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