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Environmental information systems
As average temperatures rise and rainfall patterns change, the stereotypical ‘arid’ regions of Europe may need a rethink. Increasing aridity is an important hazard for many ecosystems and economic sectors, including water management, agriculture, forestry and tourism. Unlike drought, aridity describes the long-term average dryness of a region, which leads to limited or low water content in the soil. Aridity can lead to soil erosion, salinisation and other forms of land degradation; it can also set off or exacerbate water-related conflicts in affected regions.
Key messages
Aridity is currently highest in the southernmost regions of Europe, and it generally tapers off towards the north. Broadly, Europe is likely to experience more aridity in the future, especially in areas just north of the current ‘aridity hotspots’ (the northern part of the Iberian Peninsula, Turkey and the Balkans).
The duration of dry spells has been largely stable throughout Europe so far. For the future, large increases are projected for southern Europe, smaller increases for central Europe and no significant changes for southern Europe.
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Definition
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
Relevance
The index provides information on long-term changes in soil water availability relevant for biodiversity protection, agriculture, forest management and water management.
Past and projected changes
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.
Current situation and projected changes in actual aridity in Europe
Definition
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
Relevance
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.
Past and projected changes
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.
Consecutive dry days for the European land area and sub-regions
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