Lessons from long-term forested catchment studies - potential impacts on streamflow

1. Integrated science for our changing worldwww.ceh.ac.uk Lessons from long-term forested catchment studies- potential impacts on streamflow Dr Mark Robinson Centre for Ecology & Hydrology Wallingford Oxfordshire UK Irish Natural Forestry Foundation 5th Nov 2008

2. Integrated science for our changing worldwww.ceh.ac.uk Climate change scenarios • Increase in winter rainfall by between 10 and 40% – hence more severe flooding • Reduction in summer rainfall by 50% – hence increase in frequency and intensity of droughts

3. Integrated science for our changing worldwww.ceh.ac.uk Changing climate WYE An increase in precipitation over this period has been observed at other UK upland sites

4. Integrated science for our changing worldwww.ceh.ac.uk Potential role of forestry: • Post 2007 summer flooding- British Ecological Society. The restoration of native deciduous woodland would help to: • increase the interception of rainfall by vegetation; • increase water infiltration into the soil and groundwater; • increase transpiration to the atmosphere; • reduce peak flow due to surface runoff.

5. Integrated science for our changing worldwww.ceh.ac.uk The forest area in Europe grew by 13 million hectares between 1990 and 2005. The potential for land-use management to help mitigate flooding, in an integrated approach to flood risk, in which land-use practices maximize a range of ecosystem services, including flood control.

6. Ireland and UK amongst lowest forest coverage in Europe.

7. Rises in the price of fossil fuels –renewed interest in biofuels – SRF and biodiesel. Recognition of forest as a supplier of Environmental Services, – so that multitude of benefits can be considered in relation to possible water resource costs. Forestry schemes - New policy drivers

8. Integrated science for our changing worldwww.ceh.ac.uk Traditional view of forest hydrology: • Deeper, more absorbent soil: ↓ peak flows, ↑ baseflows • Thicker vegetation canopy ↓ peak flows • Greater water use ↓ annual yield, ↓ peak flows, (↓ baseflow?) Overall: Even out flows, reduce flooding

9. Integrated science for our changing worldwww.ceh.ac.uk Is evaporation from broadleaved woodland the same as from grassland ?Intuitively you would expect it to be greater, and there is evidence due to greater observed soil moisture depletions under woodland

10. Integrated science for our changing worldwww.ceh.ac.uk Clipstone, Nottinghamshire(after Calder et al., 2000)

11. BUT – Black Wood, Hampshire – no difference! (Roberts and Rosier, 2006)

12. Reason for conflicting results: Higher evaporation per unit area of forest from fragmented woodland and along woodland edges than in the middle of an extensive forest

13. Integrated science for our changing worldwww.ceh.ac.uk Edge effects – greater drying Greater drying of soil due to higher evaporation rates at the better ventilated forest edge

14. Integrated science for our changing worldwww.ceh.ac.uk Biofuels: Short rotation coppice (Finch, 2008)

15. Integrated science for our changing worldwww.ceh.ac.uk Planting of broadleaves is increasing in Ireland and Britain but most forests (and over half of current planting) are conifers:Ireland ~85% conifersBritain ~ 70% conifers

16. Integrated science for our changing worldwww.ceh.ac.uk Conifer forests are: • Different to broadleaves • all-year canopy • Often shallow rooted • monoculture, even-aged • Complex • management cycle 40+ years • management interventions at different stages

17. Integrated science for our changing worldwww.ceh.ac.uk Case studies • Plynlimon – comparison forest vs. grass • Coalburn – time study moorland → forest Detailed surveys of catchment properties soils , geology and forest cover changes. Measurements over decades of streamflow (floods, baseflows, water yield) plus climate, water quality.

