Drought bulletin

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 ofthe 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 od 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	Extremely wet
1.5 to 1.99	Very wet
1.0 to 1.49	Moderately wet
-0.99 to 0.99	Near normal
-1 to 1.49	moderately dry
-1.5 to 1.99	Severely 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/NCAR reanalysis provides global daily precipitation data on a 2.5° grid mesh, from 1948 to present. NCEP reanalisys data are automatically downloaded and stored every month to produce monthly bulletin images. The global data cover allows to realize SPI maps on any desired area.

A brief history of APAT’s drought bulletin

This bulletin was initially set up by collaboration between the former DSTN (now APAT) 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 Drought project - INTERREG IIC programme, 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.

Resume:

• 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.
•  Considerando medie su tempi diversi è possibile monitorare indirettamente la siccità 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, prtovided they are long enough.
• This bullettin 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.
• The bullettin 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”.