How to define and monitor drought events

Unlike aridity – which is a permanent climatic condition of a region, where the lack of annual rainfall, together with high temperatures, results in a non-effective, life-promoting moisture (cf. "aridity" in AMS, 2012) – drought is a temporary meteorological condition characterized by a decrease of precipitation compared to the historical average for the area of interest (see, e.g., Rossi et al., 1992; WMO, 2006, Schmidt et al., 2012; Mariani et al., 2018). 

There does not exist a unique definition of drought, although all definitions describe the phenomenon as temporary but recurring, with associated environmental, social and economic impacts. All droughts originate from a precipitation deficiency (i.e., meteorological drought), but other types of drought and impacts cascade from this deficiency. Hence, drought can be classified as follows:

1. Meteorological drought, which is usually defined by a precipitation deficiency
2. Hydrological drought, which is characterized by reduced streamflow, inflow to reservoirs, lakes, and ponds, or groundwater levels
3. Agricultural drought, which occurs when there is not enough soil moisture to support crop production
4. Socio-economic and environmental drought, which refers to the drought impacts on society, economic goods and terrestrial and aquatic ecosystems.

The SPI–Standardized Precipitation Index used in the ISPRA Drought bulletin

Several statistical indexes are used for drought monitoring. In the present bulletin, it has been decided to adopt the SPI–Standardized Precipitation Index (McKee et al., 1993), which is one of the most used nationally and worldwide to quantify drought conditions at different timescales. In addition, the adoption of the SPI has a twofold advantage: it uses precipitation only; and it can be compared across regions with different climates. Hence, wetter and drier climates can be represented on a map in the same way. Recently, this index has been also included in the core set of index to be applied for water scarcity and drought monitoring by the Italian network of River Basin District Observatories for water resource uses (Osservatori distrettuali permanenti per gli utilizzi idrici, in Italian; Mariani et al., 2018).

By comparing for the location of interest the precipitation total for a chosen time interval against a cumulative probability distribution for the precipitation data for the identical interval (i.e., the climatological norm for the location), the SPI provides a quantification of drought. Negative SPI values indicate less than median precipitation, that is, drought conditions, more or less extreme, positive values indicate greater than median precipitation, that is, wet conditions. 

Further details on the SPI calculation can be found here.

The drought intensities resulting from the SPI are defined according to the following table (McKee et al., 1993; WMO, 2012):

SPI Values	Class
SPI ≥ 2.0	Extremely wet
1.5 ≤ SPI < 2.0	Very wet
1.0 ≤ SPI < 1.5	Moderately wet
–1.0 < SPI < 1.0	Near normal
–1.5 < SPI ≤ –1.0	Moderately dry
–2.0 < SPI ≤ –1.5	Severely dry
SPI ≤ –2.0	Extremely dry

 

Moreover, the SPI has the intrinsic ability to assess drought over a range of timescales: it can show, for instance, that a given location is presently under wet condition on a short time scale, but it is experimenting drought on a longer time scale. Hence, the SPI is calculated on different timescales (here equal to 3, 6, 12 and 24 months). The SPI values obtained for a suite of different timescales will be suitable to give useful hints on potential drought impacts: on short timescales (1 to 3 months), the SPI focuses on immediate impacts, such as the reduction in soil moisture, snowpack and streamflows of small creeks; on medium timescales (3 to 12 months), the SPI provides information about the reduction in streamflows and inflow to reservoirs; on long timescales (above 12 months), the SPI provide info about the reduction in recharge of reservoirs and groundwater.

The 12-month SPI maps over Italy provided on a monthly basis represent one of the environmental indicators – named Hydrological drought that is yearly published in the ISPRA Environmental Indicators Database (former ISPRA-SNPA Environmental Data Year Book, in Italian), under the Environmental Theme Water resources and budget, and in other related reports.

