The Standardized Precipitation Index, known as SPI, seems to be the most popular among the existing simple indices for the estimation of drought. The Standardized Precipitation Index (SPI) is widely used as drought meteorological index, to identify the duration and/or severity of a drought. The SPI is usually computed by fitting the gamma probability distribution to the observed precipitation data. The available long-term rainfall data is fitted to Gamma probability distribution, which is then transformed to a normal distribution so that the mean SPI for the location and desired period is zero (Mckee et al., 1993).
The Standardized Precipitation Index (SPI) is an index which was developed primarily for defining and monitoring drought. It allows an analyst to determine the rarity of a drought at a given time scale (temporal resolution) of interest for any rainfall station with historic data.Reference. For calculation SPI, continuous long term data of at least 30 years is required to compute SPI; it does not allow missing data. The use of different time scales by the SPI allows the effects of rainfall deficit on different water-resources components to be accounted for, making it robust.
the SPI value, which varies between -2.0 and 2.0,with extremes outside this range occurring at 5% of the time (Edwards and Mckee, 1997). If you want to calculate SPI in yearly, monthly, or moving average 3-month, 6-month, 9-month, 12-month, 18-month, 24-month, 36-month: you can use 'MDM' (Meteorological Drought Monitoring). This is a free tool.
If you want to calculate SPI in yearly, monthly, seasonally, or moving average 3-month, 6-month, 9-month, 12-month, 18-month, 24-month, 36-month and You want to calculate severity: you can use 'RDIT' (Rain-based Drought Indices Tool)
If you want to do complete monitoring and prediction of drought by SPI or any indices you can use DMAP (Drought Monitoring And Prediction)
However, you can calculate SPI in excel by using this excel file: Calculation of SPI in excel
Name: Nurul