The Standardised Precipitation Evapotranspiration Index (SPEI) is a drought index based on the difference between precipitation (P) and potential evapotranspiration (PET). The inclusion of PET means that the SPEI can account for the effects of increased temperatures on water demand. Negative (positive) values indicate water deficit (surplus).
SPEI-3 corresponds to the SPEI of one month and the previous 2 months. For example, to look at SPEI values for summer (June, July and August), select August in the drop-down menu. The SPEI values displayed are for August and the previous 2 months, so June to August