Download Effects of Land Cover, Topography, and Climate on Pacific

Effects of Land Cover, Topography,
and Climate on Pacific Northwest
Flooding and Flood Forecasting
JISAO Center for Science in the Earth System
Climate Impacts Group
and Department of Civil and Environmental Engineering
University of Washington
January, 2004
http://www.hydro.washington.edu/Lettenmaier/Presentations/2004/hamlet_coastal_management_jan_2004.ppt
Alan F. Hamlet
Dennis P. Lettenmaier
Hydroclimatology of the Pacific Northwest
Annual PNW Precipitation (mm)
Elevation (m)
The Dalles
(mm)
Winter
Precipitation
Summer
Precipitation
Hydrologic Characteristics of PNW Rivers
Normalized Streamflow
3.0
2.5
Snow
Dominated
2.0
Transient Snow
1.5
Rain Dominated
1.0
0.5
0.0
10 11 12
1
2
3
4
Month
5
6
7
8
9
Sensitivity of Snowmelt and Transient Rivers
to Changes in Temperature and Precipitation
900000
700000
600000
500000
400000
300000
200000
100000
1974
1974
1974
1974
1974
1974
1974
1974
1974
1974
1973
1973
1973
1973
1973
0
1973
•Streamflow timing is altered
• Annual volume stays about
the same
800000
Flow (cfs)
Temperature warms,
precipitation unaltered:
Water Year
900000
800000
600000
500000
400000
300000
200000
100000
1974
1974
1973
1973
1973
1973
1973
0
1973
•Streamflow timing stays about the same
•Annual volume is altered
700000
Flow (cfs)
Precipitation increases,
temperature unaltered:
Water Year
Characteristics of Flooding Events West
and East of the Cascades
Coastal and Transient Snow Basins
(West of the Cascades)
Flooding frequently occurs in Nov-Dec when intense rain
storms with temperatures above freezing are most likely.
In so-called “Rain on Snow” events that produce severe
flooding, the presence of snow is actually not the major driver.
Instead, intense and sustained precipitation over enlarged
basin areas (due to warm temperatures) with fully saturated
soils produce the major component of the runoff in the largest
events.
In moderate flooding events, snow melt and precipitation tend
to be more comparable in their contribution to peak
streamflows and antecedent snowpack is more important.
Effective basin area contributing direct runoff to
the river channel system increases in warm
winter storm events.
Skagit River Basin
Cold
Warm
Snow Melt Dominant Basins
(East of the Cascades)
Flooding mostly occurs in spring when snow melt peaks.
Severe flooding can result from extraordinarily heavy
snowpacks over large spatial areas (e.g. WY 1997), rapid
snowmelt due to extremely warm or clear weather, or from a
combination of sustained snow melt and heavy precipitation
(e.g. the Vanport Flood in 1948).
Moderate snowmelt floods can have much longer duration in
comparison with flooding produced by individual rain storms.
Note that huge snowpacks do not necessarily produce severe
flooding in spring (e.g. WY 1999).
Effects of Land Cover on Flooding in the
Pacific Northwest
Urbanization (increased impervious surfaces and
removal of active soil storage during development)
•Altered streamflows:
Increased magnitude and “flashiness” of peak flows
More rapid recession and lower base flows in late
summer
•Stream channel erosion and instability
•Capacity problems in storm water drainage systems
•Ecological problems due to erosion, scouring, or increased
nutrient and sediment loadings
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 Logging and Road Networks
•Loss of forest canopy increases total snow accumulation
•Increased exposure to wind and solar radiation increases melt
rates
•Road building and culverts alter natural drainage networks
creating “pipes” to the stream channel which increase peak flows
during moderate flooding events
•Loss of vegetation can produce larger sediment loads or trigger
debris flows
•Effects of logging and road building are roughly additive.
Effects of Forest Canopy on
Snow Accumulation
Loss of canopy increases the snow water
equivalent and increases the rate of melt.
