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EC-JRC
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McKee, T.B., N.J. Doesken and J. Kleist. 1993. The relationship of drought frequency and duration to time scale. In: Proceedingsof the Eighth Conference on Applied Climatology, Anaheim, California, 17–22 January 1993. Boston, American Meteorological Society, 179–184. https://www.droughtmanagement.info/literature/AMS_Relationship_Drought_Frequency_Duration_Time_Scales_1993.pdf
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https://edo.jrc.ec.europa.eu/gdo/php/index.php?id=2044
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Second to last full month
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Monthly
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Global
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1 decimal degree
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The layer shows the Standardized Precipitation Index for an accumulation period of 3 months (SPI-3) as implemented in the Copernicus Global Drought Observatory (GDO).
The SPI-n is a statistical indicator comparing the total precipitation received at a particular location during a period of n months with the long-term rainfall distribution for the same period of time at that location.
The long-term historic record is fitted to a probability distribution (the “gamma” distribution - based on data from 1981 to 2010), which is then transformed into a normal distribution so that the mean SPI value for that location and period is zero. For any given region, increasingly severe rainfall deficits (i.e., meteorological droughts) are indicated as SPI decreases below ‒1.0, while increasingly severe excess rainfall are indicated as SPI increases above 1.0.
Because SPI values are in units of standard deviation from the long-term mean, the indicator can be used to compare precipitation anomalies for any geographic location and for any number of time-scales.
SPI is calculated on a monthly basis for a moving window of n months, where n indicates the rainfall accumulation period. SPIs for seven different accumulation periods (1, 3, 6, 9, 12, 24 or 48 months) are available in the GDO. The corresponding SPIs are denoted SPI-1, SPI-3, SPI-6, etc. For example, SPI-3 refers to an accumulation period of three months.
Each accumulation period describes a different aspect of the drought and its potential impacts. SPIs computed for short accumulation periods (SPI-1 to SPI-3) are indicators for immediate impacts such as reduced soil moisture, snowpack, and flow in smaller creeks. SPIs for medium accumulation periods (SPI-3 to SPI-12) are indicators for reduced stream flow and reservoir storage. SPIs for long accumulation periods (SPI-12 to SPI-48) are indicators for reduced reservoir and groundwater recharge.
The SPIs are calculated from the monthly precipitation data provided by the Global Precipitation Climatology Centre (GPCC - http://gpcc.dwd.de/).
The layer displayed in the AKP shows the map for the month second to last (e.g.: in July, the layer displays the SPI-3 values for May).
More information on the product factsheet: https://edo.jrc.ec.europa.eu/documents/factsheets/factsheet_spi.pdf
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The exact relationship between the accumulation period and the drought impact depends on the natural environment (e.g., geology, soils) and the human interference (e.g., existence of irrigation schemes). In order to get a full picture of the potential impacts of a drought, the SPI should be calculated and compared for different accumulation periods. A comparison with other drought indicators is also needed, in order to evaluate the actual impacts on the vegetation cover and different economic sectors.
SPI is based only on precipitations and does not address the effects of high temperatures on drought conditions.
The SPI should be interpreted with care in arid regions (high probability of zero rainfall) where the concept of drought needs to be adapted, or analysed with alternative drought indicators.
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GOAL 13: Climate action, GOAL 13: Climate Action
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