Water

 

Climate change predictions for wetter winters and warmer annual temperatures across the UK, and drier summers in the south, can be expected to have a strong influence on forest water use and water yields. For example, interception is likely to be enhanced overall, accentuating the difference in evaporation losses between forest and non-forest land covers, although the impact on water supplies may be offset by the increase in winter rainfall. Similarly, interception losses for broadleaves could be increased (warmer temperatures extend the length of the leafed period and rising carbon dioxide levels increase total leaf area). The effect on transpiration losses is more difficult to predict. At one level, the ability of trees to limit their transpiration rates when the atmospheric demand is high could make them better able to withstand warmer summers than most other vegetation types. However, shorter, shallower rooted vegetation is more likely to suffer water stress and brown-off, limiting their water use. Another factor to consider is the impact of the rise in levels of carbon dioxide, which may increase the water use efficiency of certain species by ≥15% by promoting the closure of leaf stomata.

Riparian forests may be needed to provide shade if open waterbodies become too hot for important stream dwellers. Observations in recent summers show that stream water temperatures in open streams draining southern England can greatly exceed tolerance levels for salmon and trout. The situation is expected to get worse with average temperatures predicted to rise by between 2º and 5ºC by the end of this century.

Floodplain woodland could help to alleviate the increased flood risk resulting from wetter winters and more frequent storms. Modelling studies indicate that the hydraulic roughness created by woody debris dams within stream channels and by the presence of trees, shrubs and deadwood on the floodplain can significantly delay the progression of flood flows, to the benefit of downstream flood defence. Research is focussed on improving understanding in four key areas:

• The measurement and modelling of the impact of forestry on water yield at key sites across the UK. Long-term studies such as at Plynlimon, Coalburn and Balquhidder will help to improve our understanding of the response to climate change as well as the effects of different forestry practices and changes to forest design. These data are essential for the further development and testing of water use models.
• The development of an improved catchment hydrology model that can take account of present and predicted climate scenarios to support operational decision-making in forestry. This has performed well in initial tests and can be applied at a variety of spatial and temporal scales. This is an issue that will be addressed by the European Water Framework Directive as it seeks to establish a new integrated catchment management planning system for the protection, improvement and sustainable use of the water environment.
• The use of riparian woodland to limit the effects of climate warming on freshwater life. The shade provided by riparian trees can significantly reduce peak summer temperatures within watercourses. Riparian woodland may therefore have an increasingly important role to play. Judicious management of riparian woodland offers a means of maintaining water temperatures within a favourable range for salmonid fish and other sensitive freshwater fauna.

The measurement and modelling of the effects of floodplain and riparian woodland on flood flows. Field studies are in place at sites in England and Wales to check hydraulic roughness values for UK floodplain woodland and to provide data for testing hydraulic model predictions. Further model development work is also underway to improve the assessment of the effects of woodland design and management factors on flood flows. The results from this research will help to improve the evidence base and guidance on using floodplain woodland for flood alleviation.