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Standardized Precipitation-Evapotranspiration Index (SPEI)

The SPEI is an extension of the widely used SPI. The SPEI is designed to take into account both precipitation and potential evapotranspiration (PET) in determining drought. Thus, unlike the SPI, the SPEI captures the main impact of increased temperatures on water demand.

The SPEI can measure drought severity according to its intensity and duration, and can identify the onset and end of drought episodes. The SPEI allows comparison of drought severity through time and space, since it can be calculated over a wide range of climates, as can the SPI.

The idea behind the SPEI is to compare the highest possible evapotranspiration (what we call the evaporative demand by the atmosphere) with the current water availability. Thus, precipitation (accumulated over a period of time) in the SPEI stands for the water availability, while ETo stands for the atmospheric water demand.

Negative SPEI values represent rainfall deficit and less than median precipitation, and high potential epotranspiration (Dry), starts when the SPEI value is equal or below -1.0. Whereas positive SPEI values indicate rainfall surplus and greater than median precipitation, and low potential epotranspiration (Wet), starts when the SPEI value is equal or above 1.0, and ends when the value becomes negative.

SPEI derived from TerraClimate data

About the data

Characteristic Description
Function Display SPEI-1, 2, 3, 6, 9, 12, 18, 24, 36, 48, 60 and 72-months
Variable SPEI
Geographic coverage Global 90N-60S, 180W-180E
Spatial resolution 4-km at equator
Temporal resolution monthly.
Format GeoTIFF
Unit unitless

Symbology

The threshold and the symbology for the SPEI can follow below color codes and image.

Class Threshold Hex RGB
Exceptionally Dry -2.00 and below #760005 #760005 rgb(118, 0, 5)
Extremely Dry -2.00 to -1.50 #ec0013 #ec0013 rgb(236, 0, 19)
Severely Dry -1.50 to -1.20 #ffa938 #ffa938 rgb(255, 169, 56)
Moderately Dry -1.20 to -0.70 #fdd28a #fdd28a rgb(253, 210, 138)
Abnormally Dry -0.70 to -0.50 #fefe53 #fefe53 rgb(254, 254, 83)
Near Normal -0.50 to +0.50 #cccccc #cccccc rgb(204, 204, 204)
Abnormally Moist +0.50 to +0.70 #a2fd6e #a2fd6e rgb(162, 253, 110)
Moderately Moist +0.70 to +1.20 #00b44a #00b44a rgb(0, 180, 74)
Very Moist +1.20 to +1.50 #008180 #008180 rgb(0, 129, 128)
Extremely Moist +1.50 to +2.00 #2a23eb #2a23eb rgb(42, 35, 235)
Exceptionally Moist +2.00 and above #a21fec #a21fec rgb(162, 31, 236)

Data access

Global SPEI-12 data available at DEC S3: s3://wbgdecinternal-ntl/climate/products/spei-terraclimate

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