It is difficult to generalise about the effects of forestry on water resources, however some important distinctions can be drawn between the impact of conifers and broadleaves in the uplands and lowlands.
The factors that influence the water use of trees and how conifers and broadleaves are likely to affect water supplies in different parts of the UK are address summarized below.
- In general, conifers lose between 25 to 45% of annual rainfall by interception, compared to 10 to 25% for broadleaves and almost 0% for grass.
- Conifers tend to lose an additional 300 mm to 350 mm per year due to transpiration, compared to 300 mm to 390 mm for broadleaves and 400 mm to 600 mm for grass.
- Interception and transpiration losses vary between different species, with some species such as willow and poplar able to sustain very high transpiration losses (e.g. >500 mm/yr) when well supplied with water.
- Water use varies greatly throughout the year; maximum daily transpiration loss for large individual trees can vary between 500 l to 2000 l on a hot summer day.
- On a catchment basis in the wetter uplands, the additional water use by a complete cover of mature conifer forest can result in a 15 to 20% reduction in the annual volume of streamflow.
- The impact on water supplies can be even greater in the lowlands, where a conifer forest can reduce the annual volume of water recharging a groundwater aquifer by 70% or more compared to grass.
- The impact of broadleaved woodland is much less than conifers and some species on certain soils and geologies can actually increase the annual volume of groundwater recharge.
- It is possible to control the impact of forests on water supplies by varying species choice, forest age and the scale of forest cover within a catchment.
- The ability of some forests to reduce water supplies needs to be balanced against other benefits, including the potential to protect water quality and enhance freshwater habitats.
- Climate change could have a strong effect on water use by forests and thus on their impact on water supplies. This subject is being addressed through research and monitoring work.