Understanding Drought Indices

Raw precipitation alone can’t tell us whether conditions are normal or unusually dry. For example, 50 mm of monthly rainfall may be abundant in one region and a deficit in another. Drought indices solve this by comparing current conditions to long‑term climate patterns, allowing us to understand how unusual current moisture conditions are. Why drought indices matter Drought indices help to:

• Compare regions with very different climates

• Detect early warning signals of drought

• Track severity and duration over time

• Inform decisions in agriculture, water allocation, and risk management

Standardized Precipitation Index (SPI)¶

The SPI (McKee et al., 1993) is one of the simplest and most widely used drought indicators. What SPI measures SPI represents precipitation anomalies over a chosen timescale, normalized by long‑term climatology. Different accumulation periods highlight different types of drought:

• SPI-1 → short-term meteorological dryness

• SPI-3 → seasonal impacts

• SPI-12 → long-term hydrological stress

How to interpret SPI values (Based on WMO, 2012)

Strengths

• Very simple and robust

• Uses only precipitation data

• Comparable across climates

Limitations

• Ignores temperature and evaporative demand

Standardized Precipitation–Evapotranspiration Index (SPEI)¶

The SPEI (Vicente‑Serrano et al., 2010) builds on SPI by incorporating temperature effects through evaporative demand. What SPEI measures It’s based on the climatic water balance:

Water balance = Precipitation – Potential Evapotranspiration (PET)

Higher temperatures → higher PET → lower available moisture. SPEI captures this temperature‑driven intensification of drought, which is crucial under warming climates.

Advantages

• Includes temperature and evaporative demand

• Sensitive to heat‑stress‑related drought

• Flexible across many timescales (1–48 months)

Limitations

• PET calculation can vary depending on method

• Slightly more complex than SPI