Plynlimon, Wales 1969 -

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Location of the Plynlimon catchments, Wales, UK


Powys, mid-Wales


1969/1971 to present

Catchment size[edit]

  • Wye: 10.6 km²
  • Severn: 8.7 km²


Humid climate, with annual average precipitation of about 2650 mm.


The generalised pattern is of parent materials, which are similar throughout the catchments (Palaeozoic grits, mudstones and shales, or drifts composed of them) and therefore soil differentiation is mainly dependent on drainage. Thus the impeded drainage of the plateaux and wider interfluves have led to the accumulation of organic deposits - blanket peat - whilst the more freely draining slope soils are podzolized. In the valey bottoms, drainage is also impeded and there occurs a complex of peat and gleyed mnineral soils, the balance of which at any site is the result of local topography. Whilst the two catchmnents are almost identical in soil types, the lower parts of steeper slopes in the Wye are mantled by a free-draining brown earth; steeper slopes in the Severn have skeletal soils or patchy podzols.


The landscape of Plynlimon is dominated by rolling hills dissected by steep valleys. The altitudinal range is from 319 m ODN in the Severn and 341 m in the Wye up to 738 m. In terms of slopes, the Severn is locally steeper than the Wye catchment. Both catchments have limited development of valley flat areas associated with short alluvial reaches. Most reaches are confined within valley side slopes. Most streams are flowing over bed rock, sometimes with a shallow covering of river sediments. The irregularities in the profiles are typical of British upland channels. The steps often coincide with resistant bands of rock Such rock controls are apparent in both catchments. Although there are slope angles greater than 25° which are local to the lower parts of the Wye catchment the vast majority of slopes in both catchments are in the range 0° to 15°. The two catchments differ primarily in their land use but also geomorphologically in the degree to which the extensive Tertiary plateaux of Plynlimon are crossed by their tributary streams and the extent of periglacial head deposits.

Vegetation/Land use[edit]

The Wye catchment is 98.8% grassland, while in the Severn forestry is dominant, covering 67.5% of the catchment.


The origins of the Plynlimon study are in a paper on Hydrological Research in Britain written by Howard Penman (see Penman et al., 2007). This was in part in response to a study by Frank Law of the Fylde River Purification Board suggesting, from large lysimeter and catchment studies in the Forest of Bowland, that the water yield from forested catchments was much less than from those predominantly left as pasture (Law, 1956). At the time, there was a policy of reforesting the catchments upstream of reservoirs with a view to having runoff of better quality if both sheep and people were excluded. The question was whether this result held elsewhere. The nacent Hydrological Research Unit (HRU) at Wallingford was given the task of testing the proposition at a new site. Finding a site was difficult, McCulloch (2007) reports that a team from the HRU “scoured the land” for a suitable pair of catchments. The eventual choice was the grass headwaters of the River Wye and the mostly forested headwaters of the River Severn at Plynlimon in mid-Wales. After access to the site was negotiated, instrumentation was started in 1969 and research there has continued in various forms ever since while the HRU at Wallingford became the NERC Institute of Hydrology and later part of the Centre for Ecology and Hydrology. An on-site outpost of the Institute of Hydrology was maintained at Plynlimon until 1999 with a number staff stationed in the former forestry offices at Staylittle, north of Llanidloes. The staff were know locally as the "HydroBoys" (both collectively and as a 5-a-side football team captained by Jim Hudson).

Process studies were led there by Malcolm Newson who, after learning to speak Welsh, was chair of the local Parish Council for a while. Malcolm and Kevin Gilman carried out work on the networks of soil pipes in the catchments as well as velocities of overland and channel flow and some early isotope studies (Gilman and Newson, 1980) The catchments were also the base for a number of PhD studies, including Paul Wolfendon (UCW), Brian Knapp (Reading) who carried out early work on soil pipes in the catchments. It was also used by visiting undergraduate field trips, including the students from the University of East Angia in the early 1970s who spent a lot of time crawling on their knees over the hillslopes mapping out networks of pipes. Pipe connectivity was also investigated by Malcolm and Kevin, out in the rain, listening for water flowing in near-surface soil pipes while traversing the tops and bottoms of slopes with maps and walkie-talkies. Malcolm Newson later became Professor of Physical Geography at Newcastle University. One of his most important contributions at Plynlimon was the study of the two flood events in 1973 and 1975 (Newson, 1980). This event showed that the sequence of events could be important in that the geomorphological effect of the first was mostly on hillslopes, particularly shallow landslides, while that of the second was mostly on channel form, redistributing material from the first event. This initiated a reconsideration of notions of geomorphological effectiveness of hydrological events.