Source: 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
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
Effects of Climate Variability on Flooding in the
Pacific Northwest
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
2000
1990
1980
1970
1960
1950
1940
1930
1920
1910
150000
1900
Apr-Sept Flow (cfs)
400000
Naturalized Summer Streamflow at The Dalles
450000
WarmPDO/WarmENSO
Flow (cfs)
400000
WarmPDO/ENSONeut
350000
WarmPDO/CoolENSO
300000
250000
CoolPDO/WarmENSO
200000
CoolPDO/ENSONeut
150000
CoolPDO/CoolENSO
0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1
Probability of Exceedence
Pacific Northwest Streamflow Records
Selected for Flood Analysis
Selection Criteria:
•Unregulated Streams
•Daily Flow Records
•Records 57-65 Years Long
Daily Flow Data w/ Threshold (Oct.1,1990-Nov.30,1990)
Dungeness R near Sequim, WA
6000
Data Processing Methods
daily flow
threshold
Flow (cfs)
5000
4000
3000
2000
1000
11/30/90
11/27/90
11/24/90
11/21/90
11/18/90
11/15/90
11/9/90
11/12/90
11/6/90
11/3/90
10/31/90
10/28/90
10/25/90
10/22/90
10/19/90
10/16/90
10/13/90
10/7/90
10/10/90
10/1/90
10/4/90
0
Daily Flow Data w/ Threshold (1986-1996)
Dungeness R near Sequim, WA
Date
6000
5000
4000
•Set threshold and reset value
3000
•Determine number of peaks
above threshold for each
climate category
2000
1000
Date
5/23/96
12/6/95
6/20/95
1/2/95
7/17/94
1/29/94
8/13/93
2/25/93
9/9/92
3/24/92
10/7/91
4/21/91
11/3/90
5/18/90
11/30/89
6/14/89
12/27/88
7/11/88
1/24/88
8/9/87
2/21/87
9/4/86
3/19/86
0
10/1/85
Flow (cfs)
•Determine mean annual
flood for each basin
daily flow
threshold
•Estimate probability of
event above threshold for
each basin and climate
category
Transient Snow Basins
Probability of Flood Event Above
Threshold
1.00
0.90
0.80
0.70
0.60
0.50
0.40
0.30
0.20
0.10
0.00
EwPw
EnPw
EcPw
EwPc
Climate Category
EnPc
EcPc
Snow-Dominant Basins
Probability of Flood Event Above
Threshold
1.00
0.90
0.80
0.70
0.60
0.50
0.40
0.30
0.20
0.10
0.00
EwPw
EnPw
EcPw
EwPc
Climate Category
EnPc
EcPc
Effects of Climate Change on
the Pacific Northwest
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
Changes in Mean
Temperature and
Precipitation or Bias
Corrected Output
from GCMs
VIC
Hydrology Model
ColSim
Reservoir
Model
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
Naturalized Flow for Historic and Global Warming Scenarios
Compared to Effects of Regulation at 1990 Level Development
Historic Naturalized Flow
Estimated Range of
Naturalized Flow
With 2040’s Warming
Regulated Flow
Effects to the Cedar River (Seattle Water Supply)
for “Middle-of-the-Road” Scenarios
9000
8000
6000
Simulated 20th
Century Climate
2020s Climate
Change Scenario
2040s Climate
Change Scenario
5000
4000
3000
2000
1000
Date
9/2
8/5
7/8
6/10
5/13
4/15
3/18
2/18
1/21
12/24
11/26
10/29
0
10/1
Inflow (acre-ft)
7000
Observed Climate Change:
Trends in Temperature, Precipitation,
Snowpack, and Streamflow
Area-weighted Regional Avg=1.5 F/century
Annual Precipitation Trends
From HCN stations
Elevation (m)
Relative Trends in April 1 Snow Water Equivalent 1916-1997
Relative Trend %/yr
Relative Trend %/yr
Trends in Annual Streamflow at The Dalles from 1858-1998 are strongly downward.
350000
250000
Annual
200000
5 yr mean
10 yr mean
150000
Linear (Annual)
100000
50000
0
1858
1868
1878
1888
1898
1908
1918
1928
1938
1948
1958
1968
1978
1988
1998
Annual Mean Flow (cfs)
300000
Some Conclusions Regarding
Planning, Project Design
Specifications, and Flood Forecasting
“Past Performance is not a Good Measure of
Future Performance.”
Estimates of flood probability distributions and design specifications (e.g. the
“100 year” or “1% likelihood” flood) are a complex function of land surface
characteristics, interannual and decadal scale climate variability, long-term
climate variations (such as global warming), and water management policies,
all of which are non-stationary in time.
For convenience, estimates of flood design specifications have traditionally
been based on fixed periods of the historic record.
In the case of expensive or long-lived structures or for planning processes that
should be robust to climate variability and climate change, the use of the
historic record for flood estimation is problematic both because of relatively
small sample size and changing conditions over time.
Note that in the case non-stationary conditions over time, longer streamflow
records do not necessarily improve estimates of flood frequencies.
Problems with Forecasting Applications
Based on Statistical Relationships
Many operational streamflow forecasting applications are
currently based on statistical relationships between weather
or climate forecasts, snowpack measurements, and
streamflow.
When land cover of the basin or climate conditions change,
the skill of these forecasts can be impaired. Such problems
cannot be resolved in the short term because there is no
training data available for the altered conditions.
These problems have serious implications both for short term
flood forecasting applications and forecasts used for water
management at seasonal time scales.
Use of dynamic models can improve
estimates of hydrologic design
specifications and short-term and seasonal
streamflow forecasts.
Current or Projected
Land Surface
Conditions
Hydrologic Model
Design Criteria
Forecasts
Planning Scenarios
Updated or Projected
Streamflow
Time Series
Current or Projected
Meteorological Data