Standardized Precipitation Evapotranspiration Index (SPEI)

Description

The Standardized Precipitation Evapotranspiration Index (SPEI) is calculated using a similar methodology to the Standardized Precipitation Index (SPI). The difference between the two indices is that SPEI accounts for both precipitation and potential evapotranspiration, whereas SPI accounts for only precipitation. Climate Engine allows users to choose to calculate SPEI (and EDDI/SPI) using either a non-parametric or log-logistic standardized probability-based method. Plotting positions are used to obtain probabilities and then converted to SPEI values using an inverse-normal distribution. 

SPEI values near 0 represent near-normal conditions, while positives or negatives represent above- or below-normal conditions. SPEI is approximately the number of standard deviations the precipitation-evapotranspiration amount (accumulated over a specified time scale, i.e. 3-months, 6-month, 12-month, or 24-month) is above or below the mean. For example, SPEI values below -2 represent drought conditions. In many locations, SPEI is considered a more robust drought indicator than SPI because it accounts for important variables that affect atmospheric water demand such as solar radiation, temperature, humidity, and windspeed.

In Climate Engine, potential evapotranspiration is either calculated using ASCE Grass Reference ET (ETo) or Hargreaves Potential Evapotranspiration.

References

• Stagge, J. H., Tallaksen, L. M., Gudmundsson, L., Van Loon, A. F., & Stahl, K. (2015). Candidate distributions for climatological drought indices (SPI and SPEI). International Journal of Climatology, 35(13), 4027-4040.
• Vicente-Serrano, S. M., Beguería, S., & López-Moreno, J. I. (2010). A multiscalar drought index sensitive to global warming: the standardized precipitation evapotranspiration index. Journal of climate, 23(7), 1696-1718.