Instrumentation of the sites was a challenge. The gauging stations for the two main catchments were completed in 1968 (Wye) and 1971 (Severn) respectively. Eight additional sites were subsequently gauged on the three major tributaries of each main catchment The streams were steep with heavy sediment load and with the help of Hydraulics Research Station at Wallingford (at that time still a government research station) high and narrow steep stream flumes were developed to get more accurate estimates of discharge on a number of sub-catchments within both the Severn and the Wye (Harrison and Owen, 1967). The narrowness of the flumes however has meant that they have limited capacity and they have been by-passed in some more extreme events. The stage- discharge ratings for the flow gauges were checked by multiple depth current metering and dilution gaugings. Five sub-catchment gauges were built in the early 1970s (Cyff, Gwy, Hore, Nant Iago and Tanllwyth), the Hafren flume was completed in 1976 and the Upper Hore was instrumented in 1985 to act as an experimental control while the forest was clear-felled in the downstream part of the Hore catchment. The gauge on the Nant Iago (which is affected by old mine activities in the subcatchment) was discontinued in June 1999. In autumn 2004, a new flume was completed to monitor the unforested headwaters of the Hafren.

Measuring the input to the water balance was also a challenge since the catchments have sufficient elevation range, aspects and wind speeds. Studies were made of how the wind affected the catch of different types of gauge, relative to a new type of ground level gauge. In 1971, the pattern of monthly rainfalls was also investigated using 48 monthly gauges in a stratified sample across elevation and aspect. In the Wye, precipitation was estimated from 21 ground level gauges while in the Severn, the measurements were made from a combination of 11 canopy gauges in the forest areas and 7 ground level gauges in the higher upland pasture part of the catchment. During severe snow periods, information was provided by 9 UK standard height (30.5 cm or 12 inch) raingauges situated close to roads in or just outside the catchments. Standard height gauges were also used in place of some of the canopy gauges after forest felling. Scaling up to catchment precipitation was performed using the Thiessen polygon method. In addition to the storage gauges, sub-daily (hourly or less) data are available from between 4 and 6 tipping bucket raingauges (depending on the time period). In 1999, to lower the costs and the labour involved, the number of storage gauges was reduced from 48 to 21.

Encouraged by Jim McCulloch, the Director of IH, other innovative experimental work was carried out. Interception losses and transpiration were measured, including some large sheet under-canopy collectors, large scale plot lysimeter studies and (cut) whole tree transpiration measurements by Ian Calder, John Roberts and others. Weather data were measured at a number of sites using Wallingford Automatic Weather stations, and soil moisture using the Wallingford Neutron Probe.

The hydrological instrumentation and having hydrological scientists on site also allowed work to be carried out on runoff processes, sediment sources and transport, and water quality. Attempts were made to quantify the contribution of “old” water to the hydrograph, and to identify sources of that water. More recently, the effect of sampling frequency on a large range of water quality variables has been investigated, initially with some storm sampling, then with daily sampling (Kirchner et al. 2000) and finally with 7 hour sampling (Neal et al., 2012)

The original aim of the Plynlimon investigation was to determine whether the land use affected the water balance in the catchments. Thus, the water balance for the catchments has been revisited a number of times (see Kirby et al., 1991; Hudson et al., 1997; Marc and Robinson, 2007). A number of issues were identified with some of the subcatchment data including an apparent trend in discharge for the Tanllwyth and the effects of old mine workings on the Nant Iago. The excess evapotranspiration determinations of the forest over the grass catchment areas were outside the range of uncertainty, with the difference falling as felling of the trees started to take effect (Robinson and Dupeyrat, 2005).

