Study GCMs (runs) Colorado R Runoff by mid 21 st century Christensen et al. 2004 1 (3) -18% Milly 2005 12 (24) -10 to -20% Hoerling and Eischeid 2006 18 (42) -45% Christensen and Lettenmaier 2007 11 (22) -6% (-40 to 18%) Seagar et al. 2007 19 (49) -16% (-8 to -25%) Information from Table 5-1 in Western Water Assessment (WWA) report for Colorado Water Conservation Board “Colorado Climate Change: A Synthesis to Support Water Resource Management and Adaptation.” Oct 2008 (available online at: http://wwa.colorado.edu/publications/reports/WWA_ClimateChangeColoradoReport_2008.pdf) Prologue
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Study GCMs (runs) Colorado R Runoff by mid 21st century
Christensen et al. 2004 1 (3) -18%
Milly 2005 12 (24) -10 to -20%
Hoerling and Eischeid 2006 18 (42) -45%
Christensen and Lettenmaier 2007
11 (22) -6% (-40 to 18%)
Seagar et al. 2007 19 (49) -16% (-8 to -25%)
Information from Table 5-1 in Western Water Assessment (WWA) report for Colorado Water Conservation Board “Colorado Climate Change: A Synthesis to Support Water Resource Management and Adaptation.” Oct 2008 (available online at: http://wwa.colorado.edu/publications/reports/WWA_ClimateChangeColoradoReport_2008.pdf)
1. Seager et al. 20072. Seager et al 20133. Milly et al. 2005
Hydrology model
RCM
GCM
Stream-flow
boundaryconditions
P-E, R
Stat. down
P-E, R
flow routing
CLIMATE
LAND
SURFACE
MGMT
IMPACT
Adapted from Vano et al 2014: Figure 1
1. Seager et al. 20072. Seager et al 20133. Milly et al. 20054. Christensen et al. 20045. Christensen and Lettenmaier 20076. Cayan et al 20107. USBR 2011
Hydrology model
RCM
GCM
Stream-flow
boundaryconditions
P-E, R
Stat. down
P-E, R
flow routing
CLIMATE
LAND
SURFACE
MGMT
IMPACT
Adapted from Vano et al 2014: Figure 1
1. Seager et al. 20072. Seager et al 20133. Milly et al. 20054. Christensen et al. 20045. Christensen and Lettenmaier 20076. Cayan et al 20107. USBR 20118. Gao et al 20119. Rasmussen et al 201110.Gao et al 2012
Sensitivity: The % change in runoff for an imposed
increase in T.
Coarser spatial resolutions tend to be more sensitive to change from both warming and precipitation reduction.
Lesson #2:
Land surface representation.
Hydrology model
RCM
GCM
Stream-flow
CLIM
ATE
LAND
SURF
ACE
MGMT
IMPA
CT
Uncertainty #3:
• Grid-based simulations of land-surface processes using principles of energy and water balance
• Daily timesteps with some sub-daily processes
• Forcing data: PRCP, T, specific humidity, wind speed, air pressure, and surface incident shortwave and longwave radiation
* elasticity is a ratio of: the percent change in annual model
runoff to the percent change in annual
precipitation.
elas
ticity
* (r
unoff
:prc
p)
Adapted from Vano et al 2014: Figure 5
sens
itivi
ty*
(%∆
per d
eg C
)
* sensitivity is the percent change in runoff for an imposed increase in T.
Adapted from Vano et al 2014: Figure 5
Hydrology models show:
1. substantial differences in sensitivities to T increases;
2. similar responses to precipitation change;
3. differences in P elasticity and T sensitivity are generally smaller in headwater regions.
Lesson #3:
Statistical downscaling.
Hydrology model
RCM
GCM
Stream-flow
CLIM
ATE
LAND
SURF
ACE
MGMT
IMPA
CT
Uncertainty #4:
200 km 1 month10-20 km sub-daily
inmcm.a2giss.a2
hadcm3.a2ipsl.a2
pcm.a2mri.a2
csiro.a2mpi.a2gfdl.a2
miroc.a2cnrm.a2
-20% 0% 20% 40% 60% 80% 100% 120% 140%
15%
12%
9%
15%
0%
5%
1%
5%
5%
1%
-5% differencedelta methodBCSD
percent of historical flows
CNRMMIROCGFDLMPICSIROMRIPCMIPSLHADCM3GISSINMCM
Glob
al C
limat
e M
odel
s (A
2 em
issio
ns sc
enar
io)
+6%
The choice of downscaling method can affect the magnitude of the climate signal leading to differences in long-term projected runoff.
Lesson #4:
We identified four major reasons for discrepancies in Colorado River projections:
1. GCMs and emissions scenarios;2. Spatial scale or the ability of models to
simulate the disproportionate contributions to Colorado River discharge of the relatively small, high elevation runoff source areas;
3. Sensitivities of land surface hydrology models to P and T changes;
4. Methods used to statistically downscale GCM scenarios.
Re-cap
Implications for Decision makers
From past AR4 studies, we can say with high likelihood that in the Colorado:• Temperatures (T) will rise in the Colorado over the
coming decades• Precipitation (P) less certain, but will likely
decline on annual basis
From our analysis• Warmer T (ignoring P) will reduce runoff production
(our estimates -6.5 3.5% per C at Lees Ferry)• Change in P results in streamflow response of 2 to
3 times (5% decline in P results in 10-15% decline in streamflow)
• Coarse spatial resolution of models does not resolve high elevation hydrologic processes that dominate Colorado River basin runoff production
The diversity of approaches to projecting future streamflow in the scientific literature requires further analysis to make apples-to-apples comparisons in order to better understand sources of uncertainty.
Parting words…
Thank you!Research funded by NOAA through its RISA (Regional Integrated Sciences & Assessments) Project and its National Integrated Drought Information System
The Uncertainty Prayer*Grant us…The ability to reduce the uncertainties we can;The willingness to work with the uncertainties we cannot;And the scientific knowledge to know the difference. -
• 14 co-authors, from academia and federal agencies, many authors of divergent papers
• Reached agreement on four key sources of future uncertainties AND important certainties for decision makers in the Colorado River basin
• Documented for the larger science and management communities how to approach seemingly disparate results, particularly timely with new results being released with the 5th Assessment Report (AR5)
Summary
Where research evolving (not ranking of research priorities):• New climate change projections• Increased spatial resolution of climate models• Improved land surface simulations• New paleoclimate reconstruction and model
evaluation• Improved observational records• Strengthening connection with management
community
Future research directions
As climate science evolves, understandings will improve. However, there will be no single magic bullet. Therefore, a continued effort to communicate and incorporate uncertainty is needed.