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Streamflow/runoff sensitivity to
warming and drying in the
Colorado (Western US) River
Basin
Tapash Das, Dan Cayan, David
Pierce, Mike Dettinger
Reconciling Projections of Future
Colorado River Stream Flow
Collaborators:
Robert Webb, Bradley Udall
Martin Hoerling, Jonathan Overpeck
Holly Hartman, Dennis Lettenmaier,
Julie Vano, Dan Cayan, Tapash Das
Levi Brekke, Kevin Werner
Past Studies
Table from Colorado Water Conservation Board (CWCB) report “ Colorado Climate Change: A Synthesis to Support Water Resource Management and
Adaptation.” Oct 2008 (available online at: http://cwcb.state.co.us/NR/rdonlyres/8118BBDB-4E54-4189-A354-3885EEF778A8/0/CCSection5.pdf)
Project Objectives
1) Reconcile discrepancies in projected Colorado River flow
changes.
2) Assess the basins sensitive in runoff to changes in
temperature, in precipitation, or in both.
3) Identify the underlying mechanisms for these sensitivities
(e.g. soil moisture, ET).
4) Provide meaningful information for water managers and
policymakers that incorporate uncertainties in future
climate change projections.
DATA, Model, Methods
Data, Model
Hydrologic model:
Methods
 Compute
change
Variable Infiltration Capacity (VIC)
Liang et al. (1994), JGR
 Historic
streamflow/runoff
forcing:
Observed Gridded meteorology
 Responses to prescribed
changes in T, P imposed on
historic climate
Q ref+1 - Qref
Temp =
sensitivity
Qref
deg C
Wood and Lettenmaier (2006), B Am Meteorol Soc
Hamlet and Lettenmaier (2005), J. Hydromet.
Q ref-1% - Qref
Precip =
elasticity
Qref
%
Colorado River Flows: highly sensitive to warming
GN
BL
LE IM
MA
CA
Catchments:
GN: Green River at Greendale
MA: Yampa River near Maybell
CA: Colorado River near Cameo
GA: Gunnison River near Grand Junction
CI: Colorado River near Cisco
BL: San Juan River near Bluff
LE : Colorado River at Lees Ferry
IM: Colorado River below Imperial Dam
GA CI
-6%
Sensitivity of streamflow to
warming varies across different
catchments
Overall sensitivity approximately
6% decline in streamflow per 1°C
warming at Colorado at Lees Ferry
Climatologies of monthly streamflow:
Colorado at Lees Ferry
Baseline
Warming
Temp increases:
streamflow
decreases
annually, mainly
because
decreases flow in
spring/summer
Streamflow changes to climate change:
Colorado at Lees Ferry
The total decline in
streamflow increases each
time the temperature
increases, but the total
reductions decrease as the
temperature increases
more and more
Colorado River Flows: Sensitive to Drying
GN
MA
BL LE IM
CA GA CI
-2%
Sensitivity of streamflow to drying
varies across different catchments
Catchments:
GN: Green River at Greendale
MA: Yampa River near Maybell
CA: Colorado River near Cameo
GA: Gunnison River near Grand Junction
CI: Colorado River near Cisco
BL: San Juan River near Bluff
LE : Colorado River at Lees Ferry
IM: Colorado River below Imperial Dam
Overall sensitivity approximately
2% decline in streamflow per 1%
reduction of precipitation at
Colorado at Lees Ferry
Impact of model forcing resolution to VIC
simulations: Colorado at Lees Ferry
baseline
Annual mean streamflow (MAF)
(WY1916 through 2000)
P,T basin average
Naturalized flow
VIC simulated flow (baseline)
VIC simulated flow (p, t basin wide)
VIC simulated flow (t basin wide)
VIC simulated flow (p basin wide)
Runoff climatologies at Colorado at Lees Ferry
Substantial changes in runoff volume, timing in baseline VIC
run and VIC run using average model forcings
: 15.1
: 15.7
: 8.1
: 11.8
: 7.8
Research Objectives: Sensitivity of Runoff to
Climate Change Over The Western U.S.
Understand runoff sensitivity to warming,
and drying in the Western United States

 Compare major river basins in the West, and
understand the underlying differences
 Apply sensitivity method to IPCC AR4
climate change ensemble projection
Study domain: Western US
(+ part of British Columbia)
Columbia River
Snow dominated
oC
Northern Sierra
Rain-Snow fed
Colorado River
Snow dominated
Southern Sierra
Snow dominated
Hydrographs drawn from VIC simulated streamflow
forced by observed meteorology, 1915-2002
Sensitivity of runoff to Warming varies
greatly across the West
%oC-1
Major Basins: Upper Colorado shows the highest sensitivity
to warming, followed by Columbia, and least sensitivity in
the Californian Nevada
Hydrologic characteristics: Region by Region
Runoff
efficiency
(R/P)
Evaporative demand
(AET/P)
In Columbia and
Sierra Nevada,
about 50% of total
precipitation is
lost via ET
However, in
Colorado, about
80% of total
precipitation is
lost via ET
Actual ET and runoff are computed using VIC as
driven by observed meteorology, 1915-2002
Sensitivity of runoff to Drying varies
across the West
%%-1
Major Basins: Upper Colorado 2.2% reduction, Northern
Sierra 1.4% for 1% reduction of precipitation
Does snowpack produce higher
Sensitivity?
Snowdominated
region
Other
region
Snow-dominated
region has higher
sensitivity in
Colorado, Columbia,
and Southern Sierra,
but NOT in Northern
Sierra
Region considered as snow-dominated if VIC simulated
mean April 1 Snow Water Equivalent (SWE) in the period
1915-2002 is larger than 50 mm (VIC simulation with varying snowline)
Sensitivity of Streamflow to Warming:
Month by month
Warming in the warm
season yield larger
reduction (as compared to
reduction in cool season
warming) for Columbia,
Colorado and Southern
Sierra
Runoff fraction: Region by Region
In Columbia,
Colorado and
Southern Sierra
Nevada >70% of
total runoff
produced in
warm season
Precipitation fraction: Region by Region
In Columbia and
Sierra Nevada
>65% of total
precipitation
falls in cold
season
Summary
 Sensitivity of runoff, especially due to temperature, varies
greatly across the West
The region with the highest evaporative demand has the
greatest sensitivity. Greatest sensitivities appear in zones of
snow and snow-rain transition
 Sensitivities of Runoff to warming:
Colorado > Columbia > Southern Sierra > Northern Sierra
 Warming in the warm season yield larger reduction (as
compared to cool season) for Columbia, Colorado and
Southern Sierra
 Topography and hydrologic structure is crucial: resolving
snow vs. rain and other processes in the Colorado River
Basin is crucial. Distributing precipitation over the basin
produces less runoff, but lesser effect when temperature is
smoothed out