What is the difference between SPI-12 and annual SPI?
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, you should calculate average of rainfall in years. Then you will have a number for each year and calculate SPI with these data.
Name: ACHITE Mohammed