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Drought bulletin



This bulletin is aimed to provide the public with an instrument for quantitative monitoring of drought conditions in Italy and Europe, through monthly-updated maps of the Standardized Precipitation Index, this climatic index is widely employed in order to quantify the relative scarcity or abundance of precipitation. Maps on four areas (Italy, Europe, Mediterranean basin, and CADSES area), and for the 3-month, 6-month, 12-month and 24-month time scales from December 1989 to past month, are available through the menu above (select year, month and area).

In few words…

How to define and measure drought

Differently from aridity, which is a permanent condition of water scarcity and a local climatic feature,  drought is a temporary and relative condition, defined as a displacement from the local average (climatic) water supply. A rainy region will experience drought when receiving an amount of precipitation which elsewhere, in an arid region, would be regarded as abundant.

Given the huge relevance of water resource for the environment and people, its shortage may produce multiple impacts. Thus, several different notions of drought are employed, according to the considered class of phenomena – natural, social or economical ones:

  1. Meteorological drought is given by the relative scarcity of precipitation
  2. Hydrological drought, is given by relative water shortage in the soil, in the river flow, or in groundwater
  3. Agricultural drought is the water shortage with respect to the usual irrigation needings
  4. Socio-economical drought is referred to the complex of water consumption in the terrirory, etc.

Another key concept, concerning drought, is the time scale: about meteorological drought, average rainfall during a given time interval (e.g., three months) is compared to the climatological values of the same quantity. Usual time scales for drought vary between a month and few years, affecting the other aspects of drought in a different way. For instance, lack of precipitation during several months will affect the river flow; but when it occurs a one-two year time scale it will impact the groundwater supply. Moreover, in order to evaluate agricultural or socioeconomic drought, additional knowledge about the water use and its evolution is required.

The Standardized Precipitation Index

Several statistical indexes are employed for drought monitoring. Here, the SPI (Standardized Precipitation Index) is employed. This is a meteorological drought index, since it requires only precipitation statistics; other indexes, as for instance the Palmer Drought Severity Index (PDSI) or  the Surface Water Supply Index (SWSI) are meant to monitor hydrological drought. The purpose of SPI, based only on rainfall probability, is to assign a single numeric value to the precipitation that can be compared across regions with markedly different climates. It provides indication of what the amount of precipitation is in relation to the normal. Positive SPI values indicate greater than median precipitation, and negative values indicate less than median precipitation. This is obtained through a normalization of the rainfall probability distribution, estimated from the local historic record (click here for further details). In such a way, arid and humid regions are homogenously monitored.
The following table provides a basis to interpret the SPI values:

SPI Values Class
> 2.0 Extremely wet
1.5 to 1.99 Very wet
1.0 to 1.49 Moderately wet
-0.99 to 0.99 Near normal
-1.49 to -1.0 moderately dry
-1.99 to -1.5 Severely dry
<-2.0 Extremely dry

Moreover, SPI is defined with respect to a precipitation a time scale: it can show, for instance, that a given place is presently under wet conditions on a short time scale, but it is experimenting drought on a longer time scale. So, SPI values are usually provided for a set of time scales (in our case: 3, 6, 12 e 24 months). As seen above, SPI values for different time scales are suitable to evaluate the occurrence of hydrological, agricultural, or socio-economical drought conditions.
Anyway, very long time series are needed for a significant SPI calculation. Freely-available NCEP/DOE reanalysis provides global daily precipitation data on a 2.5° grid mesh, from 1948 to present. NCEP reanalisys data are automatically downloaded* from the Website of the Physical Sciences Division (PSD), US National Oceanic & Atmospheric Administration/Earth System Research Laboratory (NOAA/ESRL), and stored every month to produce monthly bulletin images. The global data cover allows realizing SPI maps on any desired area.

A brief history of the drought bulletin

This bulletin was initially set up by collaboration between the former DSTN (then APAT, now ISPRA) and the MEDEA (MEteorologia Dinamica Elaborazione e Analisi) group of Prof. Alfonso Sutera of the Physics Dept., University of Rome “La Sapienza”. The previous version was developed, in the frame of the INTERREG IIC “Drought” project , in order to produce a prototype to monitor current states of drought in Italy on a monthly basis. The extension to Europe has been realized in the framework of the SEDEMED INTERREG IIIB MEDOCC project. The present bulletin, extended to the four mentioned areas, has been produced in the framework of the HYDROCARE  INTERREG IIIB CADSES project.

The experience provided by the development of the drought bulletin was useful in the framework of the MIPAIS – INTERREG IIIB MEDDOC project, which focused on water demand limitations and irrigation management during drought periods. The ISPRA final report provides an overview of the irrigation, the water scarcity and the vulnerability to the cycle of drought in Italy by presenting the Italian experience in drought monitoring and risk management and the protection of water quantity and quality. In addition, the report illustrates the index-based databases employed in Italy to monitor the drought conditions, including the ISPRA drought bulletin.


  • Drought is a temporary condition of relative scarcity of water resources. Meteorological drought occurs when, locally, average precipitation are much lesser then the climatic average.
  • Hydrological, agricultural, socio-economical drought can be monitored by considering averages defined on longer or shorter time scales.
  • Standard Precipitation Index (SPI) is suitable for homogeneous (independent from local climatic regime) monitoring of relative precipitation shortage. Only precipitation series are needed, provided they are long enough.
  • This bulletin provides monthly-updated maps of SPI, computed on three-month, six-month, one-year and two-year basis, over four areas: Italy, Europe, Mediterranean basin and CADSES area. It employs rainfall time series from 210-km gridded NCEP reanalysis, freely available for the years from 1948 to present.
  • Such on-line version of the bulletin has been realized in the framework of EU project HYDROCARE; it extends a prototype developed in cooperation with the MEDEA group of the University of Rome “La Sapienza”.

*NCEP Reanalysis II data provided by the NOAA/ESRL PSD, Boulder, Colorado, USA, from their Website at: http://www.esrl.noaa.gov/psd/.