Standardized Precipitation Index (SPI) Calculator

Formula

The formula to calculate the Standardized Precipitation Index (SPI) is:

\[ SPI = \frac{P - P_{avg}}{\sigma} \]

Where:

\( SPI \) is the Standardized Precipitation Index
\( P \) is the total precipitation for a chosen period of time (mm)
\( P_{avg} \) is the average precipitation for the same period over a long-term record (mm)
\( \sigma \) is the standard deviation of precipitation for the same period over a long-term record (mm)

What is the Standardized Precipitation Index (SPI)?

The Standardized Precipitation Index (SPI) is a meteorological tool used to measure and quantify the severity of a drought or an excess of rainfall in a specific region over a specific period of time. It is calculated based on the probability of recording a certain amount of precipitation, considering the climatological probability. The SPI can be calculated for different time scales, reflecting the impact of drought on the availability of different water resources. Negative SPI values indicate less precipitation than average (drought), while positive values indicate more precipitation than average (wetness).

Example Calculation

Let's assume the following values:

Total Precipitation (P): 120 mm
Average Precipitation (Pavg): 100 mm
Standard Deviation (σ): 15 mm

Using the formula to calculate the SPI:

\[ SPI = \frac{120 - 100}{15} \approx 1.33 \]

The Standardized Precipitation Index (SPI) is approximately 1.33, indicating wetter than average conditions.