IMPLICATIONS OF NATURAL SOIL PIPING FOR BASIN …alliance.la.asu.edu/rockart/scans/PipingUK.pdf ·...

land degradation & development

Land Degrad. Develop. 15: 325–349 (2004)

Published online in Wiley InterScience (www.interscience.wiley.com). DOI: 10.1002/ldr.618

IMPLICATIONS OF NATURAL SOIL PIPING FOR BASINMANAGEMENT IN UPLAND BRITAIN

J. A. A. JONES*

Institute of Geography and Earth Sciences, University of Wales Aberystwyth, Aberystwyth SY23 3DB, UK

Received 26 November 2002; Revised 16 June 2003; Accepted 8 February 2004

ABSTRACT

Natural soil pipes are a common feature in the British upland landscape, especially in podzolic and peat soils (spodosolsand histosols). It is estimated that 30% of Britain (UK) is covered by soils susceptible to pipe development. Current re-evaluation of the use of the uplands, including moves to preserve biodiversity, rehabilitate acidified surface waters, afforest, andrecreate a ‘pre-industrial’ landscape with aesthetic planting of previously native trees and shrubs, highlights the need tounderstand the key processes of hillslope hydrology that may affect the impact of any policy changes and the effectiveness ofany rehabilitation programme.

This paper reviews the evidence that soil piping can have a significant effect upon streamflow response, water quality andacidification, as well as creating diversity in plant communities and soil types, and initiating erosion features. Evidence from theUniversity of Wales Experimental Catchment at Maesnant is compared with the results of observations elsewhere in the Britishuplands and with experience gained from research programmes in other countries. Some of the effects of piping, such asaggravation of the effects of acid rain and increases in the rate of floodflow response, may currently be regarded as negative.Conversely, its effects on landscape diversity are clearly positive. Piping is a natural process and should be regarded as anintegral element in preserving and restoring a ‘natural’ landscape. Most importantly, the presence of piping can alter the‘stormflow contributing area’ within a basin and significantly expand the area of catchment over which landuse change is likelyto impact upon the hydrological and hydrochemical response of the catchment.

The paper analyses the environmental impact of piping and considers the implications for basin management.Copyright # 2004 John Wiley & Sons, Ltd.

key words: catchment management; soil piping; hillslope hydrology; upland water quality; UK

CURRENT MANAGEMENT ISSUES IN THE BRITISH UPLANDS

The way we use our uplands in Britain is the subject of change and current debate. The impetus behind conifer

afforestation has faltered as the need for the product has declined, the economics have changed and the

contribution to acidification and aesthetic despoliation of the landscape have been recognized. Sheep-rearing

and hill farming in general suffered under the 2001 foot-and-mouth epidemic, which hit an industry already reeling

from unprofitability and the prospect of reduced subsidies as the EU expands in the coming years. Plans are now

afoot for widespread re-engineering of the upland landscape, nurturing biodiversity and perhaps recreating a more

‘natural’ land cover, returning to a pre-industrial and maybe even prehistoric landscape. In Wales, the pressure

group Tir Coed (‘Woodland’) has put forward a proposal to develop a ‘national forest’ based on deciduous trees

and shrubs, similar to that already being developed in the English midlands (Tir Coed, 2001).

Copyright # 2004 John Wiley & Sons, Ltd.

�Correspondence to: J. A. A. Jones, Institute of Geography and Earth Sciences, University of Wales Aberystwyth, Aberystwyth SY23 3DB,UK. E-mail: [email protected]

Contract/grant sponsor: Natural Environment Research Council, UK; contract/grant numbers: GR/3683, GR3/6792 and a studentship.Contract/grant sponsor: The University of Wales, Aberystwyth.

These changes in landuse are likely to have many hydrological implications. Yet, despite some of the most

detailed and informative experimental studies of the impact of conifer plantation and harvesting undertaken by the

Natural Environment Research Council (NERC) Centre for Ecology and Hydrology (CEH) in mid-Wales, our

understanding of the effects of upland deciduous plantations and any guidelines designed to limit the hydrological

impacts are rather lacking.

