Wet and dry — drought

Duration of meteorological droughts

Definition

The duration of meteorological droughts index represents the average number of months in a year in which drought conditions are experienced as determined by anomalously low precipitation values. The index is based on the Standardized Precipitation Index aggregated over three months (SPI-3), which represents the deficit or surplus of precipitation with respect to a reference period. Alternative aggregation periods for the SPI can be used depending on the type of drought considered and the specific applications. SPI values represent standard deviations of precipitation from the long-term mean. A drought event is considered to have started when SPI values fall below -1 for at least two consecutive months and to have ended when the index value returns to a positive number.

Index factsheet (ETC/CCA Technical Paper): Duration of meteorological droughts

Relevance

The SPI is the most commonly used index for detecting and characterising the duration of meteorological droughts in a wide range of water-related sectors, including agriculture, forestry, hydrology and water management.

Past and projected changes

The duration of meteorological droughts has not shown clear trends in Europe since the 1960s, except for a small decrease in northern Europe. Further decreases in meteorological droughts are projected for northern Europe. In central Europe, increasing drought duration is projected for the highest emissions scenario (representative concentration pathway (RCP)8.5), but not for the two lower emissions scenarios (RCP2.6 and RCP4.5). In southern Europe, drought duration is projected to increase for the two highest emissions scenarios (RCP4.5 and RCP8.5), but not for the lowest emissions scenario (RCP2.6).

Duration of meteorological droughts for the European land area and sub-regions

Notes:   The black lines show the annual values for 1950-2020 from reanalysis data, and the dashed horizontal lines show the means for 1986-2005. Solid grey, blue, yellow and red lines represent the ensemble medians of model simulations for the historical period and under low-, medium- and high-emissions scenarios (RCP2.6, RCP4.5 and RCP8.5, respectively) (smoothed by a 20-year moving average). Shaded areas show the 15th and 85th percentile ranges of the model ensembles. The size of the model ensemble used for each scenario is shown by the coloured numbers in the top-right corner.

Source:  ERA5 and bias-adjusted CMIP5 data.

Current situation and projected changes in the duration of meteorological droughts in Europe

Notes:   The top panel shows the 1986-2005 mean values based on the reanalysis. The central row and bottom row show the projected values and the projected changes, respectively, of the 15th and 85th percentiles for the near and far future. The size of the model ensemble used for each scenario is reported in the time-series figure above.

Source:  ERA5 and bias-adjusted CMIP5 data.

Magnitude of meteorological droughts

Definition

The magnitude of meteorological droughts index combines information about the duration and severity of droughts. It is defined as the positive sum of the Standardized Precipitation Index (SPI) for all months within drought events in a given year, thereby giving more weight to months with severe droughts than those with less severe droughts. For consistency with the duration of meteorological droughts index above, this index is also based on the SPI aggregated over three months (SPI-3) and a threshold of -1 is used to identify drought occurrences. Alternative aggregation periods for the SPI can be used depending on the type of drought considered and the specific applications.

Index factsheet (ETC/CCA Technical Paper): Magnitude of meteorological droughts

Relevance

This index is widely applicable in water-dependent sectors, including agriculture and water management. However, the drought magnitude index may be more difficult to interpret than the drought duration index owing to its slightly more complex definition.

Past and projected changes

The magnitude of meteorological droughts shows even more interannual variability than the duration of meteorological droughts, which makes the detection of past trends more difficult. Future projections suggest that there will be small declines in drought magnitude in northern Europe and substantial increases in central Europe under the highest emissions scenarios, and even larger increases in southern Europe, where drought magnitude is expected to triple by the end of the century under the high-emissions scenario (representative concentration pathway (RCP)8.5).

Magnitude of meteorological droughts for the European land area and sub-regions

Notes:   The black lines show the annual values for 1950-2020 from reanalysis data, and the dashed horizontal lines show the means for 1986-2005. Solid grey, blue, yellow and red lines represent the ensemble medians of model simulations for the historical period and under low-, medium- and high-emissions scenarios (RCP2.6, RCP4.5 and RCP8.5, respectively) (smoothed by a 20-year moving average). Shaded areas show the 15th and 85th percentile ranges of the model ensembles. The size of the model ensemble used for each scenario is shown by the coloured numbers in the top-right corner.

Source:  ERA5 and bias-adjusted CMIP5 data.

Current situation and projected changes in the magnitude of meteorological droughts in Europe

Notes:   The top panel shows the 1986-2005 mean values based on the reanalysis. The central row and bottom row show the projected values and the projected changes, respectively, of the 15th and 85th percentiles for the near and far future. The size of the model ensemble used for each scenario is reported in the time-series figure above.

Source:  ERA5 and bias-adjusted CMIP5 data.

Duration of soil moisture droughts

Definition

The duration of soil moisture droughts index represents the total number of months in a year where soil water content, as estimated from hydrological models, is below the 20th percentile of soil moisture in the same calendar month during a reference period. The depth of the soil column considered may vary depending on the desired application. Estimates of soil water content derived from hydrological models are considered more accurate and are therefore preferred over the direct outputs of climate models.

Index factsheet (ETC/CCA Technical Paper): Duration of soil moisture droughts

Relevance

The index defined over the total depth of the soil column (e.g. 2 m) is most relevant for applications in forestry and water management. The index defined over the near-surface soil layers (e.g. up to 50-cm depth) is most suitable for agricultural applications.

Past and projected changes

Modelled soil moisture content, which is considered here as a proxy for the proposed index, has significantly decreased in southern Europe and increased in parts of northern Europe since the 1950s, because of past warming and precipitation changes. Significant decreases in soil moisture content are projected for southern Europe, in particular in summer, whereas increases are projected for north-eastern Europe.

Changes in the duration of soil moisture droughts are expected to follow a similar regional pattern. Further information will be integrated once the required data are available.

Further information (EEA indicator assessment): Soil moisture