- It requires only monthly precipitation.
- It can be compared across regions with markedly different climates.
- The standardization of the SPI allows the index to determine the rarity of a current drought.
- It can be created for differing periods of 1-to-36 months.
The SPI is obtained by fitting a gamma or a Pearson Type III distribution to monthly precipitation values. The current implementation of dim_spi_n uses a 2-parameter gamma distribution fit where the shape and scale parameters are maximum liklihood estimates as described in
A Note on the Gamma Distribution Thom (1958): Monthly Weather Review, pp 117-122. specifically: eqn 22 for gamma; just above eqn 21However, there is some variation in the methods used to derive the SPI. Generally, monthly precipitation is not normally distributed so a transformation is performed such that the derived SPI values follow a normal distribution. The SPI is the number of standard deviations that the observed value would deviate from the long-term mean, for a normally distributed random variable. One interpretation of the resultant values is:
[+,-]2.00 and above/below: exceptionally [wet,dry] [+,-]1.60 to 1.99: extremely [wet,dry] [+,-]1.30 to 1.59: severely [wet,dry] [+,-]0.80 to 1.29: moderately [wet,dry] [+,-]0.51 to 0.79: abnormally [wet,dry] [+,-]0.50: near normalExplanation of the SPI at different lengths and sample spatial pattterns over the USA at different run times are available.
More information can be obtained at the ClimateDataGuide.