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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