18. Integrated science for our changing worldwww.ceh.ac.uk Plynlimon catchments • Severn (70% forest) • Wye (grass)

19. Integrated science for our changing worldwww.ceh.ac.uk Wye Catchment • Land area of 10.55km2 • Mainly moorland • Diverse land-use • Sheep farming • Rally driving

20. Integrated science for our changing worldwww.ceh.ac.uk Severn Catchment • Land area of 8.67km2 • Two-thirds planted in 1930s-1960s by the Forestry Commission • Forest felling and replanting now taking place

21. Integrated science for our changing worldwww.ceh.ac.uk Spatial Data • Spatial data included in GIS • Catchment boundaries • Streams (main, minor and artificial) • Contours and spot heights • Vegetation maps • Forestry planting maps • Soil map • Solid geology (Bell, 2005)

22. Integrated science for our changing worldwww.ceh.ac.uk Changing structure of Hafren forest – from uniform age to multi-aged

23. Integrated science for our changing worldwww.ceh.ac.uk Greater water use of conifers over plantation cycle < Forest growth > < Felling > Additional water use of the forested Severn catchment compared to the grassland Wye.

24. Integrated science for our changing worldwww.ceh.ac.uk Flooding Forestry in UK is less effective in reducing peak flows than is popularly claimed – but this depends on silvicultural practices (how the forest is felled, and if it is a plantation, then whether afforestation includes site drainage) BUT Uncertainty remains to what extent afforestation (without intrusive site preparation) will affect flood response.

25. Integrated science for our changing worldwww.ceh.ac.uk Flood frequency curves

26. Integrated science for our changing worldwww.ceh.ac.uk Peak flows (4mm h-1 ≈ 1m3km-2)

27. Integrated science for our changing worldwww.ceh.ac.uk Forests and low flows Annual 95%-ile exceedance flows before and after felling in Severn basin

28. Integrated science for our changing worldwww.ceh.ac.uk Summary – Plynlimon felling

29. Integrated science for our changing worldwww.ceh.ac.uk Coalburn - Britain’s longest running research catchmentAim: to follow the hydrological effects of a whole plantation forest cycle • Moorland calibration 1967 - 1971 • Drained and afforested 1972 - 1973 • Canopy closure 1995 - 2000 • Felling anticipated ~ 2010 • Second rotation forest ~ 2012

30. Kielder Reservoir Forest 10 km N. Tyne COALBURN Lyne Irthing FIRTH CARLISLE SOLWAY Location and main physical characteristics Part of Kielder Forest Peaty soils Area 1.5 km² Precipitation ~1200 mm / yr. Winter Hill 310 Longmoss 310 Wilcocks 1 310 B A 310 300 300 KEY Raingauge 290 Flow gauge 290 Automatic Weather Station 0 0.5 km Major unplanted areas A, B Interception sites Contours m Soil series boundary

31. Catchment changes:Forestry drainage 1972 Aerial photo

32. Forest growth across the catchmentAerial photos 1983 1989 Growth has been variable across the catchment: Sitka spruce Yield Classes 9 to 15

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36. Integrated science for our changing worldwww.ceh.ac.uk Felling

37. Integrated science for our changing worldwww.ceh.ac.uk Impact of felling on peak flows

38. Integrated science for our changing worldwww.ceh.ac.uk Effects of clearfelling lodgepole pine on flood peaks in a blanket peat catchment at Burrishoole, Co. Mayo Estimated response to a 1-hour pulse of 1 mm of net rainfall

39. Integrated science for our changing worldwww.ceh.ac.uk Managed plantation forest – changes over the cycle

40. Forest plantation cycle - changing impacts • Cultivation and planting – species, coupe size, cultivation method, landscape, buffer zones. Peak and low flows, sediment ↑ • Thinning – to provide an ‘extra’ crop, unsuitable where prone to windblow. May be selective or systematic. Water yield ↑?↓ • Growth – yield class and top height All flows ↓ • Fertiliser – perhaps 3 times per crop Water quality • Felling – age, machinery, coupe size Flows ↑ mean & extremes • Restocking – natural or replanting, original drainage system may satisfactory, revise species selection and landscape design. Flows, water quality.

41. Hydrological issues • Water Resources: • Annual water yield - depends upon scale and forest age structure • Baseflows reduced by forests • Flooding: • - Limited effect reducing peak flows – but depends on silvicultural practices (i.e. if afforestation includes drainage, how the forest is felled …..)

42. Guidance available elsewhere The Forestry Commission issued Forest & Water Guidelines to protect watercourses from the impact of forestry operations • Limit felling coupes • Minimum riparian zones corridor width • Forestry management is subject to wide consultation through forest design plans.