The SPI calculation for any location is based on the long-term precipitation record for the desired period. This long-term record is fitted to a probability distribution, which is then transformed into a normal distribution so that the mean SPI for the location and desired period is zero (Edwards and McKee, 1997). Positive SPI values indicate greater than median precipitation and negative values indicate less than median precipitation. Because the SPI is normalized, wetter and drier climates can be represented in the same way; thus, wet periods can also be monitored using the SPI.

The SPI at these timescales (12-month) reflects long-term precipitation patterns. A 12-month SPI is a comparison of the precipitation for 12 consecutive months with that recorded in the same 12 consecutive months in all previous years of available data. Because these timescales are the cumulative result of shorter periods that may be above or below normal, the longer SPIs tend to gravitate toward zero unless a distinctive wet or dry trend is taking place. SPIs of these timescales are usually tied to streamflows, reservoir levels, and even groundwater levels at longer timescales. In some locations, the 12-month SPI is most closely related to the Palmer Index, and the two indices can reflect similar conditions.

Regards

As SPI can be negative and positive with the mean value close to 0, the percentage change is generally not applicable to such variables. The trend can be simply described by the slope, like temperature change in °C/(10a).

Songhao Shang Thank you for this answer. I'm assuming the mean isn't affected by the signs of the values.