The Plynlimon catchments had a significant impact on forest policy in the UK after work by Ian Calder and Malcolm Newson extended the implications of the research nationwide. The results underlie the development of UK government approved Forest and Water Guidelines.

Reference material[edit]

Gilman, K and Newson, M., 1980, Soil pipes and pipeflow: a hydrological study in Upland Wales, Brit. Geomorph. Res. Grp. Mongraph 1, Geobooks, Norwich

J. A. Hudson, K. Gilman, and I. R. Calder, 1997, Land use and water issues in the uplands with reference to the Plynlimon study, Hydrol. Earth Syst. Sci., 1, 389-397, doi:10.5194/hess-1-389-1997.

Kirchner, J.W., Feng, X. and Neal, C., 2000. Fractal stream chemistry and its implications for contaminant transport in catchments. Nature, 403(6769), pp.524-527.

Law, F, 1956, The effect of afforestation upon the yield of water catchment areas, J. Br. Waterworks Ass., 38:484-494

Marc, V. and Robinson, M., 2007. The long-term water balance (1972–2004) of upland forestry and grassland at Plynlimon, mid-Wales. Hydrology and Earth System Sciences, 11(1), pp.44-60. J. S. G. McCulloch, 2007, All our yesterdays: a hydrological retrospective, Hydrol. Earth Syst. Sci., 11, 3-11, doi:10.5194/hess-11-3-2007, 2007

Neal, C., Reynolds, B., Rowland, P., Norris, D., Kirchner, J.W., Neal, M., Sleep, D., Lawlor, A., Woods, C., Thacker, S. and Guyatt, H., 2012. High-frequency water quality time series in precipitation and streamflow: From fragmentary signals to scientific challenge. Science of the Total Environment, 434, pp.3-12.

Newson, M., 1980. The geomorphological effectiveness of floods—a contribution stimulated by two recent events in mid‐wales. Earth Surface Processes, 5(1), pp.1-16.

H. L. Penman, H. C. Pereira, J. E. Nash, and M. Nixon, 2007, A view from the watershed, Hydrol. Earth Syst. Sci., 11, 12-25, doi:10.5194/hess-11-12-2007, 2007

Robinson, M. and Dupeyrat, A., 2005. Effects of commercial timber harvesting on streamflow regimes in the Plynlimon catchments, mid‐Wales. Hydrological Processes, 19(6), pp.1213-1226.

Forest and Water Guidelines

Links to project webpages[edit]

other Links[edit]


A complete bibliography for Plynlimon prior to 1997 is given in C. Kirby, C. Neal, H. Turner, and P. Moorhouse, 1997, A bibliography of hydrological, geomorphological, sedimentological, biological and hydrochemical references to the Institute of Hydrology experimental catchment studies in Plynlimon, Hydrol. Earth Syst. Sci., 1, 755-764, including Institute of Hydrology Reports relevant to Plynlimon. Many of these are available on-line via the archive

Two special issues on Plynlimon have been produced in Hydrology and Earth System Sciences 1(3), 1997 and 11(1), 2007

More recent significant papers arising from work with Plynlimon data are:

Johnson, R., 1998. The forest cycle and low river flows: a review of UK and international studies. Forest Ecology and Management, 109(1), pp.1-7.

Kirchner, J.W., Feng, X. and Neal, C., 2000. Fractal stream chemistry and its implications for contaminant transport in catchments. Nature, 403(6769), pp.524-527.

Stott, T. and Marks, S., 2000. Effects of plantation forest clearfelling on stream temperatures in the Plynlimon experimental catchments, mid-Wales. Hydrology and Earth System Sciences Discussions, 4(1), pp.95-104.

Kirchner, J.W., Feng, X. and Neal, C., 2001. Catchment-scale advection and dispersion as a mechanism for fractal scaling in stream tracer concentrations. Journal of hydrology, 254(1), pp.82-101.