This paper considers just one feature of the upland drainage system that currently contributes to biodiversity and

may both help and hinder the rehabilitation of acidified surface waters: natural soil piping. Natural soil pipes are

the largest and most highly connected form of macropore and as such are potentially a major source of bypass

flow within the soil body and a contributor to quickflow of comparable importance to ‘classical’ overland flow in

stream hydrographs. Jones et al. (1997) estimated that up to 30 per cent of the land surface of Britain may be

susceptible to pipe development (Figure 1). Almost all of the 70 catchments that displayed piping in their survey

were in the uplands; the only exception was a pelosol in East Anglia (eastern England) with a high cracking

potential. The piped basins are predominantly covered by podzolic soils (especially ferric stagnopodzols and

brown podzolics) or peaty soils (especially raw oligo-fibrous peats) with rock exposures, mean annual rainfall in

the range 1500–2000 mm, altitudes peaking around 500 m a.s.l. and with mainstream slopes of the order of 108.Holden and Burt (2002) confirm that piping is common in blanket peats in the British uplands.

A considerable amount of evidence has been published in recent years that supports the view that natural

pipeflow can be an important pathway for hillslope drainage and a significant process in streamflow generation,

especially in headwater catchments in the uplands. The evidence was first collected in catchments in the Welsh

mountains, but this has been supplemented, particularly during the last decade, by a number of field-monitoring

experiments covering both pipeflow and piping erosion in other countries with a variety of climatic regimes.

In comparison with the study of pipeflow and pipe erosion, research into the chemistry of pipeflow, especially

its role in the acidification of surface waters, effects on aluminium concentrations and the movement of plant

nutrients, remains very limited and the evidence available to date is highly complex. This paper attempts to review

the evidence for the effects of piping on upland landscapes, streamflow response and water quality, and considers

the possible implications for the management of piped catchments.

EVIDENCE OF THE IMPACT OF PIPING ELSEWHERE

Land Degradation

Although piping is not generally seen as a major cause of land degradation in Britain, it is instructive to note that

most research in other countries has focused on piping as an erosion process, responsible for initiating gullying,

landslips or ‘pseudokarst’ landscapes. This is the main focus of the papers presented in a Special Issue of

Geomorphology (Jones and Bryan, 1997). All of these landform processes are found in Britain (see later), but

generally on a smaller scale.

Piping has been found to be both a cause and a result of both gullying and landslides. Landslip failures caused by

piping have been reported, for example, in Canada (Bryan and Price, 1980), Japan (Tsukamoto et al., 1982) and

Romania (Balteanu, 1986), but landslips can also create voids that may develop into pipes, as postulated by

Jackson (1966) in New Zealand and Jenkins et al. (1988) in Britain. Hagerty was funded by the US Waterways

Experiment Station to survey the problem of riverbank failure due to piping in America (Ullrich et al., 1986;

Hagerty, 1991a, 1991b). Jones (1989) pointed out that riverbank failure caused by piping cannot necessarily be

predicted from geotechnical analyses of the bank material, as the discharge and shear stresses generated within the

pipes can depend on the properties of the soil well removed from the bank itself. On the hillside, complete or

partial collapse of pipe roofs can create a variety of landforms, varying from the linear collapse features reported

by Terajima and Sakura (1993) in Japan, to bridged channels or extensions of open stream channels (Jones, 1971,

1987b).

Some of the worst land degradation due to piping is reported from badlands and semiarid regions, where piping

is almost ‘symbiotically’ linked to rilling, gullying and mass movement (e.g. Parker, 1963; Bryan and Yair, 1982;

326 J. A. A. JONES

Copyright # 2004 John Wiley & Sons, Ltd. LAND DEGRADATION & DEVELOPMENT, 15: 325–349 (2004)

Figure 1. Distribution of piped catchments in Britain in relation to soil type. Winter Rainfall Acceptance Potential (WRAP) class 5 soils havethe lowest infiltration capacity; class 2 have moderately high throughflow potential.


NATURAL SOIL PIPING FOR BASIN MANAGEMENT 327

Higgins and Coates, 1990; Torri and Bryan, 1997; Gutierrez et al., 1997). The collapse of the roofs of large pipes is

a common cause of gullying, but gullying also steepens the hydraulic gradient in the banks, which favours new

pipe development (e.g. Aghassy, 1973; Harvey, 1982).

Piping has also been a scourge on agricultural land, especially where insufficient care has been taken after forest

clearance or on susceptible soils. In Arizona, Masannat (1980) estimated that almost 50 per cent of the agricultural

land in the San Pedro valley had been destroyed or severely affected by piping. Similar problems have been

reported from Australia and New Zealand (e.g. Floyd, 1974; Crouch et al., 1986). Over-irrigation is also seen as a

cause of piping erosion by Higgins and Schoner (1997) in California and by Garcıa-Ruiz et al. (1997) in Spain.

