Short answer
SPI measures drought using precipitation only. SPEI measures drought using climatic water balance, usually precipitation minus potential evapotranspiration. SPI is simpler, robust, and widely comparable across regions. SPEI is often more sensitive to warming, heat waves, and evaporative demand. In practice, SPI is a good starting point, while SPEI is useful when temperature-driven water stress is important.
Why compare SPI and SPEI?
The Standardized Precipitation Index (SPI) and the Standardized Precipitation Evapotranspiration Index (SPEI) are both designed to express drought conditions as standardized anomalies. A negative value indicates drier-than-normal conditions and a positive value indicates wetter-than-normal conditions. Because both indices can be calculated at multiple time scales, they are often used for drought monitoring, agricultural risk assessment, hydrological interpretation, and climate-change studies.
The key difference is the drought process they represent. SPI asks: How unusual is the precipitation deficit? SPEI asks: How unusual is the balance between precipitation supply and atmospheric water demand? This difference becomes important when temperature, heat waves, wind, radiation, and low humidity increase evapotranspiration demand.
Input data
SPI requires only precipitation. This makes it attractive because precipitation records are widely available and are often longer than temperature or evapotranspiration records. SPEI requires precipitation and an estimate of potential evapotranspiration (PET), which may be calculated from temperature alone or from more complete meteorological variables depending on the method.
| Index | Main input | Water process represented | Typical data challenge |
|---|---|---|---|
| SPI | Precipitation | Atmospheric water supply | Reliable long precipitation record |
| SPEI | Precipitation and PET | Climatic water balance | Consistent PET estimation and temperature data |
Because SPEI depends on PET, it is more sensitive to the method used to estimate evapotranspiration. A temperature-only PET method may behave differently from a physically based method that uses radiation, humidity, and wind speed. This does not make SPEI unreliable, but it means users should document the PET method and data source.
Interpretation
Both SPI and SPEI are commonly interpreted using similar standardized categories. Values near zero indicate near-normal conditions. Negative values represent dry conditions, while positive values represent wet conditions. A value below -1 often indicates moderate drought, below -1.5 severe drought, and below -2 extreme drought. These thresholds are useful, but they should not replace local knowledge or impact data.
For the same location and time scale, SPI and SPEI may give different signals. If precipitation is below normal and temperature is near normal, both indices may show drought. If precipitation is near normal but temperatures are unusually high, SPI may indicate near-normal conditions while SPEI may indicate dryness because evaporative demand has increased. This is one reason SPEI is often used in climate-change and agricultural drought studies.
Time scales
Both indices can be calculated at multiple accumulation periods such as 1, 3, 6, 12, or 24 months. Short time scales are useful for recent rainfall deficits, early warning, and agricultural stress. Longer time scales are useful for water resources, hydrological drought, reservoir storage, and multi-year drought persistence.
SPI-1 and SPEI-1 can change quickly and are useful for short-term monitoring. SPI-12 and SPEI-12 smooth short-term variability and help identify longer drought events. When comparing SPI and SPEI, it is important to compare the same time scale. Comparing SPI-3 with SPEI-12 would mix short-term and annual-scale drought signals.
Strengths and limitations
| Topic | SPI | SPEI |
|---|---|---|
| Strengths | Simple, widely used, precipitation-based, easy to compare across climates. | Includes atmospheric water demand, useful under warming and heat stress. |
| Limitations | Does not directly include temperature or evapotranspiration effects. | Requires PET calculation and can be sensitive to PET method and data quality. |
| Best use | Meteorological drought and baseline drought monitoring. | Agricultural, ecological, and climate-change-sensitive drought studies. |
| Data requirement | Lower. | Higher. |
SPI should not be dismissed because it does not include temperature. Its simplicity is an advantage in many operational settings. SPEI should not be treated as automatically better either. It is more physically inclusive for some questions, but it also introduces additional assumptions through PET estimation.
When should you use SPI or SPEI?
Use SPI when the primary question concerns precipitation deficit, when long precipitation records are available, or when the goal is to compare drought conditions across many locations using a simple and transparent method. SPI is often the best first diagnostic because it is easy to explain and has a long history in drought monitoring.
Use SPEI when temperature-driven water demand matters. This includes crop stress during hot seasons, warming-related drought assessment, ecosystem drought, and studies where evapotranspiration demand is expected to amplify water stress. SPEI is especially valuable when a location receives moderate rainfall but unusual heat increases evaporative demand.
How DMAP-AI uses this comparison
DMAP-AI uses drought indices as structured inputs for chart interpretation, event summaries, and AI-assisted reporting. SPI can provide a stable precipitation-based view of drought history. SPEI can add sensitivity to temperature and evaporative demand. When both indices are available, comparing them helps distinguish rainfall-driven drought from drought intensified by heat and atmospheric demand.
This comparison is important for farmer-facing and scientific outputs. A farmer may need to know whether crop stress is mainly caused by rainfall shortage or by high evaporative demand. A researcher may need to know whether long-term drought trends are consistent across precipitation-only and water-balance-based indicators. Structured metadata helps the AI report these differences without overstating certainty.
Frequently asked questions
Is SPEI always better than SPI?
No. SPEI includes additional information, but it also requires PET estimation. SPI is simpler and often more robust when precipitation is the primary drought signal.
Can SPI and SPEI disagree?
Yes. They can differ when temperature and evaporative demand are abnormal. SPEI may show drier conditions than SPI during hot periods even if rainfall is near normal.
Which index is better for climate-change studies?
SPEI is often useful for climate-change studies because it can reflect increasing atmospheric water demand. However, SPI remains valuable for separating precipitation changes from temperature-driven demand.
Which index is better for farmers?
Both can be useful. SPI helps explain rainfall deficits, while SPEI may better represent crop water stress during hot and dry periods. Soil moisture and crop-stage information should also be considered.
Selected references
- McKee, T. B., Doesken, N. J., and Kleist, J. (1993). The relationship of drought frequency and duration to time scales.
- 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.
- World Meteorological Organization. Standardized Precipitation Index User Guide.
- Beguería, S., Vicente-Serrano, S. M., Reig, F., and Latorre, B. (2014). Standardized Precipitation Evapotranspiration Index revisited.