Hydrolog-y Report No. 133 From moorland to forest: the Coalburn catchment experiment .=~~~~~~~~~~~~~~~~aua =ntozo - -- -. Reeac Counc.........il. Report No. 13 3 From moorland to forest: the Coalburn catchment experiment M. Robinson', R.E. Moore2, T.R. Nlsbet3 and J.R. Blackle' August 1998 'Institute of Hydrology 'Environment Agency 'Forestry Commission Crowmarsh Gifford North West Region Research Agency Wallingford Gillan Way Alice Holt Lodge Oxfordshire Penrith Wreccilesham OXI 0 8BB Cumbria Farnham CAI 1 9BP Surrey GUIO0 4LH Egentre fo Institute of ENVIRONMENT North Ecology & west Forestry Commission Water Hy~o1o~ Hydrology~~~~Hydrology AGENCY W.. © Copyright Institute of Hydrology 1998 ISBN 0 948540 86 9 IH Report No. 133 published by the institute of Hydrology August 1998 Cover photographs (anti-clockwise from top left): a Catchment plough drainage in 1972, prior to tree planting b Young trees in 1978 c Closed canopy forest in 1994 British Library Cataloguing-in-Publication Data A catalogue record for this book is available from the British Library Contents ACRONYMS AND ABBREVIATIONS iv ACKNOWLEDGEMENTS v EXECUTIVE SUMMARY vi 1 BACKGROUND I 1.1 History of the study 1 2 INTRODUCTION 3 2.1 Changing perceptions of the hydrological impact of forestry 3 2.2 Water industry interests 5 2.3 Forestry industry interests 6 2.4 Environment Agency interests 7 2.5 International perspective 8 3 STUDY CATCHMENT 9 4 INSTRUMENTATION AND ANALYSIS 13 4.1 P'recipitation 13 4.2 Streamflow 15 4.3 Potential evaporation 16 4.4 Water quality 18 4.5 New instrumentation initiatives 19 5 RESULTS AND DISCUSSION 21 5.1 Catchmnent water balance 21 5.2 Interception losses 24 5.3 Transpiration 32 5.4 Soil water studies 33 5.5 Flow regimes 35 5.6 \Vater chemistry 39 5.7 Fertiliser losses 45 5.8 Stream sediment 46 5.9 Biology 48 6 COMPARISONS WITH OTHER STUDIES 53 6.1 Catchimlent studies 53 6.2 Forest management at Coalburn 55 7 CONCLUSIONS 56 8 CONTINUING STUDIES AND FORWARD LOOK 58 REFERENCES 60 Acronyms and abbreviations AOD above Ordnance Datum AWS automatic weather station(s) BFI Baseflow Index CRA Cumberland River Authority DBH diameter at breast height DOC dissolved organic carbon EU European Union GIS Geographical Information Systems IAHS International Association of Hydrological Sciences IH Institute of Hydrology Met. Office Meteorological Office Mld megalitres or million litres of water per day MORECS Met. Office Rainfall and Evaporation Calculation System NRA National Rivers Authority NWW North West Water Limited NWWA North West Water Authority SHE Systeme Hydrologique Europeen THMs trihalomethanes UNECE United Nations Economic Commission for Europe iv Acknowledgements hliis long-term co-operative project to study the hydrological effects of upland afforestation in the Coalburn research catchment has been undertaken by the Institute of Hydrology, the Forestry Commission, the Environment Agency and North West Water Ltd. Over the years, the Coalburn catchment study has hosted, and benefited from, research by staff and students from Higher Education establishments, including Coventry University (Division of Geography), Lancaster University (Environmental Science Division), Leeds University (Sciool of Geography), and the University of Newcastle upon Tyne (Department of Geograpily and Department of Civil Engineering). The authors acknowledge the contribution of the many individuals in these organisations who have been involved in the study. Particular thanks are expressed for the contributions from: Environment Agency T.H. Waugh, S. Mounsey*, C. Addiss, R. Prigg-, P Kerr', R. Furnell Forestry Commission P. Gough, M. Ridley, K. Wylie, D. Durrant' Forest Enterprise G. Gill* Institute of Hydrology H.N. Davies', P.T. Rosier', R.L. Hall*, T.K. Jones (deceased) North West Water Ltd 1. Sanders* University of Newcastle M.D. Newson*, W. Stelling*, P.D. Hind (Department of Geography) J.S.G. McCulloch*, S. Dunn' (Department of Civil Engineering) I.R. Calder (Centre for Land Use and Water Resources Research) Additional contributions from J. C. Rodda* (President of IAHS). This report has been produced with financial support from the Commission of the European Communities Agriculture and Fisheries (FAIR) Research and Technical Development Programme, CT95-0235, 'The Impact of Forestry and Silvicultural Practices upon the Extreme Flows of Rivers' (FOREX). Opinions expressed in this report do not necessarily reflect the views, or anticipate future policy, of the organisations involved. Authors of subsections of this report v Executive summary The uplands of Britain are crucially important for This report covers the period 1967-96. The work water supply; although they comprise only to date has highlighted some important points approximately 200A of the