Transcript Flood Hydroclimatology and Its Applications in Western United States

A Process-Based “Bottom-Up”
Approach for Addressing
Changing Flood-Climate
Relationships
COHS Workshop
National Academy of Sciences
Global Change and Extreme Hydrology:
Testing Conventional Wisdom
January 5-6, 2010
Katie Hirschboeck
“GLOBAL CHANGE & EXTREME HYDROLOGY”
How can water managers deal with events
in the “tails” of streamflow probability
distributions, both floods
and droughts,
. . .by moving beyond conventional
wisdom and approaches . . . .
WESTERN WATER
MANAGERS :
are planning for future extreme
LOW FLOW conditions using :
-- tree-ring reconstructions
-- simulations
-- scenario-building
-- climate projection modeling
Water supply simulation
based on extreme low
flow sequences in the
paleo-record
STREAMFLOW RECONSTRUCTION for 1330-2005
2002 & 1996 = lowest annual flows in the entire record
In contrast . . .
FLOOD HAZARD MANAGERS:
have been more constrained in developing
ways to incorporate climate change
information operationally due to:
-- existing flood management policy and
practices
-- the short-term, localized, and
weather-based nature of the flooding
process itself
What’s Needed:
Information presented in an operationally
useful format for flood managers which
describes how changes in the large-scale
climatic “drivers” of hydrometeorological
extremes will affect flooding variability
in SPECIFIC WATERSHEDS 
This presentation argues:
. . . that attention to some very basic
elements at the local and regional
watershed scale
-- such as basin size, storm type,
seasonality, atmospheric circulation
patterns, and geographic setting
. . . can provide a basis for a cross-scale
approach to linking GLOBAL climate
variability with LOCAL hydrologic
variations . . .
In other words we will . . . .
. . . let the rivers “speak for themselves”
about how they respond to climate !
Santa Cruz River at Tucson, Arizona
OUTLINE
I. RE-EXAMINING CONVENTIONAL WISDOM
& ASSUMPTIONS:
The Standard iid Assumption for FFA
II. RE-THINKING
New Insights from “Flood Hydroclimatology”
III. THE “BOTTOM-UP” APPROACH
Complementary Upscaling
IV. FINAL THOUGHTS
I. RE-EXAMINING
http://acwi.gov/hydrology/Frequency/B17bFAQ.html#mixed
“Flood magnitudes are determined by many
factors, in unpredictable combinations.
It is conceptually useful to think of the various
factors as "populations" and to think of each year's
flood as being the result of random selection of a
"population”, followed by random drawing of a
particular flood magnitude from the selected
population.”
The Standard iid Assumption for FFA
The standard
approach to
Flood Frequency
Analysis (FFA)
assumes
stationarity in the
time series & “iid”
“ iid ” assumption:
independently,
identically distributed
Storm type 
hydrograph
The type of storm
influences the shape
of a hydrograph and
the magnitude &
persistence of the
flood peak
This can vary with basin
size (e.g. convective
events are more important
flood producers in small
drainage basins)
Summer
convective event
Synopticscale winter
event
Tropical storm or
other extreme event
In addition, extreme flow events can
emerge from synergism in:
The way in which rainfall is
delivered
• in both space (e.g., storm
movement, direction)
• and time (e.g., rainfall
rate, intensity)
• over drainage basins of
different sizes &
orographies
from Doswell et al. (1996)
FLOOD-CAUSING MECHANISMS
Meteorological &
climatological
flood-producing
mechanisms
operate at
varying temporal
and spatial
scales
HYDROMETEOROLOGY
 Weather, short time scales
 Local / regional spatial scales
 Forecasts, real-time warnings
vs.
HYDROCLIMATOLOGY
 Seasonal / long-term perspective
 Site-specific and regional synthesis of
flood-causing weather scenarios
 Regional linkages/differences identified
 Entire flood history context 
benchmarks for future events
Re-Examining the “iid”
Assumption
It all started with a newspaper ad . . . .
THE FFA
“FLOOD PROCESSOR”
With expanded feed tube
– for entering all kinds of flood data
including steel chopping, slicing
& grating blades
– for removing unique physical
characteristics, climatic
information, and outliers
plus plastic mixing blade
– to mix the populations together
Alternative Conceptual Framework:
Timevarying
means
Timevarying
variances
Mixed frequency
distributions
may arise from:
• storm types
• synoptic patterns
Both
• ENSO, etc.
teleconnections
• multi-decadal
circulation regimes
SOURCE: Hirschboeck, 1988
II. RE-THINKING
FLOOD HYDROCLIMATOLOGY
is the analysis of flood events within the
context of their history of variation
- in magnitude, frequency, seasonality
- over a relatively long period of time
- analyzed within the spatial framework
of changing combinations of
meteorological causative mechanisms
SOURCE: Hirschboeck, 1988
Flood Hydroclimatology Approach
 “ Bottom–Up ” Approach
(surface-to-atmosphere)
 Observed Gage Record
 Meteorological / Mechanistic /
Circulation-Linked
 Flood Hydroclimatology Framework /
Link to Flood
Distribution
3 EXAMPLES: Flood Hydroclimatology in AZ
Sample
Distributions of
Peaks-aboveBase (Partial
Duration Series)
events:
Are there
climatically
controlled mixed
populations
within?
Santa Cruz River at Tucson
Peak flows separated into
3 hydroclimatic subgroups
All Peaks
Tropical
storm
Winter
Sumer
Synoptic
Convective
Hirschboeck et .al. 2000
What does this time series look
like when classified
hydroclimatically?
What kinds of storms produced
the biggest floods?
Hydroclimatically classified time series . . .
