What is the difference between SPI-12 and annual SPI?

The Standardized Precipitation Index (SPI) is a relatively new drought index based only on precipitation. The SPI can be used to monitor conditions on a variety of time scales. This temporal flexibility allows the SPI to be useful in both short-term agricultural and long-term hydrological applications. The SPI (The Standardized Precipitation Index) was designed to quantify the precipitation deficit for multiple time scales. These time scales reflect the impact of drought on the availability of the different water resources. Soil moisture conditions respond to precipitation anomalies on a relatively short scale. Groundwater, streamflow, and reservoir storage reflect the longer-term precipitation anomalies. For these reasons, McKee et al. originally calculated the SPI for 3-, 6-, 12-, 24-, and 48-month time scales.

For calculating annual SPI, you should use summation (or average) of rainfall data in each year but in SPI-12, you should calculate average of last 12 month for each month. In fact, the SPI-12 is monthly that each month of rainfall is equal to average of last 12 month for each month.

For example you want to calculate 12-month moving average of Mar-2000, you should calculate average of monthly rainfall from Apr-1999 to Mar-2000 and for calculating 12-month moving average of Apr-2000, then calculate average of monthly rainfall from May-1999 to Apr-2000. After the process done, you will have 12 number of rainfall for each year and you should calculate SPI with these data.

For example you want to calculate annual SPI (The Standardized Precipitation Index), you should calculate average of rainfall in years. Then you will have a number for each year and calculate SPI with these data.

The SPI (The Standardized Precipitation Index) calculation for any location is based on the long-term precipitation record for a 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. Positive SPI (The Standardized Precipitation Index) 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, and wet periods can also be monitored using the SPI (The Standardized Precipitation Index).

Simple Moving Average Formula