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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
Collaborators:
Robert Webb, Bradley UdallMartin Hoerling, Jonathan Overpeck Holly Hartman, Dennis Lettenmaier, Julie Vano, Dan Cayan, Tapash DasLevi Brekke, Kevin Werner
Reconciling Projections of Future Colorado River Stream Flow
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)
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.
Project Objectives
Data, Model
Hydrologic model: Variable Infiltration Capacity (VIC)
Liang et al. (1994), JGR
Historic forcing: Observed Gridded meteorology
Wood and Lettenmaier (2006), B Am Meteorol Soc
Hamlet and Lettenmaier (2005), J. Hydromet.
Methods Compute streamflow/runoff change
Responses to prescribed changes in T, P imposed on historic climate
Temp sensitivity
Q ref+1 - Qref
Qref
deg C
=
Precip elasticity
Q ref-1% - Qref
Qref
%
=
DATA, Model, Methods
Colorado River Flows: highly sensitive to warming
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
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
-6%
GN
MA
CAGA CI
LEBL
IM
WarmingBaseline
Temp increases: streamflow decreases
annually, mainly because
decreases flow in spring/summer
Climatologies of monthly streamflow:
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
Streamflow changes to climate change: Colorado at Lees Ferry
Colorado River Flows: Sensitive to Drying
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
Sensitivity of streamflow to drying varies across different catchments
Overall sensitivity approximately 2% decline in streamflow per 1%
reduction of precipitation at Colorado at Lees Ferry
-2%
GNMA
CA GA CI LEBL IM
Runoff climatologies at Colorado at Lees FerrySubstantial changes in runoff volume, timing in baseline VIC
run and VIC run using average model forcings
Impact of model forcing resolution to VIC simulations: Colorado at Lees Ferry
Annual mean streamflow (MAF)(WY1916 through 2000)
Naturalized flow : 15.1VIC simulated flow (baseline) : 15.7VIC simulated flow (p, t basin wide) : 8.1VIC simulated flow (t basin wide) : 11.8VIC simulated flow (p basin wide) : 7.8
baseline
P,T basin average
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)
oC
Colorado River Snow dominated
Northern Sierra Rain-Snow fed
Columbia River Snow dominated
Southern Sierra Snow dominated
Hydrographs drawn from VIC simulated streamflow forced by observed meteorology, 1915-2002
%oC-1
Sensitivity of runoff to Warming varies greatly across the West
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
In Columbia and Sierra Nevada,
about 50% of total precipitation is
lost via ET
Runoff efficiency
(R/P)
Evaporative demand (AET/P)
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
Major Basins: Upper Colorado 2.2% reduction, Northern Sierra 1.4% for 1% reduction of precipitation
%%-1
Does snowpack produce higher Sensitivity?
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)
Other region
Snow-dominated region has higher
sensitivity in Colorado, Columbia, and Southern Sierra, but NOT in Northern
Sierra
Snow-dominated
region
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
Sensitivities of Runoff to warming:
Colorado > Columbia > Southern Sierra > Northern Sierra
The region with the highest evaporative demand has the greatest sensitivity. Greatest sensitivities appear in zones of snow and snow-rain transition
Warming in the warm season yield larger reduction (as compared to cool season) for Columbia, Colorado and Southern Sierra
Sensitivity of runoff, especially due to temperature, varies greatly across the West
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
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