One of the most robust drought indices is so called Standardized Precipitation Index (SPI). The SPI can be calculated at various time scales which reflect the impact of the drought on the availability of water resources. The SPI calculation is based on the distribution of precipitation over long time periods (30 years (1961-1990) was used). The long term precipitation record is fit to a probability distribution, which is then normalised so that the mean (average) SPI for any place and time period is zero. SPI values above zero indicate wetter periods and values less than 0 indicate drier periods. SPI6 0 0 379 179 31.995 10 50 48.005 10 31.995 48.005 50 urn:ogc:def:crs:EPSG::4326 urn:ogc:def:method:WCS:1.1:2dSimpleGrid 10.0500065789474 49.9499861111111 0.100013157894737 -0.100027777777778 urn:ogc:def:cs:OGC:0.0:Grid2dSquareCS SPI6_RANGE RANGE6 bilinear nearest neighbor 1 2 3 urn:ogc:def:crs:EPSG::4326 image/tiff image/png