Tuesday, 21 August 2012

The Plynlimon catchments - an open air laboratory

The Plynlimon catchments in central Wales are the most important intensively studied long-term research basins in the UK. Managed by the Centre for Ecology & Hydrology (CEH), Plynlimon is among the key European and world sites in terms of catchment science - not just for the length of the records gathered (40 years and counting) but crucially because of the breadth of measurements, spanning hydrology and including water chemistry, groundwater studies, geomorphology, ecology and atmospheric science.

With the study now in its fifth decade, and inspired by a new paper* by Dr Mark Robinson, working with former colleagues John Rodda and John Sutcliffe, who played key roles in establishing Plynlimon, we take a look at the history behind the establishment of one of CEH's most significant scientific experiments, a long-term environmental monitoring project which has evolved and adapted to stay relevant to current problems and scientific needs.

Landscape view over the Plynlimon catchments in central Wales (photo: Simon Grant / CEH)

In the mid-20th century, against a backdrop of population growth, rising living standards and post-war industrial recovery, there was growing concern in the UK about increased demand for water and possible future water shortages. A severe drought in summer 1959 and dry winters in 1962/3 and 1963/4 underlined the belief in government circles that a national plan for water use was needed, as it seemed likely that demand would soon outstrip available resources.

There were concerns too about other factors potentially affecting water availability, not least the government's policy of encouraging widescale tree planting on reservoir catchments. The rapid growth characteristics of the conifer and the cheapness of the land resulted in conifer plantations on many upland regions of Britain throughout the first half of the 20th century. However, there were conflicting scientific views on the hydrological impact of forests, with some catchment studies, particularly in the USA, indicating that tree cover could reduce water yield relative to short vegetation.

Research in the late 1950s by Frank Law Senior, a water engineer in north-west England, tried to find out more about the water balance of woodlands under British conditions. His conclusions were controversial: that afforestation of the Stocks reservoir catchment area was substantially reducing runoff, with the additional evaporation representing a reduction in reservoir yield of over one third. At a time of anticipated increasing domestic and industrial water demands, the debate about water versus forestry was set for the decades ahead.

British hydrology and water industry were still rather fragmented in the decade after the Second World War, but these national pressures meant that more rational management of water resources was required. The particular need to understand the role of vegetation in the hydrological cycle led to the creation by the Department of Scientific and Industrial Research (DSIR) of a Committee on Hydrological Research in 1961. This committee soon proposed the establishment of a specialist unit to be responsible for applied research into surface water hydrology, including studies of the water balance of catchment areas in relation to land use.

The new Hydrological Research Unit was set up in the following year and attached to the existing Hydraulics Research Station at Wallingford in Oxfordshire. A steering group was appointed to guide its studies, with a range of potentially competing stakeholder representatives from the Forestry Commission, Nature Conservancy, Water Research Association, Met Office and Agricultural Research Council.

The Unit was asked to conduct a hydrological study of catchments with contrasting land cover, preferably grassland and coniferous forest, since that was seen as the land use distinction most likely to reveal differences in behaviour and was the most common source of land use concern.  The catchments would need to be on watertight geology, and it was felt that the difference in water used by the two vegetation types would be most clearly apparent in relatively dry climate.

Where exactly to base the experiment was not a straightforward decision. It proved difficult to identify pairs of almost identical catchments that differed only in their vegetation and had landowner agreement for access. Plynlimon was not even on the original eight potentially suitable paired catchments identified. The climate criterion was later relaxed to include ‘wet’ sites, and the adjoining headwater catchments of the Severn and the Wye rivers draining the slopes of Pumlumon Fawr, i.e. Plynlimon, were added to the list, and later judged to be the most promising of the possible sites. The headwaters of each catchment provide a land use contrast between the two major upland land uses: moorland (Wye) and plantation forest (Severn).

In addition to the ongoing scientific debate concerning forest evaporation, the organisational framework for UK environmental research was undergoing major change as part of a wider government reorganisation. In 1965 the HRU was transferred to the newly formed Natural Environment Research Council (NERC). The Unit’s remit was then expanded to encompass all aspects of the hydrological cycle including physical and applied studies, instrumentation and mathematical modelling.

The Plynlimon study officially started on 1 January 1967, with the NERC Annual Report foreseeing it as being an “outdoor laboratory of immense value to all sciences”. The delays in establishing a field site actually afforded the opportunity to develop expertise in cutting edge instruments and new technologies. There was pioneering work (widely adopted elsewhere) on neutron probes for soil water content measurement, as well as automatic weather stations capable of operating at remote locations, and new raingauge designs for windy sites.

Cefn Brywn crump weir on the River Wye, late 1960s

In recognition of the growing importance of hydrological research in Britain, NERC designated the HRU as a full component institute: on 1 April 1968 it became the Institute of Hydrology, which later became part of the Centre for Ecology & Hydrology. The Plynlimon study played a crucial role in proving beyond doubt the potentially serious reduction in water flows resulting from afforestation, leading to major changes in the management of water and land use by resource managers and policy makers.

Work continues at the Plynlimon catchments to this day. CEH conducts long-term measurements of weather, streamflow and chemical fluxes. We use this information to look for time trends, understand the processes operating, and investigate the impacts of land use on water resources, floods, drought flows, stream sediment and dissolved chemicals and acidification. 

Other Plynlimon research highlights

·        The original aim of the Plynlimon catchment experiment was to make measurements which could resolve the uncertainties concerning the water use of conifer forests. It not only helped to answer this question but subsequently developed into a long-term multi-disciplinary project.

·        Building upon the established catchment infrastructure, hydrochemical measurements commenced and now more than almost 30 years are available, together with the longer record of hydrological data.

·        The Plynlimon catchments have been one of the more successful catchment studies, advancing science and its findings have directly altered UK forestry environmental policy and informed UK government policy.

·        The Plynlimon study provided a considerable stimulus to the development of the science of hydrology in Britain. The research findings have been reported in more than 500 papers in refereed journals, and much of the Plynlimon record has been freely available to the international research community for decades, and is also accessible via the internet from the CEH Information Gateway.

Summarising the importance of the Plynlimon catchments, Dr Mark Robinson told us, “We owe a great debt to the farsightedness of the stakeholders and academics who recognised the need for, and supported the establishment of, the research catchments at Plynlimon. The catchments provided an important focus in British hydrology, promoting excellence in experimental hydrology and advancing our understanding of the physical processes controlling the hydrological cycle.”

Additional information

* The full paper reference is: Robinson, M., Rodda, J.C., and Sutcliffe, J.V. (2012), Long-term environmental monitoring in the UK: origins and achievements of the Plynlimon catchment study. Transactions of the Institute of British Geographers. doi: 10.1111/j.1475-5661.2012.00534.x 

View archive photos from Plynlimon on Flickr

CEH: Why monitor upland catchments? [PDF, 4.95mb]

1 comment:

  1. Great to see thi article celebrating the breadth and continuity of the work on the Plynlimon catchments. Long may it go on!