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The impact of forests and silvicultural practices upon the extreme flows of rivers

Contract nr: FAIR-CT95-0235
Project nr: 235
Project type: SC
Starting date: 01/03/1996
Duration: 48 months
Total cost: 1,126,000 EUR
EC Contribution: 652,500 EUR
Scientific Officer: Arnaud BORCHARD
Research topic: Forest conservation and protection
Acronym: FOREX (FORests and EXtreme flows)

In many countries, it is widely believed that forests encourage water to infiltrate into the soil, increasing the recharging of groundwater and reducing surface runoff and erosion. Thus, forests are often claimed to enhance dry weather baseflows and subdue peak runoff. But there is contradictory evidence that, in dry weather periods, forest evaporation may dry out the soil and reduce baseflow, whilst in wet weather, forests may have little impact on flood flows.
Another important consideration is that many forests and woodlands are managed by people for timber production. Local silvicultural practices may disturb the soil and alter water flow pathways, with variable effects on extreme flows.
Within the European Union, the land area under forest cover is expanding: as an alternative to agriculture, to assist reform of the Common Agricultural Policy, for environmental protection and enhancement, and to reduce the impact of climate change through carbon fixation.

The objective is to quantify the effects of forestry (broadleaf and conifer) on extreme flows - both dry weather flows and peak flood flows. This covers a range of forest conditions and includes:
- stages of forest growth (planting, growth, harvesting);
- forest types (conifers, broadleaves, eucalyptus);
- silvicultural practices (drainage, harvesting);
- different climate regions in Europe (oceanic, continental, Mediterranean).

The partnership structure includes universities, governmental research organisations and national forest bodies responsible for commercial forest management and timber production. The project team includes scientific experts with extensive appropriate academic experience, as well as participants who work closely with forest managers, so that the results and experimental studies are relevant to real forest conditions.
There are nine field sites across Europe, covering a range of forest types, climates and soils. Tree species studied include Sitka spruce, Lodgepole pine, Eucalyptus, Maritime pine, spruce, beech and some oak.
The programme of work includes:
- analysis of existing long-term catchment records;
- detection of effects on streamflow of felling, coppicing or planting;
- measurement of plot parameters such as transpiration, interception losses, soil water, soil moisture, meteorological variables;
- production of a general descriptive model.

Current situation/results:
The results of forestry are complex, which accounts for the differences in opinions and observations. Nevertheless some general observations can be made.
- Pre-planting drainage is widespread on peat ground in northern European countries to aid tree establishment. Forest drainage can increase peak flows, with effects varying temporally and spatially, depending on the scale of operation. There is the potential for increased erosion from the open drains, which may block watercourses downstream and further aggravate flooding. However, deep drains may augment baseflows during the early growth of trees by providing an outlet for a deeper thickness of soil than the former natural channels.
- The generally deeper rooting of trees may potentially cause greater depletion of soil water in rainless periods, and lead to a reduction in stream baseflow
- Intercepted rainwater on the forest canopy evaporates at a faster rate than from short vegetation, and will also reduce soil water recharge.
- Due to the higher evaporation from trees, forest growth can reduce low flows and peak flow rates, especially for evergreen tree species. Changes over time are most rapid when the trees are still small. Once canopy closure has been reached, further changes in extreme flows are more difficult to detect.
- Forest cutting (harvesting) can increase stream baseflow and peak flow - or can have no clear effect. Care must be taken to identify and distinguish the effect of associated work, such as soil compaction by forest machinery.

A project brochure (FOREX Impacts of forestry on extreme flows) has been produced; copies are available from the EU, the coordinator, or from any of the project participants.
In addition, approximately 10 scientific reports have been published or are going to press.

Institute of Hydrology
Crowmarsh Gifford
UK-OX10 8BB Wallingford
Tel.: +44 1491 69 23 22
Fax: +44 1491 69 24 24


  • Yves TRAVI
    Université d'Avignon et des pays de Vaucluse
    Rue Louis Pasteur 33
    F-84000 Avignon
    Tel.: +33 4 90 14 44 00
    Fax: +33 4 90 14 44 89

  • Jorge Soares DAVID
    Universidade Tecnica de Lisboa
    Tapada da Ajuda
    P-1399 Lisboa
    Tel.: +351 21 350 20 93
    Fax: +351 21 364 50 00

  • Thomas Reyburn NISBET
    Forestry Commission - Forestry Authority
    Alice Holt Research Station
    UK-GU10 4LH Farnham
    Tel.: +44 1420 22255
    Fax: +44 1420 23653

  • Richard MC CARTHY
    The Irish Forestry Board - Coillte Teoranta
    Sidmonton Place 1-3
    IRL-Bray, Co. Wicklow
    Tel.: +353 12 86 77 51
    Fax: +353 12 86 81 26

  • Hans FEUHRER
    Hessische Landesanstalt fuer Forsteinrichtung
    Prof. Olkers Straße 6
    D-34346 Hann Muenden
    Tel.: +49 5541 70 04 24
    Fax: +49 5541 70 04 73

  • Alois ZOLLNER
    Bavarian State Institute for Forestry
    Am Hochanger 11
    D-85354 Freising
    Tel.: +49 8161 71 48 81
    Fax: +49 8161 71 49 71

  • Michael ROGERS
    National University of Ireland
    Western road
    Tel.: +353 91 52 44 11
    Fax: +353 91 52 57 00
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