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The aridity actual index is defined as the ratio between mean annual actual evapotranspiration and mean annual precipitation, typically calculated over a reference period of 30 years. Actual evapotranspiration is estimated using hydrological models and, in contrast to potential evapotranspiration, accounts for the limited water content actually available in the soil when estimating the evapotranspiration demand.
Index factsheet (ETC/CCA Technical Paper): Aridity actual
The index provides information on long-term changes in soil water availability relevant for biodiversity protection, agriculture, forest management and water management.
Annual mean actual aridity is currently highest in the southernmost regions of southern Europe, and generally decreases towards the north. Aridity is projected to increase throughout Europe, without the overall spatial pattern changing. The largest increases are expected just north of the current aridity hotspots (i.e. in the northern part of the Iberian Peninsula, Turkey and part of the Balkans), where aridity index values could double during the 21st century. This increase could exacerbate the risk of water deficits and water conflicts in this region.
The aridity estimates shown here are derived from a Copernicus Climate Change Service (CS3) Climate Data Store (CDS) data set based on the outputs of the E-HYPE hydrological model, driven by an ensemble of four bias-adjusted EURO-CORDEX simulations. Note that the use of a single hydrological model prevents estimating and considering the uncertainty of future projections for actual evapotranspiration as computed by different hydrological models. A fuller uncertainty assessment would require the outcomes from multiple hydrological models to be compared.
Notes: The top panel shows the 1971-2000 mean values based on the median of four E-HYPE hydrological simulations. The central row shows the ensemble median of projected values for the near and far future. The projected relative changes of the ensemble median compared with the 1971-2000 reference period are reported on the bottom row. The maps show the gridded values at a resolution of 0.11 ° × 0.11 ° resolution. An ensemble median is used to filter out the very local discrepancies among the four simulations. The ensemble is composed of four model simulations.
Source: C3S CDS data set — Hydrology-related climate impact indicators from 1970 to 2100 derived from bias adjusted European climate projections.
The consecutive dry days index reports the longest consecutive period in a year with daily precipitation below 1 mm. The index measures the persistence of dry conditions in a region.
Index factsheet (ETC/CCA Technical Paper): Consecutive dry days
The index is relevant for a wide range of applications related to water management and agriculture. The simple and rather general definition allows for the easy calculation and interpretation of the index.
Dry spells are most persistent in southern Europe with a duration of almost 40 days, compared with about 20 days in northern and central Europe. The maximum annual number of consecutive dry days has been almost stable throughout Europe since the 1980s. Projections suggest that there will be no changes in dry spells in northern Europe, small increases of about 5 days in central Europe and larger increases of about 15 days in southern Europe throughout the 21st century under the high-emissions scenario (representative concentration pathway (RCP)8.5). Even larger increases are possible in particular parts of southern Europe.
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.
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.
Chapters of the Europe's changing climate hazards report
For references, please go to https://eea.europa.eu./publications/europes-changing-climate-hazards-1/wet-and-dry-1/wet-and-dry-aridity or scan the QR code.
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