Foster, H.J., Lees, M.J., Wheater, H.S., Neal, C. and Reynolds, B., 2001. A hydrochemical modelling framework for combined assessment of spatial and temporal variability in stream chemistry: application to Plynlimon, Wales. Hydrology and Earth System Sciences Discussions, 5(1), pp.49-58.

Neal, C., Reynolds, B., Neal, M., Pugh, B., Hill, L. and Wickham, H., 2001. Long-term changes in the water quality of rainfall, cloud water and stream water for moorland, forested and clear-felled catchments at Plynlimon, mid-Wales. Hydrology and Earth System Sciences Discussions, 5(3), pp.459-476.

Neal, C., Reynolds, B., Neal, M., Hughes, S., Wickham, H., Hill, L., Rowland, P. and Pugh, B., 2003. Soluble reactive phosphorus levels in rainfall, cloud water, throughfall, stemflow, soil waters, stream waters and groundwaters for the Upper River Severn area, Plynlimon, mid Wales. Science of the total environment, 314, pp.99-120.

Neal, C., 2003. Dissolved beryllium in rainfall, stream and shallow groundwaters in the Upper River Severn catchments, Plynlimon, mid Wales. Science of the total environment, 314, pp.171-184.

Brandt, C., Robinson, M. and Finch, J.W., 2004. Anatomy of a catchment: the relation of physical attributes of the Plynlimon catchments to variations in hydrology and water status. Hydrology and Earth System Sciences Discussions, 8(3), pp.345-354.

Feng, X., Kirchner, J.W. and Neal, C., 2004. Spectral analysis of chemical time series from long-term catchment monitoring studies: hydrochemical insights and data requirements. Water, Air and Soil Pollution: Focus, 4(2-3), pp.221-235.

Haria, A.H. and Shand, P., 2004. Evidence for deep sub-surface flow routing in forested upland Wales: implications for contaminant transport and stream flow generation. Hydrology and Earth System Sciences Discussions, 8(3), pp.334-344.

Neal, C., Reynolds, B., Neal, M., Wickham, H., Hill, L. and Williams, B., 2004. The water quality of streams draining a plantation forest on gley soils: the Nant Tanllwyth, Plynlimon mid-Wales. Hydrology and Earth System Sciences Discussions, 8(3), pp.485-502.

Neal, C., 2004. The water quality functioning of the upper River Severn, Plynlimon, mid-Wales: issues of monitoring, process understanding and forestry. Hydrology and Earth System Sciences Discussions, 8(3), pp.521-532.

Bell, J., 2005. The soil hydrology of the Plynlimon catchments. Centre for Ecology and Hydrology.

Neal, C., Robson, A.J., Neal, M. and Reynolds, B., 2005. Dissolved organic carbon for upland acidic and acid sensitive catchments in mid-Wales. Journal of Hydrology, 304(1), pp.203-220.

Shand, P., Haria, A.H., Neal, C., Griffiths, K., Gooddy, D., Dixon, A.J., Hill, T., Buckley, D.K. and Cunningham, J., 2005. Hydrochemical heterogeneity in an upland catchment: further characterisation of the spatial, temporal and depth variations in soils, streams and groundwaters of the Plynlimon forested catchment, Wales. Hydrology and Earth System Sciences, 9(6), pp.621-644.

Marc, V. and Robinson, M., 2007. The long-term water balance (1972–2004) of upland forestry and grassland at Plynlimon, mid-Wales. Hydrology and Earth System Sciences, 11(1), pp.44-60.

Neal, C., Neal, M., Wickham, H., Hill, L. and Harman, S., 2007. Dissolved iodine in rainfall, cloud, stream and groundwater in the Plynlimon area of mid-Wales. Hydrology and Earth System Sciences Discussions, 11(1), pp.283-293.

Archer, D.R., 2007. The use of flow variability analysis to assess the impact of land use change on the paired Plynlimon catchments, mid-Wales. Journal of hydrology, 347(3), pp.487-496.

Page, T., Beven, K.J., Freer, J. and Neal, C., 2007. Modelling the chloride signal at Plynlimon, Wales, using a modified dynamic TOPMODEL incorporating conservative chemical mixing (with uncertainty). Hydrological Processes, 21(3), pp.292-307.

