Applying the Standardized Precipitation Index as a

Drought Indicator

Mark Svoboda, ClimatologistMonitoring Program Area Leader

National Drought Mitigation CenterUniversity of Nebraska-Lincoln

Mali Drought Monitoring Workshop, Bamako September 14-17, 2009

Characteristics of the Standardized Precipitation Index (SPI)

Developed by McKee et al. in 1993Simple index--precipitation is the only parameter (probability of observed precipitation transformed into an index)Being used in research or operational mode in over 60 countriesMultiple time scales allow for temporal flexibility in evaluation of precipitation conditions and water supply

How it WorksIt is NOT simply the “difference of precipitation from the mean… divided by the standard deviation”Precipitation is normalized using a probability distribution so that values of SPI are actually seen as standard deviations from the medianNormalalized distribution allows for estimating both dry and wet periodsAccumulated values can be used to analyze drought severity (magnitude)

How it Works

Need 30 years of continuous monthly precipitation data (the longer the better)SPI time scale intervals longer than 24 months may be unreliableIs spatially invariant in its interpretationProbability based (probability of observed precipitation transformed into an index) nature is well suited to risk management

SPI Methodology• Overview: The SPI is an index based on the

probability of precipitation for any time scale.• Who uses it: Many drought planners

appreciate the SPI’s versatility. • Pros: The SPI can be computed for different

time scales• can provide early warning of drought and help assess

drought severity• less complex than the Palmer.• One number/has historical context

• Cons: Based on Precipitation only• no Temp, no ET• Values based on preliminary data may change.

SPI MethodologyThe SPI was designed to quantify the precipitation deficit for multiple time scalesThese time scales reflect the impact of drought on the availability of the different water resourcesSoil moisture conditions respond to precipitation anomalies on a relatively short scale. Groundwater, streamflow, and reservoir storage reflect the longer-term precipitation anomaliesFor these reasons, McKee et al. (1993) originally calculated the SPI for 3–, 6–,12–, 24–, and 48–month time scales.

SPI MethodologyThe SPI calculation for any location is based on the long-term precipitation record for a desired period. This long-term record is fitted to a probability distribution, which is then transformed into a normal distribution so that the mean SPI for the location and desired period is zero (Edwards and McKee, 1997)Positive SPI values indicate greater than median precipitation, and negative values indicate less than median precipitationBecause the SPI is normalized, wetter and drier climates can be represented in the same way, and wet periods can also be monitored using the SPI.

NDMC SPI ProductsUpdated daily for several time frames using near real time data from the ACIS data stream from the Regional Climate CentersSPI map is generated by using Grid Analysis and Display System (GrADS). The discrete station SPI data are interpolated using a Cressman objective analysis. The grid resolution is 0.4 degreesMonthly SPI archive by climate division first of its kind and now back to 1900

SPI data used in the U.S. Drought Monitor

D0 Abnormally Dry: SPI value of -0.5 to -0.7D1 Moderate Drought: -0.8 to -1.2D2 Severe Drought: -1.3 to -1.5D3 Extreme Drought: -1.6 to -1.9D4 Exceptional Drought: -2.0 or less

NDMC Daily Gridded SPI Product

Probability of Recurrence

SPI Category # of times in 100 yrs.

Severity of event

0 to -0.99 Mild dryness

33 1 in 3 yrs.

-1.00 to-1.49

Moderate dryness

10 1 in 10 yrs.

-1.5 to -1.99

Severe dryness

5 1 in 20 yrs.

< -2.0 Extreme dryness

2.5 1 in 50 yrs.

