Standardized Precipitation Index

Standardized Precipitation Index Background

• Developed by Thomas McKee at Colorado State University
• Embraced by National Drought Mitigation Center and Western Regional Climate Center
• Adapted for use in Hawaii by NWS Honolulu Forecast Office in 2000

What is the SPI?

• A statistical method for assessing rainfall
• More representative than using the average rainfall as a representation of what is "normal".
• What's wrong with the average?
  • Rainfall distributions do not typically follow a Gaussian ("bellcurve") distribution.
  • Rainfall distributions are skewed with lower amounts being more frequent.
  • Result is that the average is biased toward higher values
• Rainfall data are better represented by a Gamma distribution
• In calculating the SPI, the observed rainfall values are first fitted to a Gamma distribution
• The Gamma distribution is then transformed to a Gaussian distribution (a "bell curve").
• The standardized anomaly is then computed with results having an average of zero and a standard deviation of one.
• BOTTOM LINE: All of the above steps makes the SPI independent of both the location and the range of values so that different seasons and climate areas are represented on an equal basis.

Why SPI?

• Palmer Drought Severity Index (PDSI) in widespread use in many parts of the U.S. BUT
  • PDSI not suited for high spatial variability of HI climate
  • PDSI depends on soil moisture and temperature data that are not routinely available in Hawaii
• SPI is dependent only on rainfall data
  • Regularly reporting network of gages available
  • Results normalized - different areas can be assessed equally
  • Can be used to monitor wet OR dry conditions
• SPI is flexible - can be applied to other types of data (e.g. streamflow, reservoir levels, etc.)

A Note on Data Sources

• 58 rain gages (out of about 300 sites) from 3 NWS networks
  • 17 gages report in real-time or near real-time
  • 41 gages manually read and report on a monthly basis
• Selection based on:
  • Location: Tried to have several sites per climate zone
  • Reporting record: should be long and reliable
• Real-time sites allow for SPI calculations right after the end of the month for 17 sites ("quick" report).
• 41 manual sites have 20 to 30 day delay in data availability. ("full" report).

SPI Product Development Process

• SPI values calculated for each site from raw data
• Climate zone averages determined
• Text bulletin created listing SPI values for each site and for each time period (1, 2, 3, 6, 12, 18, and 24 month periods)
• SPI maps created with color-coding for each climate zone.
  • Purple: Extremely wet (SPI at or above +2.00)
  • Blue: Very wet (SPI = 1.50 to 1.99)
  • Green: Moderately wet (SPI = 1.00 to 1.49)
  • Pale green: Near normal (SPI = -0.99 to 0.99)
  • Yellow: Moderately dry (SPI = -1.49 to -1.00)
  • Orange: Very dry (SPI = -1.99 to -1.50)
  • Red: Extremely dry (SPI at or below -2.00)
• Text bulletin and SPI maps posted to NWS Honolulu website.

Short Term SPI (1-2 months)

• Useful for those interested in short-term variations in rainfall
• Sensitive water systems (catchment, surface water, etc.) and certain types of crops, etc.

Seasonal SPI (3 and 6 months)

• Useful for certain crops and wildfire threat areas, etc.

Long-term SPI (1-year or more)

• Useful for livestock, certain crops and wildfire threat areas, water supply, etc.

Sample SPI maps

Big Island

5 zones based roughly on climatology, 1 zone with no data

Maui

3 zones based roughly on climatology, 1 zone with no data

Oahu

5 zones based roughly on climatology

Kauai

3 zones based roughly on climatology