EDO Standardized Precipitation Index MARSMet, 12-month accumulation period (SPI-12) (version 2.1.0)

The 12-month Standardized Precipitation Index (SPI-12) is a meteorological drought indicator to monitor precipitation anomalies over 12-month accumulation periods and is a proxy indicator for medium-term impacts, for example reduced stream flow and reservoir storage, and for long-term impacts, for example reduced reservoir and groundwater recharge. This indicator is based on the 10-km interpolated data from the EC-JRC Agri4CAST project for the MARS Crop Yield Forecasting System. The data are actualized every 10 days, thus giving a more actual overview of the indicator than the classical end-of-month computation. The accumulation periods remain however monthly, with monthly windows (28-, 29-, 30-, or 31-day periods) starting on the 1st, 11th and 21st day of the month. The SPI computation method is still provisional and might be upgraded. The SPI represents precipitation anomalies at a given location and time, as the deviation from the long-term mean in a standard-normal distribution, with more negative values representing a more severe drought. SPI is calculated by comparing the observed total precipitation amounts for an accumulation period of a number of months, indicated by a number following SPI-, with the long-term “climatological” rainfall distribution for that same period of the year, derived from historical rainfall records in a specific reference period. A given accumulation period corresponds to different potential impacts of a drought, with shorter accumulation periods generally representing fast-changing aspects of the water system such as surface soil moisture and flow in small rivers, and longer accumulation periods representing slower changing aspects of the water system such as groundwater and baseflow in larger rivers. However, the exact relationship between accumulation period and impacts depends on the natural environment (e.g. geology, soils) and human interference (e.g. irrigation). Details on the calculation of the SPI are given in the indicator factsheet.