Short answer
SPEI is a standardized drought index based on the difference between precipitation and potential evapotranspiration. Negative SPEI values indicate drier-than-normal water-balance conditions, while positive values indicate wetter-than-normal conditions. Unlike SPI, which uses precipitation alone, SPEI can reflect the effect of temperature and evaporative demand on drought stress.
What is SPEI?
The Standardized Precipitation Evapotranspiration Index was developed to extend the logic of SPI by including atmospheric water demand. Instead of standardizing precipitation alone, SPEI standardizes a climatic water-balance variable, usually precipitation minus potential evapotranspiration. This makes SPEI sensitive to both water supply and water demand.
Because warming temperatures can increase evaporative demand, SPEI is often useful in climate-change studies, agricultural drought analysis, and regions where heat stress and moisture deficit occur together.
How SPEI is calculated
A typical SPEI workflow starts with monthly precipitation and potential evapotranspiration. The difference between them is calculated for each time step. This water-balance series is then accumulated over selected time scales such as 1, 3, 6, 12, or 24 months. Finally, a probability distribution is fitted and values are transformed into standardized units.
| Step | Description | Why it matters |
|---|---|---|
| 1. Calculate water balance | Precipitation minus potential evapotranspiration | Combines water input and atmospheric demand |
| 2. Accumulate over time scale | For example, 3-month or 12-month totals | Links drought to short-term or long-term impacts |
| 3. Fit distribution | Fit a probability model to the accumulated series | Allows standardization across climates |
| 4. Transform to standardized values | Convert probability values to SPEI units | Provides categories similar to SPI |
How to interpret SPEI values
SPEI is interpreted similarly to SPI. Values near zero indicate near-normal conditions. Negative values indicate dry water-balance anomalies, and positive values indicate wet anomalies. A value below -1 generally indicates moderate drought or worse, while values below -2 indicate extreme drought conditions in standardized terms.
However, SPEI should be interpreted carefully because it depends on the method used to estimate potential evapotranspiration. A simple temperature-based PET method can behave differently from a physically based method that uses radiation, wind, and humidity.
SPEI compared with SPI
SPI and SPEI can tell different stories during warm droughts. SPI may show near-normal precipitation, while SPEI may show drought because high temperatures and atmospheric demand increase water loss. This is especially important for agricultural drought, ecological stress, and climate-change attribution.
| Feature | SPI | SPEI |
|---|---|---|
| Main input | Precipitation | Precipitation and potential evapotranspiration |
| Temperature sensitivity | No direct sensitivity | Yes, through PET |
| Best use | Precipitation drought | Water-balance drought |
| Data need | Lower | Higher |
Strengths and limitations
SPEI is valuable because it responds to both dry precipitation conditions and increased evaporative demand. It can be especially informative in warming climates. It also preserves the multi-time-scale structure that makes SPI useful.
The main limitations are related to PET estimation, data quality, and interpretation. In some regions, PET uncertainty can influence the index strongly. SPEI also remains a climatic index; it does not directly measure soil moisture, irrigation, streamflow, or crop stress.
How DMAP-AI can use SPEI
DMAP-AI currently emphasizes SPI in the Research Version, but SPEI fits naturally into the same framework. A SPEI workflow can support multi-index drought monitoring by comparing precipitation-only drought with water-balance drought. This is especially useful when AI interpretation needs to explain whether drought stress is mainly rainfall-driven or intensified by temperature and evaporative demand.
Frequently asked questions
Is SPEI better than SPI?
Not always. SPEI is better when atmospheric water demand matters, but SPI is simpler and requires fewer inputs. The best choice depends on the question.
Can SPEI be used for agricultural drought?
Yes, especially because crop stress is influenced by both water supply and evaporative demand. However, soil moisture and crop stage should also be considered.
Why can SPEI and SPI disagree?
They can disagree when precipitation is near normal but temperature or evaporative demand is unusually high.
Selected references
- Vicente-Serrano, S. M., Beguería, S., and López-Moreno, J. I. (2010). A multiscalar drought index sensitive to global warming: The Standardized Precipitation Evapotranspiration Index. Journal of Climate.
- McKee, T. B., Doesken, N. J., and Kleist, J. (1993). The relationship of drought frequency and duration to time scales.
- World Meteorological Organization and Global Water Partnership. Handbook of Drought Indicators and Indices.