I'm using the SPEI index to obtain a measure of drought conditions for various countries, across a number of years. I just want one measure, per country, per year. I am unclear about whether I should be using SPEI-1, SPEI, 3, SPEI-6 or SPEI-12. I am planning on taking the average for the year, however I've tested a number of locations and can see that the yearly average is rather different depending on which time-series (1,3,6, or 12) is used. I'm looking for the measure that gives the best overall picture of drought conditions within that year, can anyone help?
Many thanks, Orla