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Ancient trees, climate models, and the future of drought in western Colorado
Jeff Lukas - [email protected]
Western Water AssessmentCIRES, University of Colorado
38th Annual Colorado Water Workshop Western State Colorado University, Gunnison, COJuly 17, 2013
2013
Parsing our changing climate: a conceptual model
Internal (“natural, unforced”) climate variability
+External climate forcing (Greenhouse gases, etc.)
Changing future climate
=
PAST PRESENTPRESENT FUTURE
PRESENT FUTURE
change
Drought: driven by precipitation deficit, exacerbated by high temperatures
Middle photo: Jane Stulp
Need to look back and forwards to assess future drought risk
1900 2013
Instrumental record
Climate model projections
Tree-ring record
Western Colorado annual (water year) precipitation vs. Western Colorado naturalized runoff, 1900-2012
Precip Data: NOAA NCDC, CO Division 2 data
Precipitation deficits and hydrological drought
Streamflow data: Reclamation (1906-2010); 2011-12 values estimated from preliminary Reclamation data
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1012141618202224
1900 1910 1920 1930 1940 1950 1960 1970 1980 1990 2000 2010
Prec
ipita
tion,
in.
02468101214161820
Stre
amflo
w, M
AF
PrecipitationStreamflow
Western Colorado’s annual water balance – average year
Image: D. Meko, U. of Arizona
Precip: 40 MAF
ET : -30 MAF
Runoff: 10 MAF
(MAF = million acre-feet)
Western Colorado’s annual water balance – drought year
Image: D. Meko, U. of Arizona
Precip: 30 MAF
ET : -24 MAF
Runoff: 6 MAF
While precipitation declines only 25% vs. average year, runoff declines 40% due to ET component increasing as proportion of precipitation
(MAF = million acre-feet)
Using tree rings to reconstruct past droughts
Douglas-fir, south San Juans, CO
1977 1983
Tree growth is typically limited by moisture availability
So:– a dry year leads to a narrow growth ring– a wet year leads to a wide growth ring
Ancient trees and wood are abundant across western Colorado
0
5
10
15
20
25
30
750 1000 1250 1500 1750 2000
An
nu
al F
low
, MA
F
Tree-ring reconstructed annual flows, Colorado River at Lees Ferry, 762-2005
2002
1977, 2002: in the lowest 10 reconstructed annual flows since 762
Meko et al. 2007. Medieval Drought in the Upper Colorado River Basin, Geophysical Research Letters; data available at http://treeflow.info
Tree-ring reconstructed annual flows, Colorado River at Lees Ferry, 762-2005, with 20-year running mean
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17
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750 1000 1250 1500 1750 2000
Water Year
An
nu
al F
low
, M
AF
Instrumental record
Tree-ring reconstructed annual flows, Colorado River at Lees Ferry, 762-2005, with 20-year running mean
10
11
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750 1000 1250 1500 1750 2000
Water Year
An
nu
al F
low
, M
AF
0
5
10
15
20
25
1120 1130 1140 1150 1160 1170 1180
Ann
ual f
low
, MA
F
Mid-1100s megadrought
46 of 57 years were dry
Future projections from global climate models
Jan. 2070
Temperatures – Going up!
Source: Marty Hoerling, NOAA ESRL PSD; data available from http://gdo-dcp.ucllnl.org/
1950 2000 2050 2100
50°F
60°F
Western Colorado annual temperatures - ensemble of 16 GCMs, medium emissions scenario
Observed temp.
+4ºF by 2050
1950 2000 2050 2100
32”
16”
Western Colorado annual precipitation – ensemble of 16 GCMs, medium emissions
Precipitation – Up? Down?
Observed precip.
Source: Marty Hoerling, NOAA ESRL PSD
Runoff – Probably down, but continued high variability!
Observed runoff
Data: http://gis.usbr.gov/Streamflow_Projections/
0
10
20
30
40
50
1950 2000 2050 2100
Annu
al R
unoff
, MAF
Upper Colorado River Basin annual runoff – ensemble of 16 GCMs, medium emissions
Western Colorado’s annual water balance – average year, median 2050 projection (+4oF, no precip change)
Image: D. Meko, U. of Arizona
Precip: 40 MAF
ET : -31 MAF
Runoff: 9 MAF
Even though precipitation doesn’t change, runoff declines ~10% to 9 MAF due to increasing ET
+
-
Projections for Western Colorado from all GCM runs used in the most recent studies
Source: Denver Water & Joint Front Range Climate Change Vulnerability Study, as printed in Kerr, Science, 25 Nov 2011
More runoff
Less runoff
Median 2050
The “new normal” in a (yet) warmer climate
• Increased evaporation and transpiration• More rain and less snow in fall and spring• Reduced spring snowpack• Earlier peak runoff • Reduced annual flows (unless precip gains cancel out)• Reduced summer soil moisture• Greater plant moisture stress and crop water use
• These are all impacts currently associated with drought…they will occur more often in a warming climate
• Any future drought will be more severe, and will tend to persist longer, for a given precipitation deficit
Don’t fixate on trend: impacts will still be most deeply felt in low-precipitation (drought) years
Middle photo: Jane Stulp
Dust-on-snow also makes snow melt sooner, and probably reduces overall runoff
• Already has significant hydrologic impact; likely to get worse in future with warming/drying of source regions
• Impacts are in same direction as those caused by warming
Photos by Chris Landry, CSAS
Future drought: expectations from tree rings, instrumental record, and climate model projections
750 1000 1250 1500 1750 2000 1950 2000 2050 21001900 2000
Take-home
• Ancient trees tell us that the natural hydroclimatic variability is larger than the last 100 years would suggest (extreme short-term droughts & megadroughts)
• The instrumental record tells us precipitation drives drought, but temperature (ET) is also a big factor
• The climate models tell us that the temperature (ET) dial will continue to get turned up, but much more uncertain about precipitation
• Drought conditions will likely be more frequent and severe in the future
• Given the uncertainties in future climate, “scenario-based” planning is more appropriate than traditional “single-target” planning
• Please contact me ([email protected]) with questions or requests for information