Summary

1
Assessing the Implications of Climate Change for
Western Water Resources Andrew W. Wood1, Dennis
P. Lettenmaier1, and Tim P. Barnett2 1.
Department of Civil and Environmental
Engineering, Box 352700, University of
Washington, Seattle, WA 98195 2. Climate
Research Division, Scripps Institution of
Oceanography, La Jolla, CA 92093
CCSP Workshop Arlington, Virginia November 14
16, 2005
Summary The hydrology and water resources of the
western U.S. are highly sensitive to climate. As
part of the Accelerated Climate Prediction
Initiative (ACPI) we coupled the Variable
Infiltration Capacity (VIC) macroscale hydrology
model to output from the NCAR/DOE Parallel
Climate Model (PCM) to generate plausible future
streamflows for the next century. These
streamflows were then input into reservoir models
to assess the implications of climate change on
managed water resources. A key step in this
process is the removal of bias in climate model
predictions using a probability mapping
procedure, and downscaling from the spatial scale
of GCMs to the hydrology model scale. This step
is facilitated by knowledge of the climatology
(probability distribution) of the hydrology model
forcings from both the climate model and
observations. The three focus areas for the ACPI
study were the Columbia, the Sacramento-San
Joaquin, and the Colorado river basins. The
Columbia and Sacramento-San Joaquin results are
strongly sensitive to the seasonal shifts in
streamflow associated with climate change. These
hydrologic shifts are reflected in reductions of
firm hydropower production, and/or reduction in
the systems ability to meet minimum spring and
summer streamflow targets for fisheries
protection and enhancement in the case of the
Columbia. For the Sacramento-San Joaquin, the
fraction of the time that the system would be in
critically dry status more than doubles by
mid-century. In contrast, the Colorado River
reservoir system is almost completely insensitive
to seasonal runoff shifts, but is highly
sensitive to the approximately 10 percent
reduction in annual runoff predicted by
mid-century under the PCM scenarios. This change
would reduce the systems ability to meet water
supply delivery targets, and to meet U.S.-Mexico
treaty obligations.
Hydrologic Model (VIC)
Bias Correction Downscaling
Overall Approach
Downscaling Bias Correction
The hydrologic model used was the VIC macroscale
land surface model (see http//www.hydro.washingto
n.edu/ for model details). The model was run in
a 24 hour timestep Water Balance mode at 1/8
resolution. Forcing variables are daily
precipitation, maximum and minimum temperatures
(from NCDC cooperative observer stations), and
wind from NCEP Reanalysis. Soil parameters are
taken from the Penn State Soil Geographic
Database (STATSGO) and land cover is from the
University of Maryland 1-km Global Land Cover
product (derived from AVHRR). Water Balance mode
assumes that the soil surface temperature is
equal to the air temperature for the current time
step. The exception to this is that the snow
algorithm still solves the surface energy balance
at three hour timesteps to determine the fluxes
needed to drive accumulation and ablation
processes.
1. Bias Correction On a monthly and PCM grid
cell specific basis, a percentile mapping
approach was used to correct for bias in GCM
output biases.
Climate Scenarios
Hydrologic Model (VIC)
Global climate simulations, next 100 yrs
Monthly Precip, Temp
Natural Streamflow
Reservoir Model
Note future scenario temperature trend (relative
to control run) removed before, and replaced
after, bias-correction step.
Performance Measures
DamReleases, Regulated Streamflow
Reliability of System Objectives
2. Spatial Disaggregation PCM scale monthly
anomalies were interpolated to the 1/8 degree
hydrology model grid and applied the 1/8 degree
observed climatological mean fields.
Reservoir Models Separate reservoir models that
represents the physical and operational features
of each basin were utilized to derive the water
resource results. The schematic is for ColSim, a
model of the Columbia River reservoir System.
Similar models were developed and used for the
Colorado and California systems
ColSim
Climate Model Scenarios
The following Department of Energy/National
Center for Atmospheric Research Parallel Climate
Model (DOE/NCAR PCM) runs were utilized Histori
cal B06.22 (greenhouse
CO2aerosols forcing) 1870-2000 Climate
Control B06.45 (CO2aerosols at 1995 levels)
1995-2048 Climate Change B06.44
(BAU6, future scenario forcing)
1995-2099 Climate Change B06.46 (BAU6,
future scenario forcing) 1995-2099
Climate Change B06.47 (BAU6, future scenario
forcing) 1995-2099
Inflow
Consumptive use
Inflow
Inflow
Consumptive use

• Storage Reservoirs
• Releases Depend on
• Storage and Inflow
• Rule Curves (streamflow forecasts)
• Flood Control Requirements
• Energy Requirements
• Minimum Flow Requirements
• System Flow Requirements

