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Drought Indicators

Drought monitoring is based on the analysis of a series of drought indicators, representing different components of the hydrological cycle (e.g. precipitation, soil moisture, reservoir levels, river flow, groundwater levels) or specific impacts (e.g. vegetation water stress) that are associated with a particular type of drought. The indicators generally represent statistical anomalies of the current situation with respect to the long-term climatology at a given location and period of time, and so they provide a measure of the probabilistic severity of a given event.

Amongst the indicators of meteorological drought, the Standardized Precipitation Index (SPI) and Standardized Precipitation-Evapotranspiration Index (SPEI) are the most well-known. They are probabilistic measures of the severity of anomalous dry events, which can be calculated for various rainfall accumulation periods (e.g. 1 to 48 months) and statistically linked to impacts in different economic and environmental sectors. Snowpack or snow water equivalent (SWE) is another important variable in Northern Europe and in mountainous regions, which affects water availability over the year, by snow accumulation and melt. Drought indicators based on soil water content, such as EDO’s Soil Moisture Anomaly (SMA), the Drought Severity Index (DSI), or the Palmer Drought Severity Index (PDSI), aim to characterize the impact on plant water stress. Indicators of hydrological drought, such as EDO’s Low Flow Index (LFI), are usually based on threshold approaches to quantify the volume of water deficit in rivers and reservoirs. Finally, combined indicators blend several physical indicators into one high-level indicator of hazard (e.g. US Drought Monitor; EDO’s Combined Drought Indicator). The recently published “Handbook of Drought Indicators and Indices” (*) provides further information on commonly used drought indicators.

EDO currently produces the following continuously updated drought indicators at the European scale:

  • Standardized Precipitation Index (SPI): This indicator measures anomalies of accumulated precipitation during a given period (e.g. 1, 3, 12 months), and is the most commonly used indicator for detecting and characterizing meteorological droughts.
  • Standardized Snowpack Index (SSPI): This indicator measures anomalies of the snow water equivalent (SWE) that is contained in the “snowpack” in Europe’s cold regions, and can be used to identify in advance areas at high risk of hydrological drought in early summer.
  • Soil Moisture Anomaly (SMA): This indicator measures anomalies of daily soil moisture (water) content, and is used to measure the start and duration of agricultural drought conditions.
  • Anomaly of Vegetation Condition (FAPAR Anomaly): This indicator measures anomalies of satellite-measured FAPAR (Fraction of Absorbed Photosynthetically Active Radiation), and is used to highight areas of relative vegetation stress due to agricultural drought.
  • Low-Flow Index (LFI): This indicator, which is derived from daily river discharge outputs produced by the JRC’s in-house hydrological rainfall-runoff model (LISFLOOD), is used for near real-time monitoring of hydrological streamflow drought at European scale.
  • Heat and Cold Wave Index (HCWI): This indicator is used to detect and characterize extreme temperature anomalies, such as heat waves (during the warm season) and cold waves (during the cold season), and is computed based on daily minimum and maximum temperatures.
  • Combined Drought Indicator (CDI): This indicator integrates information on anomalies of precipitation, soil moisture and satellite-measured vegetation condition, into a single index that is used to monitor both the onset of agricultural drought and its evolution in time and space.

The above listed drought indicators, produced within the framework of EDO, are complemented by regionally or locally important indicators at larger scales (e.g. national, regional, river basin), which are provided through a network of partners for their area of competence, based on their own data, and made available through interoperable web-mapping services.

Note that for each of the EDO continental-scale drought indicators listed above, a brief description in the form of a factsheet, which provides the key message, a scientific method, quality information, and references, is available at the EDO Factsheets page (under Reference Data).

* World Meteorological Organization (WMO) and Global Water Partnership (GWP). 2016. Handbook of Drought Indicators and Indices. (M. Svoboda and B.A. Fuchs). Integrated Drought Management Programme (IDMP), Integrated Drought Management Tools and Guidelines Series 2. Geneva. ISBN 978-92-63-11173-9.
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