One of the most extensive studies of pipe erosion was undertaken by Beckedahl (1998). This showed the extent

of piping erosion mainly in the uplands of KwaZulu-Natal and Transkei, where he surveyed 148 pipe networks

at 66 sites and estimated total soil loss due to piping in the area at between 1300 and 1750 tonnes, mainly from the

B-horizon. Studying three sites in detail, Beckedahl estimated that piping caused widely varying erosion rates,

from 0�67 to 14�2 tonnes ha�1 a�1 (per annum).

Hillslope Drainage and Streamflow

Field-monitoring experiments on pipeflow in a wide variety of environments broadly suggest that pipeflow

contributes significantly to streamflow when it is present. This evidence comes from Japan (Yasuhara, 1980;

Tanaka, 1982; Tsukamoto et al., 1982; Sidle et al., 1995; Terajima et al., 1996, 1997; Uchida et al., 1999; Uchida,

2000; Terajima et al., 2000), from Canada, in the arid badlands of Alberta (Bryan and Harvey, 1985), in southern

Quebec (Roberge and Plamondon, 1987), and the subarctic tundra (Woo and diCenzo, 1988; Carey and Woo,

2000), from the Loess Plateau of Shanxi, China (Zhu, 1997; Zhu et al., 2002), and in the Western Ghats in southern

India (Putty and Prasad, 2000).

The highest percentage contributions, over 75 per cent of basin runoff, come from small headwaters in the Tama

Hills near Tokyo (Yasuhara, 1980; Tsukamoto et al., 1982). Tanaka (1982) produced estimates in the Tama Hills

that are almost identical to those reported for the Maesnant in this paper. The monitoring programmes in China,

Canada and India have all produced estimates in the range 20–35 per cent.

Again, there is abundant evidence of wide differences between mean and peak contributions, from highs of

76 per cent during snowmelt in Quebec (Roberge and Plamondon, 1987) and 59 per cent in India (Putty and

Prasad, 2000) compared with 78 per cent on Maesnant. Other aspects of pipeflow are reviewed in Jones (1990,

1994, 1997b) and Bryan and Jones (1997).

THE MAESNANT EXPERIMENTS

Most of the detail in the following review is based on research in the University of Wales experimental catchment

on the Maesnant, a headwater stream of the River Rheidol in mid-Wales. By chance, this basin appears to be

typical in all respects of the average piped basin in Britain described above.

The Maesnant Stream is a second order stream draining a 0�54 km2 basin on the western flank of Plynlimon in

the Cambrian Mountains. The catchment ranges from 752 m a.s.l. at the peak of Plynlimon to 465 m a.s.l. at the

combined v-notch gauging station. It is underlain by Ordovician greywacke, mudstone and grits, with soliflucted

drift largely forming a small river terrace. Annual rainfall is around 2200 mm, with an annual water surplus of

1800 mm. The gauged section of the stream is 750 m long between upper and lower weirs (Figure 2).

Intensive field monitoring was undertaken in the Maesnant catchment for a total of eight years during the 1980s

and 1990s (e.g. Jones and Crane, 1984; Jones, 1987a, 1988; Hyett, 1990; Richardson, 1992; Connelly, 1993; Jones,

1997b). This involved continuous data logging at ten-minute intervals at up to 17 pipeflow sites, three riparian

seepage zones, two stream weirs and two tipping bucket raingauges. Initially, monitoring at the basin outfall was

based on a Welsh Water the regional water authority weir that was enlarged for this research. This was

subsequently replaced by an Institute of Hydrology weir. Water quality was monitored over a three-year period

using auto-samplers covering up to 13 pipeflow sites and at stream sites above and below the main inputs from the

perennial pipes, plus spot sampling of soil water at the dipwell sites marked on Figure 2 (Hyett, 1990). This was

328 J. A. A. JONES


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supplemented by a single basin-wide survey of surface soil water extracts taken from 234 sites (Richardson, 1992).

The most recent research has concentrated on the factors controlling the development of the pipe networks (Jones

et al., 1997) and on devising a physically-based hydrological simulation model for pipeflow (Jones and Connelly,

2002). The site has recently been designated a Site of Special Scientific Interest (SSSI) to preserve the soil pipe

features (Jones, 1997d).

