Transcript Climate change effects on surface hydrology in the Puget Sound Basin

Climate Change Effects on Surface Hydrology In the Puget Sound Basin
Lan Cuo1, Nathalie Voisin1, Julie Vano1, Marketa E.McGuire2, Eric Salathe2, Dennis P. Lettenmaier1,2
1 Department of Civil and Environmental Engineering, University of Washington
2 Climate Impacts Group, University of Washington
AGU Fall Meeting, San Francisco, CA, Dec. 15-19, 2008
Paper Number: GC430-0759
Abstract
Analysis of historical temperature trends shows that the Puget Sound basin has experienced about one degree C of
warming over the past century. This warming has resulted in reductions in winter snow accumulation, and appears to
have changed the seasonal patterns of runoff from the Cascade and Olympic Mountains into Puget Sound, especially
from river basins with much of their area within the intermediate elevation zone (so-called transient snow zone, where
the form of precipitation transitions from rain to snow during the winter). This warming trend is expected to continue in
the 21st century. The objective of the study is to investigate the streamflow, snow water equivalent (SWE) and
evapotranspiration (ET) change in the 21st century associated with a changing climate, and their implications for the
performance of reservoir systems that supply water to the major metropolitan areas within the basin. We used a
calibrated regional hydrological model - the Distributed Hydrology-Soil-Vegetation Model (DHSVM) to simulate the
effects of future climate change on the surface hydrology using the delta method of downscaling global climate model
(GCM) output. The delta method defines a steady state climate with statistics that are appropriate to some future time.
Differences of monthly temperature and ratios of monthly precipitation are taken from GCM output archived for the
IPCC Fourth Assessment Report (2007) for two global greenhouse gas emissions scenarios (A1B and B1) and
historical conditions for three periods – 2020s, 2040s and 2080s. Twenty and nineteen GCMs were used to provide
future climate ensembles. We examine projected seasonal streamflow changes, changes in winter and spring SWE,
and total ET as compared with historical hydrologic conditions for the period 1915 - 2006. We also evaluate the
implications of projected changes for performance of water supply to the major metropolitan areas (Seattle, Tacoma,
and Everett) using reservoir system models.
Models
CGCM 3.1 t47
CGCM 3.1 t63
Institutions
Canadian Center for Climate Modelling and Analysis Canada
Canadian Center for Climate Modelling and Analysis Canada
CNRM_CM3
ECHAM5
Centre National de Recherches Meteorologiques France
Max-Planck-Institut for Meteorology Germany
HADCM
HADGEM1
IPSL_CM4
BCCR
CCSM3
CSIRO_3_5
Met Office, UK
Hadley Center Global Environment Model, v 1., UK
IPSL (Institute Pierre Simon Laplace), Paris, France
Bjerknes Centre for Climate Research Norrway
National Centre for Atmospheric Research USA
Australia's Commonwealth Scientific and Industrial Research
Organisation Australia
Meteorological Institute, University of Bonn, Germany
Meteorological Research Institute of KMA, Korea
Model and Data Groupe at MPI-M, Germany
Institute of Atmospheric Physics China
Geophysical Fluid Dynamics Laboratory USA
Geophysical Fluid Dynamics Laboratory USA
Goddard Institute for Space Studies USA
Goddard Institute for Space Studies USA
Institute for Numerical Mathematics Russia
National Institute for Environmental Studies Japan
National Institute for Environmental Studies Japan
National Centre for Atmospheric Research USA
ECHO_G
FGOALS_0_G
GFDL_CM2_0
GFDL_CM2_1
GISS_AOM
GISS_ER
INMCM3_0
MIROC_3.2
MIROC3_2_hi
PCM1
1. Study Area
The Fourth Assessment Report (AR4) of the Intergovernmental Panel on Climate Change (IPCC) states that climate
warming is unequivocal and that anthropogenic use of fossil fuels has contributed to increasing carbon dioxide
concentrations and thereby warming in the atmosphere (IPCC, 2007). Climate change effects on water resources are
likely to be especially pronounced in areas where snowmelt contributes to streamflow, as is the case in the Puget
Sound region.
Figure 7. Projected total evapotranspiration changes in 2020s, 2040s and 2080s in Sultan,
Tolt, Cedar, and Green River basins.
Table 2. GCMs
The Puget Sound basin, bounded by the Cascade Mountains to the east and the Olympic Mountains to the west, is
located in western Washington State (Figure 1). The basin, with a drainage area of about 31,000 km 2, has 13 upland
sub-basins and numerous smaller lowland sub-basins. The basin has a maritime climate. Annual precipitation, mostly
falling from October – April, ranges from 600 – 3000 mm depending on the location. Elevation ranges from sea level
to 4400m at the top of Mountain Rainer. Figure 1 shows the geographic location of the basin, and a digital elevation
map of the basin is shown in Figure 2. We focus on four catchments within the Puget Sound basin which provide
inflows to the water supply systems for three major municipalities. These are the Sultan River and Tolt Rivers, which
are in the Snohomish River basin, and the Cedar and Green Rivers. These four rivers provide inflows to the Everett
(Sultan River), Seattle (South Fork Tolt and Cedar Rivers), and Tacoma (Green River) water supply systems. In
general, all systems store water in the winter and spring when precipitation and/or snowmelt is abundant and release
water in the summer where fresh water demands are high and precipitation is minimum. Figure 3. shows the location
of the four catchments.
Figure 1. Geographic location
Figure 2. Elevation map
Figure 3. Selected river basins in
the Puget Sound (By Robert
Norheim)
Figure 4. Ranges of GCMs in their projection of
temperature and precipitation changes in the
2020s for A1B and B1 emissions scenarios.