country's land area they regarding the hydrological impacts of plantation provide about 50%0 of the water. They also contain forestry: the bulk of the nation's plantation forestry and the impact of this land use on water resources has * Effects of different forestry activities differ been a cause of major concern. It is well known substantially. Ground preparation ploughling that water yields are lower from forested increased annual total flows (especially by catchments than from other vegetation types. augmenting low flows) and enhancedl peak Whilst the majority of land-use studies on forest storm flows (although shortening their hydrology have concentrated on well-established duration). In contrast, the growth of the trees mature trees, the extensive conifer plantation reduced water yields and peak flows, andcl base forests in upland B13ritain are managed on a crop flows declined. The overall effect of mature cycle of about 50 years, and the relatively slow forests on extreme flows is still the subject of growth rates in this environment ensure that a considerable debate. significant proportion of the cropping cycle (and therefore the total area uinder forest) is occupied * Effects of forestry vary with time. In the by immature trees. early years of afforestation, the cultivation andc drainage system will exert the dominant The Coalburn research catchment provides unique hydrological influence, whilst in later years the information on the long-term effects of forest trees will become progressively more important establishment and growth on upland water hydrologically. At present, with the forest supplies. Established in late 1966, Coalburn is reaching canopy closure, Coalburn appears to now B13ritain's longest-running experimental be in a 'transitional' state. Compared to the catchment, with over 30 years of observations original moorland, the total water yields are fromn open moorland, through tree establishment, now lower, storm flow peaks have returnecd t) in 1973, to canopy closure. It is situated in the similar levels and baseflows, altlhough slowly Kiielder Forest, B3ritain's largest forest, which declining, still remain higher than the pre- provides 5% of the nation's homie-produced drainage pattern. timber. The tree species planted, the peaty soil types in the catchment, and the needc for extensive * Effects of forestry ploughing are very long groundl cultivation and drainage to aid tree lasting. Forestry drainage continues to exert establishment are typical of many upland forests. an important hydrological role after half of the forest cropping cycle has elapsed. This suggests As in any long-term environmental study, the that a similar proportion of the area of upland consistency of data collection and analysis is forests may exert environmental impacts at crucial. The data have been subject to rigorouis variance with commonly held views of forest checks, for internal consistency and, where effects. The study underlines the great value of possible, by comparison with indlependent long-term observations if we are to understand measurements. For example, precipitation is properly the changes and interactions of a landcl measured at gauges sited in the middle of large use with a rotation length of half a century. The unplanted areas; comparisons with independent long-term extent of the enhanced low flows gauges confirm that their catch has not been may be due, in part, to the dclepth of the dcirains affected by the growing tree crop. Similarly, the and furrows. Recent moves by foresters to consistency of potential evaporation estimates by adopt shallower cultivation techniquei s may automatic weather stations on the catchment was result in a smaller and shorter-lived confirmed by correlation with values at a nearby enhancement of base flows; research into this is vleteorological Office climate station. Streamflow currently being carried out at Coalburn. mieasurements have been corroborated by current metering. vi * Young forest interception losses are low marked differences in water chemistry which and variable. Ahlhouglh the young forest has are related to soil parent material; this m:ly now reached canopy closure, the measured provide a way to manipulate the catchment to interception losses are somewhat lower than the improve the poor water quality conditions. values observed in other studies of more malture trees; losses appear to be increasing · The findings show the benefits of a mixed each year and further measurements are aged forest structure. The impact continuing. downstream of areas undergoing the mosi malrked environment;al disturbance - such as * Forest transpiration is limited by cultivation and drainage resulting in locally environmental conditions. Catchment water increased sediment yieldls