EFFECTS OF PIPEFLOW YIELDS ON STREAMFLOW RESPONSE: EVIDENCE FROM

MAESNANT AND OTHER SITES IN THE BRITISH UPLANDS

The British uplands have been the principal world source of information on pipeflow. The experimental evidence

comes mainly from three catchments in Wales and three in northern England: in Wales, the Maesnant (Jones, 1978,

1981, 1982; Jones and Crane, 1984; Jones, 1987a, 1988; Jones et al., 1991; Jones, 1997a, 1997c, 1997d; Jones and

Connelly, 2002), the adjacent Centre for Ecology and Hydrology Upper Wye catchment on the eastern slope of

Plynlimon (Gilman and Newson, 1980; Chapman, 1994; Sklash et al., 1996; Chapman et al., 1993, 1997), and the

Nant Llwch Basin in the Brecon Beacons of South Wales (Wilson and Smart, 1984; Smart and Wilson, 1984); in

England all the basins lie within the Pennines, the Slithero Clough, Derbyshire (McCaig, 1983, 1984), Shiny

Brook, near Huddersfield (Gardiner, 1983; Burt et al., 1990) and the Little Dodgen Pot Sike (LDPS) in the Moor

House National Nature Reserve (Holden and Burt, 2002). The earliest published study of piping in Britain comes

from Jones (1971) on the Burbage Brook tributary of the River Derwent in Derbyshire.

These observations generally indicate that pipeflow can be a very important contributor to streamflow in many

upland headwater basins, although there are wide differences in the amount contributed in both space and time

(Table I).

Two important results from the Maesnant have been: (1) proof that the response time for pipeflow can be quick

enough to contribute storm runoff to the surface streams; and (2) proof that the volumes of pipe discharge can be

sufficient to supply a significant and, on occasions, a dominant proportion of flow to the rivers. The first of these

disproves the earlier view of Whipkey and Kirkby (1978) that rain infiltration would take too long to reach the

depth of the pipes to contribute to storm runoff in a basin. Their hypothesis would be more likely to be true if

‘conventional’ diffuse infiltration were the only process, but it is now clear that bypass flow is more important,

feeding the pipes directly through cracks or blowholes in their roofs or indirectly via crack flow feeding the

phreatic surface and raising it to pipe level (Jones and Connelly, 2002). The detailed monitoring shows that there is

a wide range of response times within the pipes, some before the stream, some after, but centring around the

average response time of the stream in terms of both start of response and peak flows (Jones, 1988). Indeed, there is

Table I. Average pipeflow contributions to total streamflow measured in British upland basins

Basin Authors Quickflow (%) Baseflow (%) Overall (%) Comments

Maesnant, Cambrian Jones et al. 49 46 47 Max 70%Mountains (1991) higher in recessionNant Llwch, Brecon Wilson and — — 68 IndirectBeacons Smart (1984) estimateUpper Wye, Cambrian Gilman and — — 34* Rain notMountains Newson (1980) streamflow

Chapman 38 — 10y One pipe onlyet al. (1993, 1997)

Shiny Brook, Gardiner (1983) — — 1 Smaller pipesPennines Burt et al. (1990) only; Deep peatLittle Dodgen, Pot Sike, Holden and 30* — 10�5 *In streamPennines Burt (2002) recession;

Deep peat

330 J. A. A. JONES


some evidence, comparing flows at the upper and lower stream weirs, that the pipe discharge forces stream

response to be earlier as it passes the main outlets of the pipes. The Maesnant pipeflow may reach peak runoff rates

that are comparable to saturation overland flow in small basins of 0�001 up to 0�2 km2, although on average they

are only about one-fifth those of saturation overland flow (Jones, 1997a).

The second result counters the suggestion by Gilman and Newson (1980) that although pipes may be important

as drains for hillslopes, they do not contribute much to streamflow because they end at the edge of the valley

bottom. The Maesnant pipes generally discharge on the edge of the river terrace and dye-tracing experiments

indicate lags of only around 10 minutes for the outflows to reach the stream. In other basins, like the Burbage

Brook, around 150 pipes issue directly from the banks into the stream.

In the Maesnant, pipeflow contributes around 49 per cent of stormflow and 46 per cent of baseflow to the stream,

with the figure rising to over 50 per cent and even 70 per cent in individual storms when moderately heavy rain

(c. 10–60 mm) falls on a moderately wet catchment (with seven-day antecedent rainfall between c. 30 and

160 mm). In wetter conditions, saturation overland flow contributions reduce the percentage coming from the

pipes. In drier conditions, the percentage contributed by riparian seepages rises. These results are corroborated

by Wilson and Smart’s (1984) calculation that ephemeral pipeflow contributes an average of 68 per cent of

streamflow in the Nant Llwch basin in the Brecon Beacons, South Wales, although this figure was based on indirect

estimates, partly based on artificial pumping experiments on pipe-flow-capacities, rather than on direct measure-

ment of natural pipeflow.

The Maesnant research has also shown that there is great spatial diversity in pipe yields. There is nearly an order

of magnitude difference between yields from individual perennially flowing pipes in the basin in both mean

stormflow discharge and peak discharges, and a 60-fold difference between the smallest ephemerally flowing and

the largest perennial pipe.