Inflow
Inflow

3. Temporal Disaggregation 1/8 degree monthly
fields were disaggregated to a daily time step by
resampling observed daily patterns by month and
rescaling/shifting them to reproduce the
bias-corrected and spatially disaggregated
monthly mean fields.
Run of River Reservoirs (inflowoutflow
energy)
System Checkpoint
Conclusions / Comparative analysis 1) The mean
of the distribution of annual precipitation does
not change much over the 21st century, although
the variability increases somewhat after the
2030s, with consequent increases in annual runoff
variability. 2) The trend in annual
temperatures is evident throughout the west, but
decadal variations in the rate of increase are
also prominent. 3) Columbia River reservoir
system primarily provides within-year storage
(total storage/mean flow 0.3). California is
intermediate ( 0.3), and Colorado is an
over-year system (4). 4) Climate sensitivities
in Columbia basin are dominated by seasonality
shifts in streamflow, and may even be beneficial
for hydropower. However, fish flow targets would
be difficult to meet under an altered climate,
and mitigation by altered operation is
essentially impossible. 5) The California
system operation is dominated by water supply
(mostly agricultural), reliability of which would
be reduced significantly by a combination of
seasonality shifts and reduced (annual) volumes.
Partial mitigation by altered operations is
possible, but would be complicated by flood
related constraints. 6) The Colorado system is
sensitive primarily to changes in annual
streamflow volumes. A low runoff ratio makes the
system highly sensitive to modest changes in
precipitation (in winter, especially in the
headwater sub-basins). Sensitivity to
alternative operations is modest, and mitigation
possibilities by increased storage would be
ineffective (even if otherwise feasible).
Selected Results
Columbia River Basin
California
Colorado River Basin
Downscaled California average annual temperature,
precipitation and derived runoff. Blue line
shown is 1950-2000 simulated historic average
dotted is static 1995 control climate gray lines
are the three future climate ensembles and the
red thick line is their average.
Downscaled Columbia River basin average annual
temperature, precipitation and derived runoff.
Blue line shown is 1950-2000 simulated historic
average dotted is static 1995 control climate
gray lines are the three future climate ensembles
and the red thick line is their average.
Downscaled Colorado River basin average annual
temperature for PCM ensemble climate simulations.
Blue line shown is 1950-2000 simulated historic
average while red is static 1995 control climate.
The spatial distribution of predicted changes in
mean annual runoff for the control and BAU
periods 1-3 (averaged over the 3 ensembles)
relative to simulated historic conditions.
Percent change of future ensemble mean snow water
equivalent (SWE) for the first five decades of
the 21st century relative to control run (1990s
climate) averages.
Percent change of future ensemble mean snow water
equivalent (SWE) for two decades (2050s and
2090s) relative to 1950-99 averages.
86
82
83
90
References Christensen, N.S., Wood, A.W., Voisin,
N., Lettenmaier, D.P. and R.N. Palmer, 2004,
Effects of Climate Change on the Hydrology and
Water Resources of the Colorado River Basin,
Climatic Change Vol. 62, Issue 1-3, 337-363,
January. Liang, X, D.P. Lettenmaier, E. F.
Wood, and S. J. Burges, 1994 A simple
hydrologically based model of land surface water
and energy fluxes for general circulation
models. J. Geophys. Res.(D7), 14,415-14,
428 Payne, J.T., A.W. Wood, A.F. Hamlet, R.N.
Palmer and D.P. Lettenmaier, 2004, Mitigating the
effects of climate change on the water resources
of the Columbia River basin, Climatic Change Vol.
62, Issue 1-3, 233-256, January. Van
Rheenen,N.T., A.W. Wood, R.N. Palmer and D.P.
Lettenmaier, 2004, Potential Implications of PCM
Climate Change Scenarios for Sacramento - San
Joaquin River Basin Hydrology and Water
Resources, Climatic Change Vol. 62, Issue 1-3,
257-281, January Wood, A.W., L.R. Leung, V.
Sridhar and D.P. Lettenmaier, 2004, Hydrologic
implications of dynamical and statistical
approaches to downscaling climate model outputs,
Climatic Change Vol. 62, Issue 1-3, 189-216,
January Zhu,C., D.W. Pierce, T.P. Barnett, A.W.
Wood, and D.P. Lettenmaier, 2004, Evaluation of
Hydrologically Relevant PCM Climate Variables and
Large-scale Variability over the Continental
U.S., Climatic Change Vol. 62, Issue 1-3, 45-74,
January
Selected Water Resources Results (right) Fish
flow target performance under different operating
strategies for future Periods 1-3 (2010-2040
2040-2070 2070-2098) (below) Trade-offs between
(left) flood protection benefits and hydropower
revenues, and between (right) fish flows and
hydropower, in Periods 1 and 2 (Period 3 not
shown).
Reservoir system results derived from routing the
simulated streamflows from VIC through the
Colorado River Reservoir Model (CRRM). CRRM
represent 9 of the rivers reservoirs (85 of
total Colorado River storage lies in Lake Powell
and Lake Mead) and operates on a monthly
timescale.
Fractions of water year type determination that
governs water allocations in the Central Valley
for an historical period and three future
periods.
Monthly depiction of reservoir storage in the San
Joaquin R. system for the control climate and
five future periods.
Figure A Simulated total January 1
storage. Figure B Simulated average annual
release from Imperial Dam to Mexico and
probability that annual release targets are
met. Figure C Simulated total energy
production.
Acknowledgements The US Department of Energys
Accelerated Climate Prediction Initiative
provided funding for this research. Publications
were also supported by the Joint Institute for
the Study of the Atmosphere and Ocean (JISAO)
under NOAA Cooperative Agreement NA17RJ1232.
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