosystems,
e.g., plant water use. This would be reflected in a change in the
relative isotopic composition of groundwater.
Acknowledgements
Funding was provided by NSF through LTER and SAHRA and by EPA through CHIMP
project.