2. Approach
Figure 5. Historical and future seasonal
cycles of temperature and precipitation.
We use the Distributed Hydrology-Soil-Vegetation Model (DHSVM) to study climate change effects on hydrology and
water resources management in the Puget Sound Basin. DHSVM was originally designed for application to mountainous
watersheds in the western U.S. (Wigmosta et al, 1994; 2002). The model was calibrated and validated in all Puget
Sound sub-basins (Cuo et al. 2008). To examine the model sensitivity to precipitation and temperature change, observed
and simulated precipitation elasticities and simulated temperature sensitivities were calculated (Table 1). The elasticity
and temperature sensitivity simulations were performed for precipitation increase of 10%, temperature minima (Tmin)
and maxima (Tmax) increase of 1˚C, and temperature maxima increase (no increase in minima) of 2˚C, respectively
(therefore both temperature changes reflect 1˚C increase in the mean temperature). Climate change effects were
studied using downscaled GCM forcings for the 2020s, 2040s and 2080s periods for IPCC SRES A1B and B1 emissions
scenarios, respectively . For the 2020s, individual GCM predicted climate conditions and their composite mean were
used to force DHSVM. For the 2040s and 2080s, composite means of all selected GCMs were used to drive the model.
Periods 2020s, 2040s and 2080s each represent 30-year means centered on 2020, 2040 and 2080, respectively. Table 2
shows the GCMs used in the study. We used the delta method to downscale the GCM forcings to a 1/16 degree grid;
each grid node was treated as a psuedo-station for purposes of producing hydrological model inputs. The delta method
uses mean monthly differences and ratios in temperature (˚C difference) and precipitation (% change) between the GCM
grids and the historical conditions in the study area (Hamlet and Lettenmaier 2000). Daily variability of the future climate
is implicitly assumed to resembles that of the historical conditions and only the monthly mean temperature and
precipitation are shifted up or down. Figure 5 shows the differences in mean monthly temperature and precipitation
between the historical period and the 2020s, 2040s, and 2080s for individual GCMs (blue swath) and composite GCM
means.
3. Results
Investigated Points
Obs
Cedar A
Cedar E
Green A
Green C
Sultan A
Tolt A
1.08
1.38
1.42
1.33
1.06
1.12
Historical Sim. Precip+10%
(fraction)
1.17
1.28
1.22
1.36
1.37
1.61
1.43
1.63
1.12
1.17
1.00
1.20
TminTmax+1C
(%)
-1.1
-1.1
-2.4
-2.3
-0.7
-0.7
Tmax+2C
(%)
-2.8
-3.0
-5.6
-5.6
-1.7
-1.5
Table 1. Precipitation elasticity (fractional change in annual runoff divided by fractional change in
annual precipitation) and temperature sensitivity (fractional change in annual runoff per degree C
increase in annual temperature) for selected locations in Cedar, Green, and Sultan River basins
4. Conclusions
Figure 8. Projected streamflow change and the reservoir storage change in
2020s, 2040s, and 2080s (Oct. 1-7 is the week 1, which is the start of the
water year).
1. All GCMs projected seasonal temperature increases in the Puget Sound region. Projected
precipitation changes vary substantially among the individual GCMs, however the composite
scenarios in the 2020s, 2040s and 2080s show slightly, but progressively higher precipitation in
the winter and lower precipitation in the summer compared to historical conditions.
2. Model and observed runoff elasticities to precipitation are in general agreement, in the range 1.0
– 1.5 as estimated from observations, and in the range 1.0-1.4 from model estimates.
Temperature sensitivities to mean air temperature increases when both Tmax and Tmin are
increased (which in the model, changes vapor pressure deficit and downward and outgoing
longwave radiation, but not downward solar radiation) are in the range 0.7-2.4 percent per
degree C. When only Tmax is increased (which fixes dew point while increasing downward
shortwave and downward and outgoing longwave radiation), sensitivities are substantially higher,
in the range 1.5-5.6 percent per degree C.
3. Snow water equivalent progressively declines through the century in all watersheds.
4. Total evapotranspiration progressively increases, with the most pronounced increases occurring
at high elevations.
5. In the future climate, there are substantial changes in streamflow seasonal patterns for all basins.
In general, winter peaks become progressively higher through the century. Summer flows peaks
become lower, and the spring snowmelt peak is progressively reduced, and in the 2080s
composite scenario, eliminated altogether.
6. Summer reservoir storage decreases progressively through the century for all three water supply
systems.
7. For all variables, changes associated with the A1B emissions scenario are larger than for B1,
however the differences are modest in the 2020s, but become progressively pronounced through
the century as the emissions differences increase.
5. References
Figure 6. Projected snow water equivalent change for 2020s, 2040s and 2080s
Cuo, L., D.P. Lettenmaier, M. Alberti, and J.E. Richey, 2008" Effects of a century of land cover and climate change on the hydrology of Puget Sound
basin.Hydrological Processes (accepted)
Hamlet, A.F., Lettenmaier, D.P., 2000. Long-range climate forecasting and its use for water management in the Pacific Northwest region of North America, J.
Hydroinformatics, Volume 02.3, pp 163-182
IPCC 2007, Snythesis Report, Summary for Policymakers
Wigmosta, M.S., L. Vail, and D. P. Lettenmaier, 1994: A distributed hydrology-vegetation model for complex terrain, Wat. Resour. Res., 30, 1665-1679.
Wigmosta, M.S., B. Nijssen, P. Storck, and D.P. Lettenmaier, 2002: The Distributed Hydrology Soil Vegetation Model, In Mathematical Models of Small
Watershed Hydrology and Applications, V.P. Singh, D.K. Frevert, eds., Water Resource Publications, Littleton, CO., p. 7-42.