GDO Standardized Precipitation Index GPCC, 12-month accumulation period (SPI-12) (version 1.2.0)

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 calculated with twelve months total precipitation accumulation period (SPI-12), derived from the Global Precipitation Climatology Centre (GPCC - http://gpcc.dwd.de/) monthly total precipitation data; using a reference period between the year 1981 and 2010. It has a spatial resolution of 1 decimal degree and a temporal resolution of 1 month. 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.