GDO Standardized Precipitation Index ERA5, 1-month accumulation period (SPI-1) (version 2.0.0)

The 1-month Standardized Precipitation Index (SPI-1) is an indicator used to monitor meteorological drought based on precipitation anomalies over 1-month accumulation periods. SPI-1 serves as a proxy indicator for immediate impacts of droughts such as reduced soil moisture, snowpack, and flow in smaller creeks. This indicator is derived from the fifth generation ECMWF reanalysis for the global climate and weather ERA5 (https://confluence.ecmwf.int/display/CKB/The+family+of+ERA5+datasets) hourly precipitation data; using a reference period between the year 1991 and 2020. Data have a monthly computation and they are provided 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 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.