EDO Standardized Precipitation Index, 9-month accumulation period (SPI-9), blended and interpolated (version 1.2.0)

The 9-month Standardized Precipitation Index (SPI-9) is a meteorological drought indicator to monitor precipitation anomalies over 9-month accumulation periods and is a proxy indicator for medium-term impacts, for example reduced stream flow and reservoir storage. This indicator has a temporal resolution of 1 month and is derived from daily rainfall observations at SYNOP (Surface Synoptic Observations) stations from the MARS database (https://ec.europa.eu/jrc/en/mars) interpolated at 0.25 decimal degrees, and blended with Global Precipitation Climatology Centre (GPCC - http://gpcc.dwd.de/) monthly total precipitation at 1 decimal degree. Interpolation at 0.25 decimal degree resolution and blending with GPCC data are according to Barnes iterative correction objective analysis (Koch, S. E., M. DesJardins, and P. J. Kocin, 1983: An interactive Barnes objective map analysis scheme for use with satellite and conventional data. J. Climate Appl. Meteor.,22, 1487–1503.). 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-9, 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 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 and blending and interpolation procedures are given in the indicator factsheet.