Anyway, very long time series are necessary for the SPI calculation: a more robust precipitation climatology is essential to obtain a more statistical robust index, leading to less uncertainty in evaluating drought conditions. WMO (2012) suggested to consider at least 30 years of continuous monthly precipitation data. For this reason, having also in mind to monitor drought at European and national scale, it has been decided to use for the ISPRA Drought bulletin the daily precipitation dataset provided by the NCEP/DOE Reanalysis 1 project (Kalnay et al., 1996) from 1948 to present on a global grid with a 2.5° grid mesh. The NCEP reanalisys data are freely-available online* from the website of the Physical Sciences Division (PSD), US National Oceanic & Atmospheric Administration/Earth System Research Laboratory (NOAA/ESRL), and they are used to produce the monthly bulletin. The global data coverage allows realizing SPI maps on any area of interest.

A brief history of the ISPRA Drought bulletin

This bulletin was initially set up by collaboration between the former DSTN-Department of Italian National Technical Services (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”. It was developed in the frame of the EU INTERREG IIC “Drought” project in order to produce a prototype to monitor on a monthly basis drought events in Italy. The extension of the bulletin to Europe was then realized in the framework of the EU SEDEMED – INTERREG IIIB MEDOCC project. The present operational bulletin, extended to the four above-mentioned areas, was designed in the framework of the EU HYDROCARE – INTERREG IIIB CADSES project and it became fully operational afterwards.

The experience provided by the development of the drought bulletin was also useful in the framework of the EU 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. More recently, the SPI maps of this bulletin have been used for a hydrological analysis reported in the ISPRA report “Analisi e valutazione dello stato ambientale del Lago di Bracciano riferito all’estate 2017” (in Italian), requested by the Italian Ministry of Environment for the evaluation of the effects of drought on the environmental status of the Bracciano Lake. 

To summarize:

• Drought is a temporary meteorological condition due to a reduction of precipitation over a certain time period with respect to what is considered normal (climatic average) for the area of interest. Drought can cause several impacts depending to occurrence, severity and duration of the event.
• Meteorological drought is defined by a precipitation reduction, with reduced cloud coverage and an increased solar radiation, and usually by a temperature increase in comparison to some “normal” or average amounts.
• Impacts on human activities and the environment can be evaluated by monitoring drought over a range of timescales.
• The SPI–Standard Precipitation Index is suitable for evaluating the deficiency of precipitation (or abnormal wetness) across regions with different climates. Precipitation is the only required input parameter, providing that sufficiently long time series are available.
• The ISPRA Drought bulletin provides monthly-updated maps of SPI, computed for 3-, 6-, 12- and 24-month timescales over four areas: Italy, Europe, Mediterranean basin and EU CADSES area. The SPI calculation is here based on daily precipitation data from the NCEP/DOE Reanalysis 1 project, which are freely-available only from 1948 to present.
• The operational version of this bulletin was originally realized in the framework of the EU project HYDROCARE, on the basis of a prototype that was developed in the framework of the INTERREG IIC “Drought” project, with the support of the MEDEA group of the University of Rome “La Sapienza”.

*NCEP Reanalysis data provided by the NOAA/OAR/ESRL PSD, Boulder, Colorado, USA, from their Web site at https://www.esrl.noaa.gov/psd/.  

 

Bibliography

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Mariani, S., G. Braca, E. Romano, B. Lastoria, and M. Bussettini, 2018: Linee Guida sugli indicatori di siccità e scarsità idrica da utilizzare nelle attività degli Osservatori permanenti per gli utilizzi idrici (in Italian). Guidelines puplished in the framework of the project CReIAMO PA, 66pp.

McKee, T.B., N.J. Doesken, and J. Kleist, 1993: The relationship of drought frequency and duration of time scales. In Proc. of Eighth Conference on Applied Climatology, American Meteorological Society, January 17–23, 1993, Anaheim CA.

Rossi, G., M. Benedini, G. Tsakiris, and S. Giakoumakis, 1992: On regional drought estimation and analysis. Water Resources Management, 6, 249–277.

Schmidt, G., J.J. Benítez, and C. Benítez, 2012: Working definitions of water scarcity and drought. EU & CIS Document in the framework of the activities of the EU CIS "Expert Group on Water Scarcity & Droughts", 11pp.

WMO–World Meteorological Organization, 2006: Drought monitoring and early warning: concepts, progress and future challenges. WMO-No. 1006, Geneva, 24pp.

WMO–World Meteorological Organization, 2012: Standardized Precipitation Index User Guide (M. Svoboda, M., Hayes, M., Wood, D.). WMO-No. 1090, Geneva, 24pp.