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. It quantifies observed precipitation as a standardized
departure from a selected probability distribution function that models
the raw precipitation data.
https://climatedataguide.ucar.edu/climate-data/standardized-precipitation-index-spi
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