Standardised Precipitation Index (SPI) data for different levels of Integrated Hydrological Units (IHU). SPI is a drought index based on the probability of precipitation for a given accumulation period as defined by McKee et al . SPI is calculated for different accumulation periods: 1, 3, 6, 12, 18, 24 months. Each of these is in turn calculated for each of the twelve calendar months. Note that values in monthly (and for longer accumulation periods also annual) time series of the data therefore are likely to be autocorrelated. The standard period which was used to fit the gamma distribution is 1961-2010. The datasets cover the period from 1961 to 2012.  McKee, T. B., Doesken, N. J., Kleist, J. (1993). The Relationship of Drought Frequency and Duration to Time Scales. Eighth Conference on Applied Climatology, 17-22 January 1993, Anaheim, California.