Download Hydrologic Forecasting - University of Washington

Hydrologic Forecasting:
Effects of Climate Variability,
Climate Change and Land Use
Center for Science in the Earth System
Climate Impacts Group
and Department of Civil and Environmental Engineering
University of Washington
Feb, 2005
http://www.hydro.washington.edu/Lettenmaier/Presentations/2005/hamlet_AAAS_feb_2005.ppt
Alan F. Hamlet Philip W. Mote
Dennis P. Lettenmaier
Natural Variability and
Global Warming
Pacific Decadal Oscillation
El Niño Southern Oscillation
A history of the PDO
A history of ENSO
warm
warm
cool
1900 1910
1920
1930 1940 1950
1960 1970 1980
1990 2000
1900 1910
1920
1930 1940 1950
1960 1970 1980
1990 2000
Effects of the PDO and ENSO on Columbia River
Summer Streamflows
PDO
450000
Cool
Cool
Warm
Warm
350000
300000
250000
200000
high
high
low
low
Red = warm ENSO Green = ENSO neutral Blue = cool ENSO
2000
1990
1980
1970
1960
1950
1940
1930
1920
1910
150000
1900
Apr-Sept Flow (cfs)
400000
The earth is warming -- abruptly
Mostly human induced
Mostly natural variability
Hydrologic Climate Change
Scenarios
Water
Management
Model
Downscaled
Temperature and
Precipitation from
GCMs
VIC
Hydrology Model
Time Scales:
Monthly
Weekly
Daily
Four Delta Method Climate Change Scenarios for the PNW
Delta T, 2020s
Delta T, 2040s
5
5
~ + 1.7 C
~ + 2.5 C
4
hadCM2
3
hadCM3
2
PCM3
ECHAM4
1
Degrees C
Degrees C
4
mean
0
hadCM2
3
hadCM3
2
PCM3
ECHAM4
1
mean
0
J
F
M
A
M
J
J
A
S
O
N
D
J
-1
F
M
A
Precipitation Fraction, 2020s
J
J
A
S
O
N
D
Precipitation Fraction, 2040s
1.75
1.75
1.5
1.5
hadCM2
hadCM3
1.25
PCM3
1
ECHAM4
Fraction
Fraction
M
-1
hadCM2
hadCM3
1.25
PCM3
1
ECHAM4
mean
0.75
mean
0.75
0.5
0.5
J
F
M
A
M
J
J
A
S
O
N
D
J
F
M
A
M
J
J
A
S
O
N
D
Somewhat wetter winters and perhaps somewhat dryer summers
The main impact: less snow
VIC Simulations of April 1 Average Snow Water Equivalent
for Composite Scenarios (average of four GCM scenarios)
Current Climate
2020s
Snow Water Equivalent (mm)
2040s
600
Area Average Water
(depth in mm)
Seasonal
Water Balance
Naches River
700
500
precipitation
400
swe
runoff+baseflow
soil storage
300
evapotranspiration
200
100
Current Climate
sep
aug
jul
jun
may
apr
mar
feb
jan
dec
600
500
precipitation
400
swe
runoff+baseflow
soil storage
300
evapotranspiration
200
100
sep
aug
jul
jun
may
apr
mar
feb
jan
dec
nov
0
oct
2040s Scenario
(+ 2.5 C)
700
Area Average Water
(depth in mm)
More runoff in
winter and
early spring,
less in
summer
nov
oct
0
Basin Averaged Runoff (mm)
Changes in Runoff in the Naches River Basin Associated with
Systematic Warming
180.00
160.00
140.00
120.00
100.00
+ 2.5° C
current climate
composite 2040
80.00
60.00
40.00
20.00
0.00
oct
nov dec
jan
feb mar apr may jun
jul
aug sep
Observed Climate Change:
Hydrologic Impacts for the West
Trends in April 1 SWE 1950-1997
Source: Mote et al. (2005), Declining Snowpack in the Western U.S., BAMS
DJF AVG T (C)
1916-1997
Relative Trend in April 1 SWE
(% per year)
DJF AVG T (C)
1916-1997
Effects of Temp
Relative Trend in April 1 SWE
(% per year)
DJF AVG T (C)
1916-1997
Effects of Precip
Relative Trend in April 1 SWE
(% per year)
a) 10 % Accumulation
b) Max Accumulation
c) 90 % Melt
Trends
in
SWE
19161997
Change in Date
Change in Date
Change in Date
DJF Temp (C)
DJF Temp (C)
DJF Temp (C)
Change in Date
Change in Date
Change in Date
Trends in fraction of annual runoff 1947-2003 (cells > 50 mm of SWE on April 1)
March
Relative Trend (% per year)
June
As the West warms,
winter flows rise
and summer flows
drop
I.T. Stewart, D.R. Cayan, M.D.
