1. What Is The SPEI Equation?

The SPEI (Standardized Precipitation Evapotranspiration Index) is a drought index that considers both precipitation and potential evapotranspiration. It provides a standardized measure of climatic water balance, useful for drought monitoring and assessment.

2. How Does The Calculator Work?

The calculator uses the SPEI equation:

Where:

• \( P \) — Precipitation (mm)
• \( PET \) — Potential Evapotranspiration (mm)
• \( \text{Mean} \) — Mean of the climatic water balance (mm)
• \( \text{Std Dev} \) — Standard deviation of the climatic water balance (mm)

Explanation: The equation standardizes the difference between precipitation and evapotranspiration relative to the historical mean and variability, providing a normalized drought index.

3. Importance Of SPEI Calculation

Details: SPEI is crucial for drought monitoring, water resource management, and climate impact assessments. It helps identify drought severity, duration, and spatial extent across different time scales.

4. Using The Calculator

Tips: Enter precipitation and PET in mm, along with the historical mean and standard deviation of the water balance (P-PET). Ensure standard deviation is not zero for valid calculation.

5. Frequently Asked Questions (FAQ)

Q1: What time scales can SPEI be calculated for?
A: SPEI can be computed for various time scales (e.g., 1, 3, 6, 12 months) to assess short-term and long-term drought conditions.

Q2: How is SPEI interpreted?
A: Negative values indicate drier conditions (drought), while positive values indicate wetter conditions. Severity increases with more negative values.

Q3: What are the data requirements for SPEI?
A: Long-term monthly precipitation and PET data are needed to compute the mean and standard deviation for the standardization process.

Q4: How does SPEI differ from SPI?
A: SPEI incorporates temperature effects through PET, making it more sensitive to climate change impacts than the precipitation-only SPI.

Q5: What are typical SPEI value ranges?
A: SPEI values typically range from -3 to +3, with values below -1 indicating moderate drought and below -2 indicating severe drought.