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Environmental information systems
The duration of meteorological droughts represents the average number of months in a year experiencing drought conditions as determined by anomalously low precipitation values. The index is based on the Standardized Precipitation Index aggregated over three months (SPI-3), which accounts for the deficit or surplus of precipitation with respect to a reference period. Alternative aggregation periods for SPI can be used depending on the type of drought considered and the specific applications. SPI values represent standard deviations of precipitation from the long-term mean. A drought event is considered to start when SPI values fall below -1 for at least two consecutive months and to end when the index returns to a positive number (see the ETC-CCA Technical Paper for details).
