Water Balance and Tree-Ring Approaches to
Understanding Climate Change Impacts
On the Yellowstone River
Stephen T. Gray
University of Wyoming
Gregory J. McCabe
US Geological Survey
Predictions for future precipitation
-Small to moderate changes by mid 21st century
-Predictions within the range of variability over
the past 1,000 years
Predictions for future climate in the Upper Colorado River Basin
Based on regionalized output from seven leading climate models
temp change vs. 1951-1980 mean
2 to 4 °C (3.6 to 7.2 °F)
within our lifetimes
All assessments agree — the West will be WARMER!
Warming = Perpetual Drought?
No significant change
In precipitation plus
1.4 °C temp. increase
Small increases in
temperature lead to
increased evaporation
and decreased
water yield to streams
No significant change
In precipitation plus
2.8 °C temp. increase
Calculated Palmer Drought Severity Index Values
Source: Hoerling and Eischeid - SW Hydrology, 2007
Yellowstone River Sensitivity Study
How might river
flows change when
increasing temps
are combined with
natural precipitation
variability?
McCabe and Gray, In Prep.
Yellowstone River Sensitivity Study
Tree-ring precipitation
(range of natural variability)
Temperature Scenarios
(Past, Present and Future)
RIVER FLOW MODEL
Estimates of Discharge
(Flow at Corwin Springs)
McCabe and Gray, In Prep.
Upper Yellowstone River
Yellowstone @
Corwin Springs
•Headwaters above Corwin
Springs, Montana
•Drains approx. 6794 km2
•Majority in Yellowstone
National Park
•Unregulated flows
•Gage in continuous operation
since 1911
Monthly PRISM
Monthly PRISM
Water Balance
Accounting Model
Monthly or
Water Year
Stream Flow
(Discharge)
Wolock and McCabe,1999
McCabe and Wolock, 2007
Modeling Water Year Runoff: Yellowstone at Corwin Springs
r = 0.85
Climate Scenarios
Temperature
Precipitation
• 1896-1995 Averages
• Predicted 2025, 2050
and 2100
– IPCC AR4
– 50th percentile A1B
• Reconstructions
– Moberg et al. 2005
– D’Arrigo et al. 2004
• Tree-ring reconstruction
– 1173-1996
– Originally annual
– Disaggregated to
monthly values using:
• 1896-1995 Climatology
• 1896-1995 Climate subdivided into Drought,
Wet and “Normal”
Scenarios
Reconstructed Annual Precipitation: Upper Yellowstone
Gray et al. 2007, Quat. Res.
Disaggregated Tree-ring Precipitation
Jan
Feb
Mar
Apr
May
Jun
Corr
0.399
-0.133
0.097
0.077
0.358
0.335
P-value
0.000
0.186
0.336
0.445
0.000
0.001
Jul
Aug
Sep
Oct
Nov
Dec
Corr
0.365
0.173
0.285
0.263
0.386
0.437
P-value
0.000
0.084
0.004
0.008
0.000
0.000
Annual r = 0.684
P-value < 0.0001
Water Balance Estimates of Yellowstone River Runoff
Results: Scenario Testing
Baseline Scenario: Tree-ring Precip & Temperatures from Climatology
Baseline Scenario: Tree-ring Precip & Temperatures from Climatology
Baseline Scenario: Tree-ring Precip & Temperatures from Climatology
Average Long-term Runoff: All Scenarios
Average Long-term Runoff: All Scenarios
10-14%
Decline
Average Runoff: Driest 10th percentile
Average Runoff: Driest 10th percentile
Additional
17-24%
Decline
Scenario Comparison: 25-year Moving Averages
Baseline
2025
2050
2100
Scenario Comparison: 25-year Moving Averages
Baseline
2025
2050
2100
Scenario Comparison: 25-year Moving Averages
Baseline
2025
2050
2100
Summary of Results
• Natural precipitation variability combined with
20th Century temperatures shows the potential
for extended low-flow periods far below the
gage average, even in the absence of additional
warming
• Small amounts of warming would greatly
intensify the impact of droughts seen in the
tree-ring scenarios
• The tree-ring + future temperature scenarios
show the potential for extreme dry years and
extended dry periods far outside the range of
gage observations
Applications
• Means to convert climate predictions into the
“currency” of natural resource management
• Tree-ring studies appeal to managers:
– Intuitive
– Strong physical basis for tree-climate
relationships
– Long, rich history of successful tree-ring work
• Provides “plausible” and defensible scenarios for
water planning
• Studies emphasize the need for sustainable resource
management, even in the absence of anthropogenic
climate change
Thanks!
Sensitivity to Changes in Seasonality?