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?
There are different types of droughts, regarding the subject that is concerned.
The short-term reflects the precipitation and is called meterological drought and best corresponds to SPEI1 (affects only sensitive plants like grass). When the drought holds on, it affects the growing of more robust plants as well - therefore called agricultural drought. So here SPEI3 would be best, using a value at the end of the growth season. Longer lasting so called hydrological droughts also affects ground water and river levels and therefore shipment and/or trees. SPEI6 would be fine using for this. Droughts lasting a year and longer finally affe ct societies and economic - SPEI12.
So, which SPEI best to use depends on the use case.