Standardized Precipitation Evapotranspiration Index
Also known as: SPEI
The Standardized Precipitation Evapotranspiration Index (SPEI) is a climate index that combines precipitation and temperature (via reference evapotranspiration) to characterize water deficits and droughts. Developed by Vicente-Serrano and colleagues in 2010, SPEI extends the SPI framework to account for the combined effect of precipitation deficiency and increased evaporative demand from warming, providing a more physically-based drought metric than precipitation-only indices.
Key highlights
- Physically motivated: includes both water supply and demand, reflecting real hydrological stress
- Sensitive to warming: captures drying trends driven by increased evapotranspiration, even if precipitation remains constant
- Flexible timescale: supports 1-, 3-, 6-, and 12-month aggregations for assessing impacts at different time horizons
- Improved drought characterization: compared to precipitation-only metrics, SPEI better correlates with streamflow, groundwater, and soil moisture changes
Intuition
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How it works
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When to use it
Use SPEI for drought assessment when temperature is expected to influence water availability (semi-arid and arid regions where evaporative demand is high), for climate change impact assessments (SPEI better captures warming-driven drying than SPI), and for operational drought monitoring where both precipitation deficit and temperature anomalies matter. For tropical humid regions where evapotranspiration is less sensitive to temperature variation, SPI may be adequate.
Strengths & limitations
- Physically motivated: includes both water supply and demand, reflecting real hydrological stress
- Sensitive to warming: captures drying trends driven by increased evapotranspiration, even if precipitation remains constant
- Flexible timescale: supports 1-, 3-, 6-, and 12-month aggregations for assessing impacts at different time horizons
- Improved drought characterization: compared to precipitation-only metrics, SPEI better correlates with streamflow, groundwater, and soil moisture changes
- Requires temperature data in addition to precipitation, limiting application in regions with sparse temperature observations
- Reference evapotranspiration (ET0) parameterization introduces uncertainty; different ET0 formulae yield different SPEI values
- Probability distribution fitting can be problematic in arid regions with many months of zero precipitation
- SPEI projections inherit large uncertainty from GCM temperature and precipitation projections, particularly at regional scales
Common pitfalls
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Applications
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Frequently asked
What is reference evapotranspiration (ET0) and how is it calculated?
ET0 is the evapotranspiration rate from a reference crop (grass) under well-watered conditions and optimal growing conditions. It represents the atmospheric demand for water. The Hargreaves method estimates ET0 from temperature alone; the Penman-Monteith method uses temperature, humidity, wind, and radiation for greater accuracy. For a given temperature, ET0 increases with aridity (lower humidity) and wind, and with stronger radiation.
How does SPEI differ from the Palmer Drought Severity Index (PDSI)?
Both combine precipitation and temperature. PDSI uses a soil water balance model with prescribed soil properties; SPEI uses simple accumulation of (P - ET0). SPEI is faster to compute and doesn't require soil data. SPEI time series at different timescales are independent; PDSI values at different timescales are derived from a single underlying process. For operational monitoring, SPEI is more practical.
Can SPEI project future drought under climate change?
Yes. Downscaled GCM precipitation and temperature projections can be input to SPEI calculation, yielding SPEI projections for 2050 or 2100. However, large uncertainty exists due to GCM spread and downscaling method. Ensemble SPEI from multiple GCMs is recommended.
What are typical SPEI thresholds for drought classification?
Similar to SPI: SPEI > 1.5 is extremely wet; 1.0–1.5 very wet; 0.5–1.0 moderately wet; -0.5–0.5 near normal; -0.5 to -1.0 moderately dry; -1.0 to -1.5 severe drought; -1.5 to -2.0 very severe drought; < -2.0 extreme drought.
Sources
- 1.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.
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ScholarGate. (2026, June 3). Standardized Precipitation Evapotranspiration Index. ScholarGate. https://scholargate.app/geophysics/standardized-precipitation-evapotranspiration-index