Blackie, J.R. and Robinson, M., 2007. Development of catchment research, with particular attention to Plynlimon and its forerunner, the east African catchments. Hydrology and Earth System Sciences Discussions, 11(1), pp.26-43.

Neal, M., Neal, C., Wickham, H. and Harman, S., 2007. Determination of bromide, chloride, fluoride, nitrate and sulphate by ion chromatography: comparisons of methodologies for rainfall, cloud water and river waters at the Plynlimon catchments of mid-Wales. Hydrology and Earth System Sciences Discussions, 11(1), pp.294-300.

Shand, P., Darbyshire, D.F., Gooddy, D. and Haria, A.H., 2007. 87 Sr/86 Sr as an indicator of flowpaths and weathering rates in the Plynlimon experimental catchments, Wales, UK. Chemical Geology, 236(3), pp.247-265.

Kirchner, J.W., 2009. Catchments as simple dynamical systems: Catchment characterization, rainfall‐runoff modeling, and doing hydrology backward. Water Resources Research, 45(2).

Godsey, S.E., Aas, W., Clair, T.A., De Wit, H.A., Fernandez, I.J., Kahl, J.S., Malcolm, I.A., Neal, C., Neal, M., Nelson, S.J. and Norton, S.A., 2010. Generality of fractal 1/f scaling in catchment tracer time series, and its implications for catchment travel time distributions. Hydrological Processes, 24(12), pp.1660-1671.

Bulygina, N., McIntyre, N. and Wheater, H., 2011. Bayesian conditioning of a rainfall‐runoff model for predicting flows in ungauged catchments and under land use changes. Water Resources Research, 47(2).

Neal, C., Reynolds, B., Norris, D., Kirchner, J.W., Neal, M., Rowland, P., Wickham, H., Harman, S., Armstrong, L., Sleep, D. and Lawlor, A., 2011. Three decades of water quality measurements from the Upper Severn experimental catchments at Plynlimon, Wales: an openly accessible data resource for research, modelling, environmental management and education. Hydrological Processes, 25(24), pp.3818-3830.

Halliday, S.J., Wade, A.J., Skeffington, R.A., Neal, C., Reynolds, B., Rowland, P., Neal, M. and Norris, D., 2012. An analysis of long-term trends, seasonality and short-term dynamics in water quality data from Plynlimon, Wales. Science of the Total Environment, 434, pp.186-200.

Neal, C., Reynolds, B., Rowland, P., Norris, D., Kirchner, J.W., Neal, M., Sleep, D., Lawlor, A., Woods, C., Thacker, S. and Guyatt, H., 2012. High-frequency water quality time series in precipitation and streamflow: From fragmentary signals to scientific challenge. Science of the Total Environment, 434, pp.3-12.

Svensson, T., Lovett, G.M. and Likens, G.E., 2012. Is chloride a conservative ion in forest ecosystems?. Biogeochemistry, 107(1-3), pp.125-134.

Robinson, M., Rodda, J.C. and Sutcliffe, J.V., 2013. Long‐term environmental monitoring in the UK: origins and achievements of the Plynlimon catchment study. Transactions of the Institute of British Geographers, 38(3), pp.451-463.

Kirchner, J.W. and Neal, C., 2013. Universal fractal scaling in stream chemistry and its implications for solute transport and water quality trend detection. Proceedings of the National Academy of Sciences, 110(30), pp.12213-12218.

Halliday, S.J., Skeffington, R.A., Wade, A.J., Neal, C., Reynolds, B., Norris, D. and Kirchner, J.W., 2013. Upland streamwater nitrate dynamics across decadal to sub-daily timescales: a case study of Plynlimon, Wales. Biogeosciences, 10, pp.8013-8038.

Benettin, P., Kirchner, J.W., Rinaldo, A. and Botter, G., 2015. Modeling chloride transport using travel time distributions at Plynlimon, Wales. Water Resources Research, 51(5), pp.3259-3276.