00.1

0.20.30.40.5

0.60.70.8

0.91

0 5 10 15 20 25

SPI Months

Cor

rela

tion

(r)

Correlation between the PDSI and different SPI series as a function of the time scale of the SPI

Source: ACIS/HPRCC/NDMC

RESOLUTION:

StandardizedPrecipitationIndex (SPI)

by Climate Division (above), and by 0.4º grid (below)

Hilo 6-Month SPI (1950-1998)

-3

-2

-1

0

1

2

3

Jan-50

Jan-55

Jan-60

Jan-65

Jan-70

Jan-75

Jan-80

Jan-85

Jan-90

Jan-95

Time

SPI

Hilo 12-Month SPI (1950-1998)

-3

-2

-1

0

1

2

3

Jan-50

Jan-55

Jan-60

Jan-65

Jan-70

Jan-75

Jan-80

Jan-85

Jan-90

Jan-95

Time

SPI

Using the SPI Program

• Monthly PC version is coded in C++• Available at the NDMC• Easy to use• UNIX version also available

(more robust)• Weekly version now available as well

‐4

‐3

‐2

‐1

0

1

2

3Year

1909

1911

1913

1915

1918

1920

1922

1924

1927

1929

1931

1933

1936

1938

1940

1942

1945

1947

1949

1951

1954

1956

1958

1960

1963

1965

1967

1969

1972

1974

1976

1978

1981

1983

1985

1987

1990

1992

1994

1996

1999

2001

2003

9 Month SPI for Sikasso, Mali

9 Month SPI

‐2

‐1.5

‐1

‐0.5

0

0.5

1

1.5

2

2.5

Year

1948

1949

1950

1952

1953

1954

1956

1957

1958

1960

1961

1962

1964

1965

1966

1968

1969

1970

1972

1973

1974

1976

1977

1978

1980

1981

1982

1984

1985

1986

1988

1989

1990

1992

1993

1994

1996

1997

1998

2000

2001

2002

2004

9 Month SPI for Tessalit, Mali

9 Month SPI

A Case Study:

Monitoring Drought in Hungary with the Standardized Precipitation Index

A. Bussay1, M. Hayes2, Cs. Szinell1, and M. Svoboda2

1. Hungarian Meteorological Service2. National Drought Mitigation Center,

University of Nebraska-Lincoln

Goals of the Study

One of the first applications of the SPI in Hungary and in EuropeTo identify the relationship of the SPI with streamflow, groundwater levels and soil moisture valuesTo assess and compare the monitoring capabilities between the SPI and the Palmer Drought Severity Index

The Study AreaThe Study Area

Table 1. Classification scale for SPI values (after Edwards and McKee 1996)

SPI Values Category Time in Category (%) ≥ +2.00 Extremely Wet 2.3

+1.50 to +1.99 Very Wet 4.4 +1.00 to +1.49 Moderately Wet 9.2 -0.99 to +0.99 Near Normal 68.2 -1.00 to -1.49 Moderately Dry 9.2 -1.50 to -1.99 Severely Dry 4.4

≤ -2.00 Extremely Dry 2.3

Classification Scale for SPI Values

-3-2.5

-2-1.5

-1-0.5

00.5

11.5

2

1981 1982 1983 1984 1985Years

SPI

-6-5-4-3-2-101234

PD

SI

3-month SPI 18-month SPI PDSI

The course of 3-month SPI, 18-month SPI and PDSI during the 1983 drought in the

southeast of Hungary

0

50

100

150

200

250

3001981 1982 1983 1984 1985

Years

Wat

er ta

ble

dept

h (c

m)

-2.5-2-1.5-1-0.500.511.5

SPI

Water Table 6-month SPI 12-month SPI

The course of the 6-month SPI, 12-monthSPI and water table depth during the 1983

drought in the southeast of Hungary

Findings

The SPI and PDSI showed the strongest relationship at around 6 or 7 monthsShorter time scales worked best with soil moisture and streamflow (2 to 3 months)Longer time scales worked best with groundwater levels (12 to 24 months)In all cases, the SPI captured tendencies and characteristics of these variables

The free PC-based version of the SPI can be found at the NDMC: http://drought.unl.edu/monitor/spi/program/spi_program.htm

The free UNIX version of the SPI can be found at:http://ccc.atmos.colostate.edu/standardizedprecipitation.php

OR..the free weekly SPI code is also at:http://greenleaf.unl.edu/downloads

How to get the SPI Program

Thank you!

Questions?

For SPI monthly code (PC or UNIX based),

E-mail me at: msvoboda2@unl.edu