Santa Cruz at Tucson
52700 (cfs)
50000
45000
C onv e ctiv e
Discharge in (cfs)
40000
Tropical S torm
35000
S y noptic
30000
25000
20000
15000
10000
5000
0
1915
1920
1925
1930
1935
1940
1945
1950
1955
1960
1965
W ater Year
1970
1975
1980
1985
1990
1995
2000
Verde River below Tangle Creek
Peak flows separated into
3 hydroclimatic subgroups
Tropical
storm
All Peaks
Sumer
Convective
Winter
Synoptic
Hirschboeck et .al. 2000
Historical Flood
Sample frequency curve defined by plotting
observed flood magnitudes vs their
empirical probability plotting positions,
separated by flood type
Probability
analysis
based on
hydroclimatically
separated
flood series
Alila & Mtiraoui 2002
Empirical plotting positions
computed separately for
each hydroclimatic type
Annual flood
peaks only:
Thinking Beyond the Standard iid
Assumption for FFA . . . .
Based on these
results we can reenvision the
underlying probability
distribution function
for Arizona floods to
be not this . . . .
. . . but this:
Alternative Model to Explain How
Flood Magnitudes Vary over Time
Schematic for Arizona floods based
on different storm types
Varying mean and standard deviations
due to different causal mechanisms
HOW MIGHT CLIMATE
CHANGE AFFECT THESE
DISTRIBUTIONS?
Change in Frequency or
Intensity of Tropical Storms?
Some Important FloodGenerating Tropical
Storms
Tropical storm
Octave Oct 1983
Latitudinal Shifts in
Winter Storm Track?
Roosevelt Dam
Jan 1993
Sabino Creek
July 2006
Winter flooding
on Rillito in Tucson
More Intense Summer
Monsoon?
. . . or this:
Conceptual Framework for
Circulation Pattern Changes
When the dominance of different types of floodproducing circulation patterns changes over time, the
probability distributions of potential flooding at any
given time (t) may be altered.
Blocking
Zonal
Regime
Regime
La Nina
year
El Nino year
. . . or this: Conceptual Framework for
Low-Frequency Variations and/or Regime Shifts:
A shift in circulation or
SST regime (or anomalous
persistence of a given
regime)
will lead to different
theoretical frequency /
probability distributions
over time.
Hirschboeck 1988
Flood Hydroclimatology for
Floods of Record
after Costa (1985)
Extreme Floods of Record
evolved from:
• uncommon (or unseasonable) locations of
typical circulation features
(a future manifestation of climate change?)
• unusual combinations of atmospheric
processes
• rare configurations in circulation patterns
(e.g. extreme blocking)
• exceptional persistence of a specific
circulation pattern.
EXAMPLE:
Rare configurations in circulation patterns
(extreme blocking)
Lane Canyon flash flood
EXAMPLE:
exceptional
persistence of a
specific circulation
pattern.
Jimmy Camp Creek flood of 1965
OVERALL:
Unusually large floods in drainage
basins of all sizes are likely to be
associated with circulation anomalies
involving quasi-stationary patterns
such as blocking ridges and cutoff
lows in the middle-level flow.
III. THE BOTTOM-UP
APPROACH
DOWNSCALING
Interpolation of GCM
results computed at
large spatial scale fields
to higher resolution,
smaller spatial scale
fields,
and eventually
to watershed processes
at the surface.
Hirschboeck 2003 “Respecting the Drainage Divide”
Water Resources Update UCOWR
“Scaling up from local data
is as important as scaling down
from globally forced regional
models.”
— Pulwarty, 2003
PROPOSED COMPLEMENTARY APPROACH:
RATIONALE FOR
PROCESS-SENSITIVE UPSCALING:
Attention to climatic driving forces & causes:
-- storm type seasonality
-- atmospheric circulation patterns
with respect to:
-- basin size
-- watershed boundary / drainage divide
-- geographic setting (moisture sources, etc.)
. . . can provide a basis for a cross-scale linkage
of GLOBAL climate variability
with LOCAL hydrologic variations
at the individual basin scale . . .
• Process-sensitive upscaling . . .
can define relationships that may not be
detected via precipitation downscaling
• Allows the imprint of a drainage basin’s
characteristic mode of interacting with
precipitation in a given storm type to be
incorporated into the statistics of the flow
event’s probability distribution as it is
“scaled up” and linked to model output
and /or a larger scale flow-generating
circulation pattern
IV. FINAL THOUGHTS
Is this evidence of climate change?
Extreme events have a legacy of confounding us!
Overall Recommendation:
A systematic compilation of watershedspecific information about spatially and
temporally varying hydroclimatic
extremes is proposed as a starting place
for making operationally useful
decisions about prospective climatic
changes.
1. The impact of climate change on a
flood distribution is likely to be more
complex than a simple shift in mean or
variance
2. Climatic changes can be
conceptualized as time-varying
atmospheric circulation regimes that
generate a mix of shifting streamflow
probability distributions over time
Recommendation: We need new and
evolving statistical tools that can address
this behavior.
3. The interactions between storm properties
and drainage basin properties also play an
important role in the occurrence and
magnitude of large floods both regionally
and seasonally.
Recommendation:
Watershed–based hydrometeorology
studies should be a key component of
watershed and flood management practice.
4. Shifts in storm track locations and other
anomalous circulation behavior are clearly
linked to unusual flood (and drought )
behavior.
They are likely to be the factors most
directly responsible for projected
increases in hydrologic extremes under a
changing climate.
Recommendation: Use process-sensitive
upscaling to link circulation patterns
directly to flood–producing mechanisms
and to complement downscaling