In contrast, pipeflow has been reported to be rather less significant in a number of other upland basins which

contain deep, blanket peats. In a highly eroded peat bog in the English Peak District, Gardiner (1983) and Burt

et al. (1990) estimated that only about 1 per cent of streamflow was derived from pipeflow. The density of piping

was certainly lower there than on Maesnant. However, the study omitted to monitor flows from the larger pipes.

More convincing evidence from a deep blanket peat catchment in northern England comes from Holden and Burt

(2002) who monitored only a 10 per cent contribution, plus about 0�5 per cent from hand-sampled pipes (Holden,

2001, pers. comm.). However, the percentage contributions from Holden and Burt’s pipes rose to 30 per cent

during stream recession, suggesting that the real difference between these pipes and those of Maesnant lies more in

the timing of contributions: flows from the pipes in the deeper blanket peat of their LDPS catchment are more

delayed and miss the peak streamflow. Indeed, Holden and Burt’s graphical comparison with the Maesnant data

(Holden and Burt, 2002: Figure 11) shows that the pipes in the deep peat are slower to start flowing and take longer

to peak than the Maesnant pipes, but their peak runoff rates are an extension of the Maesnant data; exactly what

would be expected if the Maesnant data extended down to the smaller catchment areas of the LDPS pipes.

Chapman (1994) and Chapman et al. (1997) also report only a 10 per cent contribution but from a single

ephemeral pipe to a first-order stream in a 4 ha tributary of the Afon Cyff within the CEH’s Upper Wye catchment,

adjacent to the Maesnant basin. However, there were other pipes in the subcatchment and the authors estimated

that 17–53 per cent of storm runoff was derived from ‘other waters’, these including pipe water from other outlets,

throughflow and overland flow from other sources. This and the fact that data are available for only seven storm

events rather limits the value of the results. There is, however, a major difference between the Cyff tributary and

the Maesnant, in that the latter contains many large perennially-flowing pipes. Though these exist in parts of the

Upper Wye, they have not been monitored and included as pipeflow in any CEH reports. Interestingly, the direct

contribution to streamflow from ephemeral pipes on Maesnant is very similar to this amount, if the large

proportion of ephemeral pipeflow that feeds through the perennial pipes is excluded. Also, although the early work

in the Nant Gerig Basin in the Upper Wye catchment reported by Gilman and Newson (1980) did not record flow in

the perennial pipes either, nor, more importantly, the flow in the adjacent stream, it was estimated that an average

of 34 per cent of rainfall drained through the ephemeral pipes, during 12 monitored storms. It is also notable that

contributions from these ephemeral pipes rise markedly during storm runoff and Chapman (1994) reported levels



reaching 32 per cent of streamflow in peak flow, or 38 per cent in Chapman et al. (1993) and even 50 per cent in

one event in Chapman et al. (1997: Table 3).

The important point for this paper is that the majority of monitoring programmes have concluded that pipeflow

is a significant contributor to streamflow and that average contributions are commonly in excess of 40 per cent.

The response patterns in terms of peak lag times and peak runoff rates per unit of drainage area also tend to fall

in-between saturation overland flow and matrix throughflow (Jones, 1997a, 1997b, 1997c). Pipes are by no means

present in all basins and even where pipes are present they may not flow in all storms. Also, the large contrasts in

both yields and regimes between adjacent pipes complicates the task of obtaining representative samples and of

extrapolating the results from one basin to another (cf. Jones, 1997c). Nevertheless, the fact that nearly 30 per cent

of the land area of Britain is susceptible to piping (Jones et al., 1997) does suggest that the current physically-based

models of basin hydrology, which are used in upland catchments, none of which attempt to model pipeflow, are

ignoring a potentially important process.

EFFECTS OF PIPEFLOW ON STREAMWATER QUALITY

Studies of the hydrochemistry of pipeflow, streamflow, rainfall and soil moisture in the Maesnant basin, reported

by Jones and Hyett (1987), Hyett (1990), Richardson (1992) and Jones (1997b), largely corroborate and extend

many of the conclusions drawn by CEH staff from their detailed studies in the nearby Upper Severn and Upper

Wye experimental basins.

