Water-retention potential of Europe`s forests

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Courtesy of European Environment Agency (EEA)

A European overview to support natural water-retention measures

One third of European territory is covered by forests (210 million ha). Approximately 296 million European inhabitants live in — or close to — forests. European forests are also closely connected to much of the hydrological network, and serve large groundwater bodies and many river sources. Forests provide more than 4 km3 of water annually to European citizens by hosting 870 000 km of rivers (the total length of European rivers is about 3.5 million km), and almost 33% (or 92 000 km2) of 71 000 lakes are located in forested catchments.

Natural water-retention measures are measures implemented to prevent extreme hydrological events. Among the major ecosystem types, forests have a large potential for water retention. Forests retain excess rainwater, and help to moderate run‑off patterns, preventing extreme run‑offs. This in turn reduces damage from flooding, and also helps to mitigate the effects of droughts. It is also vital to understand these functionalities in the context of the broader discussion of ecosystem services, which includes the MAES project (Mapping and Assessment of Ecosystem Services), a project that was identified in the European Union (EU) Biodiversity Strategy to 2020.

This report provides for the first time a European overview of the role of forests in water retention, based on the Water Accounts Production Database developed at the EEA. The results represent 287 sub‑basins hosting more than 65 000 catchments across Europe. As the data are not available at the same level of detail across Europe the results are highly aggregated and restricted to some key parameters. The impact of forests on water retention is measured according to three parameters/ characteristics: forest cover (measured in hectares), forest types (coniferous, broad‑leaved, mixed), and the degree of management of the forests ('protected' versus unprotected/commercial forests). The estimation of the water‑retention potential is derived from the relationships between input (rainfall) and output (water run‑off into rivers and lakes) as affected by these three forest characteristics.

Data interpretation is difficult due to the complexities of forest hydrology, which are still the subject of scientific debates on issues such as water yield and water quality. Nevertheless, the first preliminary results confirm the importance of forest cover on water retention. In water-basins where the forest cover is 30%, water retention is 25% higher than in basins where the forest cover is only 10%. In basins where the forest cover is 70%, water retention is 50% higher than in basins where the forest cover is only 10%. The results in this report also confirm that water retention in any sub‑basin (whether it has 80% forest cover, 50% forest cover, or 30% forest cover) is typically about 25 % greater in summer time than in winter time. The findings also reveal the role of the types of trees in the forests in determining the degree of water retention. Coniferous forests in general retain 10% more water than broadleaved forests or mixed forests. It is more difficult to draw any conclusions about the impacts of forest-management practices on the water-retention potential of forests. Some contradictory results have been obtained when comparing protected forests against non-protected forests.

In general, forests in Alpine and Continental regions provide the highest water-retention potentials, while Atlantic and Mediterranean regions register lower water-retention potentials. Due to insufficient data coverage in the Mediterranean region, water retention could not be clearly linked to forest cover. This relationship needs further investigation with the involvement of additional data.

This shows that water retention cannot be promoted by a one-size-fits-all solution of encouraging forest cover across Europe. Instead, water retention needs to be considered on a case-by-case basis according to local and regional ecological and hydrological conditions, as proposed in the natural water-retention measures catalogue of the European Commission.

The European Environment Agency is continuously improving the European Water Accounts Production Database by means of reported data from its member countries under different data flows such as SoE and WFD. These data combined with Corine 2012 and the high resolution of forest layers of the Copernicus programme will enable the EEA to further develop analyses of water-forest interactions from the ecosystem services perspective. This will also provide more robust results on the role of forests on water retention.

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