Severity-area-duration analysis of 20th century drought in the conterminous United States
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Severity-area-duration analysis of 20th century
drought in theconterminous United States
Climate Impacts Group Weekly SeminarOct. 18, 2004
Elizabeth Clark, Konstantinos Andreadis, Dennis Lettenmaier
http://nm.water.usgs.gov/drought/photos.htm
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Outline Background
Motivation VIC Model Drought Definitions
Technique of drought classification Preliminary results for continental U.S.
1930s, 1950s, current drought Comparison of most severe agricultural and
hydrologic droughts Implications for water managers Future research
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Motivation Recent availability of precipitation and
temperature data make model simulation of hydrologic conditions for long periods possible.
Such simulations provide a spatially and temporally continuous data set.
They also allow us to investigate historical droughts in new ways.
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VIC ModelSoil Parameterization
• 3 soil layers
• Variable infiltration curve in upper layer partitions subsurface and quick storm response
• Gravity-driven vertical soil drainage• Non-linear baseflow drainage from lowest layer
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6 Sample HydrographsGood agreement of
•Seasonal cycle•Low Flows•Peak FlowsModel
Obs.
Maurer et al., 2002
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Comparison with Illinois Soil Moisture19 observing stations are compared to the 17 1/8º modeled grid
cells that contain the observation points.
Persistence
Moisture Level
Moisture Flux
Variability
Obs.Model
Maurer et al., 2002
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Drought Definitions Meteorological Drought
Precipitation and Temperature
Agricultural Drought Soil Moisture
Hydrologic Drought Streamflow/Runoff
Socioeconomic Drought Disparity between
supply and demand
http://nm.water.usgs.gov/drought/photos.htm
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Palmer Drought Severity Index
PDSI : measures meteorological drought using a method that accounts for precipitation, evaporation, and soil moisture conditions.
Dai & Trenberth (2004) find correlation between annual PDSI and streamflow and correlation between PDSI and soil moisture during warm season. Snow interferes with soil moisture calculations.
Despite standardization, dependence on termination criteria results in questionable distribution of severe droughts.
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PDSI-based studies Cook et al. (1999) used tree-ring
chronologies to reconstruct US droughts from 1700-1978
EOFs used for regionalization purposes Examined PDSI signal over those regions “Dust Bowl” dominated the entire period Other notable droughts: 1950, 1965, 1977.
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PDSI-based studies Dai et al. (1998) used a monthly PDSI
dataset from 1860-1995 2.5o x 2.5o grid over the globe Major droughts identified: 1930s, 1950s
and 1988 Correlation between PDSI and ENSO
signals Increase in percentage areas of severe
drought during the last 2-3 decades, over many ENSO-sensitive regions
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Drought spatial analysis from other studies
Most other studies have used station data Pre-defined climate regions Statistical methods such as
Correlation analysis, (Oladipo 1986) Empirical Orthogonal Functions (EOF), (Cook
et al. 1999, Hisdal and Tallaksen 2003) Simulation provides continuous spatial
and temporal mapping of hydrologic variables
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Hydrologic Simulations Based on physical processes Not dependent on scattered or
temporally disjoint station data Allow for direct analysis of parameters
of interest, i.e. runoff and soil moisture Use of percentile values standardizes
over heterogeneous regions and is independent of initialization and termination criteria
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How can we use information from long term hydrologic
model simulations to synthesize the following drought characteristics:
severity, intensity, extent, and duration?
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Severity-Area-Duration Analysis
Based on the Depth-Area-Duration technique from probable maximum precipitation analysis
Replace depth with measure of drought severity
S=(1-ΣP/t) S=severity, ΣP = total percentile (soil moisture or
runoff), t = duration
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How do we define drought extent?
Severity
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Droughts change over time!
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SAD Construction
1. Rank cells by severity & identify potential drought centers
2. Search 3x3 neighborhood of drought center
3. Average severities & add areas
4. Output severity and area at specified area intervals
5. Compare the severity at ~25,000 km2 for each potential drought center and select center with maximum severity
Modified from WMO (1960) computational method of DAD analysis
1)
2)
3)
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MethodologyVIC model output
Total column soil moisture
Runoff
Weibull percentiles
Threshold
20th percentile and lower soil moisture
30th percentile and lower runoff
Spatial contiguity
Initial drought classification
Temporal contiguity
Final drought and subdrought classification
Severity-Area-Duration
SAD curves for each event
Highest severities
Envelope curve for each duration
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1930s Drought Soil Moisture
80 Percent severity 100
Soil moisture-defined drought
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1930s Drought Runoff
80 Percent severity 100
Runoff-defined drought
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1950s Drought Soil Moisture
80 Percent severity 100
Soil moisture-defined drought
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1950s Drought Runoff
80 Percent severity 100
Runoff-defined drought
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Current Drought Soil Moisture
Soil moisture-defined drought
80 Percent severity 100
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Current Drought Runoff
80 Percent severity 100
Runoff-defined drought
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Soil Moisture
3 month
6 month
1 year
2 years
4 years
8 years
Apr 1934-June 1934
Feb 1955-Feb 1956
July 2002-July 2002
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Runoff
3 month
6 month1 year
2 years
4 years
8 years
Jun 2002-Jun 2003
Dec 1932-Dec 1939
Nov 1953-Nov 1956
Feb 1977-Feb 1977
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Rising Temperatures and Declining Streamflow in West
US Higher temperatures are resulting
in earlier snow melts, up to 20-30 days earlier (Pagano et al., 2004).
Upper Colorado River basin reported to be experiencing worst streamflow deficit in 80 years & 7th worse in past 500 years (Piechota et al., 2004).
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Implications for Water Management
Similar to Depth-Area-Duration analysis, Severity-Area-Duration analysis provides a basis for a sort of “design drought” estimation.
This estimates an upper bound for anticipated drought severities.
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Future Research Real-time applications!
Figure from Andy Wood.
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Conclusions The most severe historical US
droughts occurred during the 1930s and 1950s.
The current drought ranks among the most severe droughts, especially when averaged over smaller areas.
Future research promises to provide water managers with new tools for real-time drought forecasting.
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AcknowledgementsThanks to Alan Hamlet, Andy Wood,
and HyoSeok Park.
http://www.usask.ca/geology/classes/geol243/243notes/sedlab3.html