In an excellent review of CEH work, Neal (2000) found: (1) that storm rainfall tends not to pass directly through

the system as quickflow (as indicated by the dampening of the chloride signal as it passes from rainfall to runoff);

(2) that this implies that there is a major body of groundwater that contributes significant amounts of mainly

neutral-to-alkaline water to baseflow in catchments previously considered impermeable; (3) that this contribution

comes predominantly via fissure flow within the bedrock; (4) that soil water is the main source of acid waters and

this contribution increases in storm flow; (5) that both soil water and groundwater exhibit large variations in

chemistry that can overlap; and (6) that chemical equilibrium is rarely reached and that there are large spatial and

temporal fluctuations in water quality, which complicate the selection of an appropriate scale for monitoring and

modelling and make explanation ‘ . . . difficult, if not impossible, to pin-down in a detailed mechanistic sense’.

Neal (2000) was not explicitly referring to soil piping, but it is clear that pipeflow acts within the soil like fissure

flow in the bedrock, and can be an important source of spatial and temporal heterogeneity in water quality.

Hyett (1990) was particularly frustrated by the inscrutability of pipeflow water quality on Maesnant, its high

temporal and spatial variability and the difficulties of explaining the chemical origins of the water at the detailed

scale of individual pipes. Neal’s, Hyett’s and Chapman et al.’s (1997) experiences all point to the potential hazards

of applying End Member Mixing Analysis (EMMA) to identify the relative contributions from different runoff

pathways, a technique that has acquired some degree of popularity in recent years (cf. Burns et al., 2001; Soulsby

et al., 1998).

Despite this high variability in pipewater quality, it is possible to make a number of generalizations.

Acidity

The most significant effect of piping appears to be in the acidification of surface streams. Piping reduces the

buffering of acid rainfall by reducing residence times and by directing flow through the upper organic horizons,

reducing contact with weathering mineral surfaces (Jones and Hyett, 1987; Gee and Stoner, 1989). It may also

encourage the release of sulphates and organic acids from the peaty horizons by draining and aerating sections of

the hillside (Jones, 1997b).

Flows in the ephemeral pipes are more acid than the rainfall. Hyett (1990) reported an average pH of 4�8 for the

rainfall and mean pHs of 3�8 to 4�3 in the ephemeral pipes. These pipes typically flow at a depth of 150 mm near

the base of the peat O horizon and, like those described in the Nant Gerig Basin by Gilman and Newson (1980),

are principally formed by desiccation cracking in the peat. The remnants of these cracks offer surface inlets that

allow rapid infiltration of rainwater into the pipe network. Although recent modelling simulations suggest that the

332 J. A. A. JONES


storm response in these ephemeral pipes can best be modelled by assuming that pipeflow is initiated by a rising

phreatic surface (Jones and Connelly, 2002), the chemistry suggests that they derive a significant amount of acidity

from the thin peaty cover. These apparently conflicting pieces of evidence can be resolved by the fact that pipe

response is relatively rapid. During average stormflows lasting 25�5 h, the mean lag time between start of rain and

start of pipeflow at the outfalls of the ephemeral pipes is 10�5 h, but they reach their peak flow at about the same

time as the rainfall peak and cease flow 8 h before the end of the rain (Jones, 1988). Peak lag time at the head of the

ephemerals is slightly longer at 4 h.

These statistics suggest that residence times are short and that pipeflow is initiated by rapidly infiltrating rainfall

that raises the phreatic surface above the pipe beds within a few hours. Calculations of the time needed for

rainwater to infiltrate down to pipe level, based on field measurements of infiltration capacity ‘using a double-ring

infiltrometer and a Guelph permeameter’ and laboratory measurements of hydraulic conductivity, support this.

They show that transmission via ‘standard’ micropore seepage is too slow to feed this rapid response: the pipes and

the pheatic surface must be fed by bypass flow through cracks and macropores.

Yields at the outfalls of the ephemeral pipes are also more acid than the discharges at the outfalls of the perennial

pipes (Table II), because the perennial pipes derive a certain amount of water from resurgent groundwater. Acidity

tends to increase downstream along the perennial pipes (Figure 3), partly because the pH is raised to around 5�0 at

the head of the perennial pipes by the resurgence of deep groundwater (Table II). This groundwater is the source of

most baseflow in the pipes and has passed through the Ordovician bedrock, albeit a weathering source low in base

cations. The pipes subsequently collect water from effluent seepage along the pipe walls and from whole

tributaries that originate within the blanket peat. These perennial pipes flow below a peat cover of around 500 mm

and although they tend to run along the peat/drift interface, they derive relatively little discharge from the clayey

drift.

The overall pattern along the pipes tends to remain the same during baseflow (Figure 4), but there are

considerable differences between storms in the level of acidity and concentration of solutes. Consistent patterns

tend to disappear during stormflow as water arrives from different sources.

