[Part-1] Determination of SPI Drought Index Using SPI Generator, RStudio and Microsoft Excel(PART-1)

Meteorological Drought Indices (The video is about SPI)
There are many rain-based meteorological drought indices:
SPI – Described in McKee et al. 1993, 1995
DI (Deciles Index) – Defined by Gibbs and Maher (1967)
PN (Percent of Normal Index) – Described in Willeke et al. (1994)
RAI (Rainfall Anomaly Index) – Developed by Van Rooy (1965)
EDI (Effective Drought Index) – Developed by Byun and Wilhite (1999)
What is SPI?
The SPI is a drought index first developed by T. B. McKee, N.J. Doesken, and J. Kleist and in 1993 (McKee et al. 1993)
Standardized Precipitation Index (SPI) expresses the actual rainfall as a standardized departure with respect to rainfall probability distribution function and hence the index has gained importance inrecent years as a potential drought indicator permitting comparisons across space and time.
SPI is expressed as standard deviations that the observed precipitation would deviate from the long-term mean, for a normal distribution and fitted probability distribution for the actual precipitation record.
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 streamflow, groundwater and reservoir storage. Soil moisture condition responds to precipitation anomalities at short timescale. i.e 1 to 6 months.
SPI quantifies observed precipitation as a standardized departure from a selected probability distribution function that models the raw precipitation data.
The raw precipitation data are typically fitted to a gamma or aPearson Type III distribution, and then transformed to a normal distribution. Because of precipitation is not normally distributed, a transformation is first applied, followed by fitting to a normal distribution.
The computation of SPI requires long term data on precipitation (longer than 30 years is desirable) to determine the probability distribution function which is then transformed to a normal distribution with mean zero and standard deviation of one.
Positive SPI (Standard Precipitation Index) values indicate greater than mean precipitation and negative values indicate less than mean precipitation.