How can I calculate standard precipitation index?

The standard precipitation index refers to SPI. The Standardized Precipitation Index (SPI) is a widely used index to characterize meteorological drought on a range of timescales. On short timescales, the SPI is closely related to soil moisture, while at longer timescales, the SPI can be related to groundwater and reservoir storage. The SPI can be compared across regions with markedly different climates.

Agrimetsoft has developed different tools for calculating SPI (Standardized Precipitation Index), namely Drought Monitor and Prediction Tool (DMAP), precipitation-based drought indices (RDIT), and MDM by Salehnia et al., (2017), for better clarification let's read the RDIT tool. RDIT can calculate eight indices of meteorological drought. You can draw graphs and estimate severity, onset, and end of each period's drought. Input and output data are in excel format file. For input data, RDIT is smart. It can assess your data and check blank cells and produce data for 'null' cells. This software calculates SPI (Standardized Precipitation Index), DI (deciles index), PN (Percent of Normal Index), RAI (Rainfall Anomaly Index), EDI (effective drought index), CZI(China-Z index), MCZI (modified CZI), ZSI (Z-Score Index) in form of yearly, seasonally, monthly and moving average for 3, 6, 9, 12, 18, 24, 48 months. You can save graphs to image files. By RDIT, you can calculate severity of drought in each indices.

About SPI (Standardized Precipitation Index): In simple terms, SPI is a normalized index representing the probability of occurrence of an observed rainfall amount when compared with the rainfall climatology at a certain geographical location over a long-term reference period. Negative SPI values represent rainfall deficit, whereas positive SPI values indicate rainfall surplus. Intensity of drought event can be classified according to the magnitude of negative SPI values such that the larger the negative SPI values are, the more serious the event would be. For example, negative SPI values greater than 2 are often classified as extremely dry conditions. Studies have shown that the SPI is suitable for quantifying most types of drought events (Mishra and Desai 2005; Szalai and Szinell 2000). Calculated at various time scales (from 1 to n months), SPI values can be efficient for the description of streamflow (on 2-6-month time scales), agricultural drought (on 2-3-month time scales), and groundwater level (on 5-24-month time scales) (Lloyd-Hughes and Saunders 2002). Some workers have stated that, because it depends only on precipitation, the SPI is not affected by topography (Hayes et al. 1999; Lloyd-Hughes and Saunders 2002; Lana et al. 2001).