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Drought Indicators

Drought monitoring relies on the analysis of a set of indicators, representing different components of the hydrological cycle (precipitation, soil moisture, reservoir levels, river flow, groundwater levels) or specific impacts (vegetation stress)
The indicators generally represent statistical anomalies of the current situation with respect to the long-term average at a given location and period of time.
Amongst the indicators of meteorological drought, the Standardized Precipitation Index (SPI) and the Standardized Precipitation-Evapotranspiration Index (SPEI) are the most well-known and used. They are measures of the severity of anomalous dry events, which can be calculated for various precipitation accumulation periods (e.g., 1 to 48 months) and statistically linked to impacts in different economic and environmental sectors. Snowpack extent and snow water equivalent (SWE) are other important variables in Northern Europe and in mountainous regions. Snow contributes to water availability over the year also over far away wide regions that draw water from snow reservoirs. Drought indicators based on soil water content, such as the Drought Observatories Soil Moisture Index Anomaly (SMA), the Drought Severity Index (DSI), or the Palmer Drought Severity Index (PDSI), aim to characterize the risks of plant water stress. Indicators of hydrological drought, such as the European Drought Observatories 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., European Drought Observatories Combined Drought Indicator). "The Handbook of Drought Indicators and Indices" (*) provides further information on commonly used drought indicators.
The European and Global Drought Observatories currently produces the following continuously updated drought indicators at scales varying between 5 by 5 kilometer to 1 decimal degree (+/- 100 kilometers).
  • 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.
  • Soil Moisture Anomaly (SMA): This indicator measures anomalies of daily soil moisture (water) content, and is used to estimate agricultural drought conditions.
  • Anomaly of Vegetation Condition (FAPAR Anomaly): This indicator quantifies anomalies of satellite measured FAPAR (Fraction of Absorbed Photosynthetically Active Radiation) and is used to highlight areas of relative vegetation stress due to drought.
  • Low-Flow Index (LFI, only available for Europe): This indicator s used for near real-time monitoring of hydrological streamflow drought at European scale. LFI is derived from daily river discharge outputs produced by the JRC hydrological rainfall-runoff model (LISFLOOD) within the Copernicus EMS European Flood Awareness System. The indicator is useful to monitor hydrological drought.
  • Heat and Cold Wave Index (HCWI): This indicator is used to detect and characterize positive and negative extreme temperature anomalies (heatwaves as well as warm and cold spells) and is based on daily minimum and maximum temperatures.
  • Combined Drought Indicator (CDI; only for Europe): This indicator integrates information on anomalies of precipitation, soil moisture and satellite-measured vegetation conditions, into a discrete classification index. CDI is used to monitor the onset of agricultural drought, its evolution in time and space, and the recovery phase.
  • Risk of Drought Impact for Agriculture (RDrI-Agri; only on Global Drought Observatory): This is a categorized risk index, indicating the probability of having impacts from drought, with particular focus on vegetation. The RDrI-Agri combines hazard, exposure (in terms of total population, livelihood and assets), vulnerability (i.e., the propensity of exposed elements to suffer adverse drought-induced effects). The hazard is expressed as the combination of precipitation anomaly (SPI), anomaly of photosynthetic activity (fAPAR) and soil moisture anomalies.
  • Indicator for Forecasting Unusually Wet and Dry Conditions: This indicator provides an early warning of unusually wet and dry cumulative periods forecasted over the next 1-, 3-, and 6-months in Europe (EDO) and the world (GDO). The indicator is derived from the statistical analysis of predicted Standardized Precipitation Index values (SPI-1, SPI-3, and SPI-6), calculated from the forecast of precipitation given by the ECMWF seasonal forecasting system (SEAS5). Only regions where an unusual wet or dry period (i.e., meeting a threshold SPI value) is forecasted with sufficient robustness (i.e., 40% of forecast members meet the threshold) are indicated. Levels of the indicator correspond to the return period of the forecast intensity and coherence.
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.
http://www.droughtmanagement.info/literature/GWP_Handbook_of_Drought_Indicators_and_Indices_2016.pdf
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