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Automatic Generation of Drought Index “Standard Precipitation Index” (SPI): Spreadsheet

The Standardized Precipitation Index (SPI) is a widely used drought index that provides a measure of meteorological drought based on precipitation data. The SPI is a statistical tool that can be automatically generated using historical precipitation records. Here's a high-level overview of the automatic generation process for the SPI:

1. Data Collection: Historical precipitation data is collected from weather stations or other sources. The data should cover a sufficiently long period to provide a representative record of precipitation variability in the region of interest.

2. Time Scale Selection: The SPI can be calculated for different time scales, such as 1 month, 3 months, 6 months, 12 months, etc. The selection of the time scale depends on the specific application and the characteristics of the region being studied.

3. Probability Distribution Fitting: The precipitation data for the selected time scale is fitted to a probability distribution, typically the gamma distribution, to characterize the precipitation patterns.

4. Calculation of SPI: Once the probability distribution is fitted to the precipitation data, the SPI values are calculated using the cumulative distribution function of the fitted distribution. This involves transforming the precipitation values into standard normal deviates, resulting in SPI values that represent the deviation of the precipitation for a specific time scale from the long-term average.

5. Interpretation: The calculated SPI values are interpreted to identify the severity and duration of drought conditions. Negative SPI values indicate below-average precipitation (drought conditions), while positive values indicate above-average precipitation (wet conditions).

6. Automation: To automate the SPI generation process, software tools and programming scripts can be utilized to collect, process, and analyze the precipitation data, fit the probability distribution, and calculate the SPI values for the selected time scales.

Automating the SPI generation process allows for the efficient and timely assessment of drought conditions, aiding in drought monitoring, early warning systems, and drought risk management. Numerous software packages and programming libraries are available for automating the calculation of the SPI, making it accessible to researchers, hydrologists, and policymakers.