There is also considerable variation in solute concentrations during individual stormflow events. Much of this

can be explained by variations in the quality of water entering the pipes from the surface, either from rainfall or

from washing off of dry deposition. The evidence collected by Hyett (1990) suggests that this variation in the

quality of surface water inputs severely limits the ability to distinguish separate sources of pipe waters on the basis

of chemistry. Jones and Crane (unpublished data) measured the highest concentration of solutes based on the

electrical conductivity of pipeflow during the first major rainstorms in September, following the summer dry

period. Monitoring at other times also seemed to show a flushing effect during the initial stages of stormflow

Table II. Mean water quality parameters for rainfall, streamflow and pipeflow sites on Maesnant

Pipe StormsRainfall Stream Perennial pipe outfalls Heads of perennials Ephemeral pipe outfalls average only

(n¼ 63)Pipe 2 Pipe 3 Pipe 4 Pipe 5 Pipe 9 Pipe 13 Pipe 14 Pipe 4

pH 4�84 5�16 4�90 4�48 4�52 5�20 5�50 4�26 4�10 4�58 4�27Conductivity 39�5 34�1 35�8 41�9 40�0 39�9 33�4 57�4 47�0 41�5 47�0(mS cm�1

at 208C)Dissolved — 0�211 0�162 0�238 0�295 0�104 0�208 0�524 0�370 0�271 0�206aluminium(mg l�1)Dissolved — 2�69 4�11 4�95 3�19 1�60 1�40 2�20 15�60 3�81 3�48organic carbon(mg l�1)

Note: Pipe reference numbers as used in Figure 2. Based on data presented by Jones and Hyett (1987).



response, followed by lower solute concentrations at peak and during recession, which may also suggest the

removal of a limited supply of weathering products.

Overall, the pipes provide a major source of stream acidity even under baseflow conditions, but during storms

the pH of pipeflow drops further. Despite this, the average stream acidity remains lower than the mean rainfall

acidity by only a small margin. The basin therefore displays a small acid neutralizing capacity, which is due to the

Figure 3. Trends in water chemistry down Pipe 2 under baseflow conditions. Weir 15 (W15) is at the outfall of the ephemeral pipe section andWeir 5 (W5) the head of the perennially-flowing section. See Figure 2 for location of these weirs.

Figure 4. Trend in acidity and aluminium concentrations down Pipe 4.

334 J. A. A. JONES


amount of groundwater entering the stream directly from the riparian zone. The ‘purest’ contributions from

groundwater come from riparian seepage zones, which contribute around 36 per cent of stream quickflow in an

average 30 mm storm. Most of these zones yield diffuse seepage, predominantly from bedrock in the upper half of

the monitored reach. Some are also fed by ephemeral pipes, which would lower the pH during the stormflows when

these pipes are active; approximately 1 in every 3 storms that yield storm runoff from the perennial pipes.

Aluminium Concentrations

The pipes are also a significant source of aluminium in the streamwater. Table II shows that the ephemeral pipes

tend to display the highest mean levels of aluminium. This is partly due to the fact that these pipes only flow under

storm conditions. It is also partly due to the fact that these pipes run wholly within the peaty surface horizon where

Richardson (1992) and colleagues measured the highest levels of aluminium within the soil. Indeed, both total

aluminium and the levels of the more toxic monomeric form are highest in the ephemeral pipes. Nevertheless,

Table III shows that even the perennial pipes can be a significant source of aluminium for streamwater, with higher

concentrations than either matrix throughflow or overland flow. Levels of monomeric aluminium in the perennial

pipes also frequently exceed the toxic threshold for fish of around 0�12 mg l�1 in this low-calcium environment

(see Figures 3 and 4).

Although Hyett (1990) found wide variations in aluminium concentrations between pipes, and frequently a poor

relationship with pH, Figures 4 and 5 clearly show a reasonable overall correlation. Figure 5 also shows how a

small storm following a larger storm can result in a disproportionately high response once the hillslope system is

thoroughly wetted. In the nearby Upper Wye catchment, Chapman et al. (1997) also noted the increase in

aluminium, particularly inorganic aluminium, downslope within the pipe, and they conclude that aluminium

discharges are proportional to the length of pipe. Muscutt et al. (1993) also concluded that pipeflow was the main

source of aluminium in this same stream catchment during storms.

Overall Effects on Streamwater Quality

Pipes are a particularly important source of ‘dirty water’, i.e. brown-stained water in the stream, which occurs after

heavy rains and especially during the first rains of autumn following a dry summer. Hyett (1990) found the

following average ranking in the effects of pipeflow on streamwater quality: colour> pH> conductivity>aluminium. Pipeflow is not always the single most important control on streamwater quality, but it is when the

volumetric contributions from pipeflow are greatest, e.g. 50–70 per cent of streamflow.

