CORDIS provides links to public deliverables and publications of HORIZON projects.
Links to deliverables and publications from FP7 projects, as well as links to some specific result types such as dataset and software, are dynamically retrieved from OpenAIRE .
Deliverables
The key result of this project is an improved understanding of the impact of forests and silvicultural practices on the generation of extreme flows. This information is very important for the sustainable planning and management of forestry operations within water catchments so there is no detrimental effect on the freshwater environment or on other water users. The problem is that while international research conclusively shows that many types of forests are likely to reduce catchment water yield, the question of how this affects the within-year distribution of stream flow is much less well understood. Most catchment studies have found forests to reduce flood flows, resulting from higher interception losses, slower rates of snowmelt, higher soil infiltration rates and greater soil water storage capacities. However, other studies suggest that these factors may have little influence on the generation of very large flood events. The situation is even less clear concerning drought flows. Traditionally, forests were believed to enhance low flows as a result of the higher infiltration and water storage capacities of forest soils. While this view still holds in some parts of the world, it has been increasingly challenged in recent years by catchment studies in North America and Australia. The latter have recorded marked increases in dry season low flows following the clear felling of native forests, although in some instances the increases have lasted for only a few years due to rapid re-vegetation. Conflicting evidence of the effects of conifer forests on low flows is also available from studies in upland Britain.
The project has shown clearly the need to disaggregate the forest cover by type and location since these reflect the different conditions of tree species, climate regime and management practices. The most notable effects are in wetter northern Europe. There the effect of conifer forests on extreme flows is complicated for plantation crops by the tendency to cultivate and drain wet soils prior to planting, which can have a significant influence on the generation of flows. Drainage can result in shorter response times and higher peak flows. The magnitude of this response depends on the relative timing of stream flows from component sub-catchments and has been shown to decline through time with tree growth and drain infill. In terms of low flows, there is good evidence to show that these can be increased by the drainage of peaty soils, due to a combination of factors, including initial dewatering of saturated soils, decreased evapotranspiration associated with the partial removal of the previous vegetation cover and lowering of soil water tables, and the increased effective depth of soil contributing to run-off.
Cultivation and drainage operations could therefore help to offset any subsequent decline in peak or low flows arising from the increasing evaporation rates of growing forest plantations. The subsequent forest growth phase will tend to offset these effects, by lowering both peak flows and low flows, but this depends upon the balance between the hydrological effects of the drains and the trees. Despite evidence of a suppression of moderate flood peaks by mature forest there is a lack of evidence that forests can reduce the most extreme peak flows. Similarly forest felling results in an increase of moderate peak flows but probably not the largest peak flows.
In Central Europe slow growing mixed deciduous forests provide a continuous cover with just small areas felled or planted each year. There is evidence of small changes at a local scale of felling increasing medium peaks and enhancing baseflows, but the impact at the larger catchment scale appears to be minimal.
In Mediterranean areas eucayptus plantations can have a dramatic effect locally, with peak flows increased when the trees are cut and baseflows enhanced, but both of these effects are very short-lived due to the extremely rapid growth of the trees. Overall, for a managed area of eucalyptus at different stages of growth it seems that this crop will reduce peak flows and may reduce baseflows. For Mediterranean open forest there was a small increase in peak flows after felling, but little evidence of a change in baseflows.
The findings of this project have helped to clarify the likely impact of forests and silvicultural practices on extreme flows within a European context. The result highlights how judicious forest design planning can be used to even out and thus control the contrasting effects of different forest practices and growth stages on extreme flows. Such action is especially warranted within sensitive water catchments where low or flood flows are a cause for concern to other water users. Forest managers and forest and water regulators can now be more confident about the likely effects of forest expansion and management operations and thus plan accordingly to protect the freshwater environment and downstream receptors. The result also demonstrates that forestry as a land use could be used to increase water retention and decrease surface flows in small basins and therefore assist flood control at a local scale. Forestry can reduce moderate peaks but its role on controlling extreme events on large catchments appears to be limited.
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