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

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

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.

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

6 Sample HydrographsGood agreement of

•Seasonal cycle•Low Flows•Peak FlowsModel

Obs.

Maurer et al., 2002

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

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

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.

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.

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

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

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

How can we use information from long term hydrologic

model simulations to synthesize the following drought characteristics:

severity, intensity, extent, and duration?

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

How do we define drought extent?

Severity

Droughts change over time!

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)

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

1930s Drought Soil Moisture

80 Percent severity 100

Soil moisture-defined drought

1930s Drought Runoff

80 Percent severity 100

Runoff-defined drought

1950s Drought Soil Moisture

80 Percent severity 100

Soil moisture-defined drought

1950s Drought Runoff

80 Percent severity 100

Runoff-defined drought

Current Drought Soil Moisture

Soil moisture-defined drought

80 Percent severity 100

Current Drought Runoff

80 Percent severity 100

Runoff-defined drought

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

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

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).

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.

Future Research Real-time applications!

Figure from Andy Wood.

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.

AcknowledgementsThanks to Alan Hamlet, Andy Wood,

and HyoSeok Park.

http://www.usask.ca/geology/classes/geol243/243notes/sedlab3.html