Dettinger, 2004, Changes toward
earlier streamflow timing across
western North America, J. Climate
(in review)
Figure courtesy of Iris
Stewart, Scripps Inst. of
Oceanog. (UC San Diego)
Some Hydrologic Effects Associated with Changes in
Land Use
1) Urban Development
•
Development or isolation of natural flood plains (dikes and levies) :
Loss of ground water recharge area—reductions in summer streamflow and
increased water temperatures
•
Increased impervious surface and soil removal:
Increased runoff production in wet season, loss of groundwater recharge
and storage, loss of water availability in dry season
2) Forest Disturbance
•
Loss of forest canopy due to logging, fire, insects, or disease:
Modest increases in flooding, net increases in water availability (lower
annual ET), potential for increased sediment loads until vegetation
recovers.
•
Forest roads
Modest increases in flooding. Modeling studies suggest that the effects are
roughly additive when combined with the effects of clear cut logging.
Typical Effects of Urbanization on a Small Watershed
Des Moines Creek
Source: Booth D.B., 2000, Forest Cover, Impervious-Surface Area, and the Mitigation
of Urbanization Impacts in King County, WA
http://depts.washington.edu/cwws/Research/Reports/forest.pdf
Effects of Harvest Strategies on
Magnitude of Flood Peaks
Modeling studies (Storck 2000)
and comparative analysis of
observations in paired
catchments (Bowling et al. 2000)
show that large scale clearcutting
results in increased flood peaks
on the order of 10% for small
basins in the transient snow zone
of the Cascades.
Sources: Storck, P., 2000, Trees, Snow and Flooding: An Investigation of Forest Canopy Effects on Snow
Accumulation and Melt at the Plot and Watershed Scales in the Pacific Northwest, Water Resources Series
Technical Report No. 161, Dept of CEE, University of Washington
Bowling, L.C., P. Storck and D.P. Lettenmaier, 2000, Hydrologic effects of logging in Western Washington,
United States, Water Resources Research, 36 (11), 3223-3240
Effects of Roads Networks on
Peak Flows
Bowling and Lettenmaier (1997)
estimated that the 10-yr flood peak
increased ~10% in two small
transient snow basins due to road
networks alone.
Roads and logging together were
estimated to increase the 10-yr
flood peak on the order of 20% in
the same two small transient snow
basins.
Bowling, L.C. and Lettenmaier, D.P., 1997, Evaluation of the Effects of Forest Roads on
Streamflow in Hard and Ware Creeks, Washington, Water Resources Series Technical Report
No. 155, Dept of CEE, University of Washington
Conclusions
•Large-scale changes in the seasonal dynamics of snow accumulation and
melt have occurred in the West as a result of increasing regional
temperatures. These changes are not well explained by natural variability.
•Precipitation changes remain uncertain and (unlike temperature) seem most
clearly associated with decadal variability rather than long-term trends.
•Hydrologic changes in the West include earlier and reduced peak
snowpack, more runoff in winter, earlier snowmelt and peak flows, less runoff
in summer, and reduced late summer low flows. Because these effects are
shown to be predominantly due to temperature changes, we expect that they
will both continue and increase in intensity as global warming progresses in
the 21st century, precipitation variability not withstanding.
•Except in areas where permanent changes are made (e.g. urbanization),
one can argue that changes in climate are likely to be more important
hydrologic drivers overall than changes in land use. Climate affects entire
river basins at once and is frequently much more persistent than vegetation
disturbances (e.g. PDO vs. logging).
Natural AND human influences explain the observations best.
Temperature changes are much more certain than precipitation changes.
Natural Climate Influence
Human Climate Influence
All Climate Influences
Decadal Climate Variability Doesn’t Explain the Loss of
SWE Due to Warming
1916-97
1947-97
1925-46
with 1977-95
Relative SWE Trends Due to Temperature Effects Alone (% per year)
Decadal Climate Variability Does Explain Some of the
Changes of SWE Due to Precipitation Changes
1916-97
1925-76
1947-97
Relative SWE Trends Due to Precipitation Effects Alone (% per year)