This also supports the view of Neal et al. (1997b) that classifying the vulnerability of streams to acidification

purely on the basis of the sensitivity of the soils is inadequate, and that flow pathways need to be considered.

However, although Neal et al. focused on the effect of the extra acid neutralizing capacity provided by resurgent

groundwaters, soil pipes have the opposite tendency—to increase vulnerability. Yet they are not included in

normal tests of soil sensitivity.

The exact processes which determine pipeflow chemistry remain elusive and highly variable in space and time

(Hyett, 1990), matching the conclusions of Neal (1997b, 2000) on the detailed chemistry of catchment processes

Table III. Contemporaneous water quality samples from six hillslope flowpaths on Maesnant during storm runoff

Determinand Stream Pipe 2 Pipe 3 Pipe 4 Matrix Overland flow

pH 5�5 5�3 4�7 4�7 4�3 4�3Conductivity (mS cm�1 208C) 32�0 32�0 34�0 38�0 39�0 36�0Total hardness (mg l�1 CaCO3) 4�7 4�4 3�7 4�9 2�4 2�0Chloride (mg l�1) 5�0 6�0 6�0 7�0 6�0 5�0Dissolved silicate (mg l�1) 1�5 1�8 1�9 1�3 0�3 0�3Dissolved potassium (mg l�1) 0�18 0�19 0�16 0�19 0�83 0�35Dissolved aluminium (mg l�1) 0�077 0�118 0�167 0�257 0�109 0�122Dissolved organic carbon (mg l�1) 1�7 3�6 3�3 5�4 10�6 8�8



Fig

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336 J. A. A. JONES


in general. However, although the exact processes may not be understood or even measurable, the overall effect of

piping on stream chemistry is clear and offers some credence to the view that lumped models may still be valid

despite the heterogeneity at the detailed scale, as Neal and Robson (1997) concluded from their study of cation-

exchange modelling.

The Old or New Water Controversy: Evidence from Pipeflow Chemistry

Conflicting views have been expressed as to whether pipe quickflow is predominantly derived from the current

storm or from previous storms. Sklash et al. (1996) report what seems to be overwhelming evidence from

hydrogen isotope analysis that the ephemeral pipes in the CEH Upper Wye catchment are draining predominantly

‘old’ water from previous storms. Neal and Rosier (1990) also report that 80 per cent of stormflow in one major

pipe in the Upper Wye appears to be ‘old’ on the basis of deuterium content. However, Biggin (1971) estimated

that only 45 per cent of peak discharge in the Plynlimon pipes was ‘old’. In the Maimai experimental catchment in

New Zealand, McDonnell (1990) concluded that pipeflow was a mixture of old and new water, and whilst Mosley

(1979) concluded macropore flow was mostly new water there, deuterium analyses at the same sites by Sklash et al.

(1986) indicated that old water was predominant. However, the fact that the Maesnant pipes respond to short-

term, within storm variations in rainfall chemistry and that a significant proportion of acidity is derived from the

surficial peaty layers suggests short residence times and limited contact with the mineral layers here (Jones,

1997b).

The Maesnant monitoring programme has provided very clear evidence that fresh rainwater is getting through

to the pipes during the storm. Figure 6 displays one of the best pieces of chemical evidence to support this view,

taken, not from a shallow, ephemeral pipe, but from the largest perennial pipe running at a depth of 500 mm. The

graph shows the close relationship between sodium concentrations in the rainwater and sodium concentrations in

pipeflow shortly afterwards. As Neal and Kirchner (2000) demonstrated, sea-salt contaminated rainfall is the

predominant source of sodium on the slopes of Plynlimon and very little is derived from weathering, so it is a good

indicator of a rainfall source. Good correlations between sodium levels in rainwater and groundwater have also

been found by Neal and Kirchner (2000) in the adjacent CEH catchments but at weekly or longer timescales,

which they considered could be partly explained by the highly fracture-flow dominated groundwater system.

However, fluctuations with lag times of hours strongly suggest that the pipeflow is draining short residence time

waters, either from direct supply via soil macropores or from shallow groundwater fed by rapid bypass flow

through these macropore routeways.

Although the amplitude of variation is somewhat dampened, the pattern sequence is very well preserved, until

the last two rainfall measurements, by which time in the storm the pipeflow may have been diluted by older

groundwater issuing into the pipe (Figure 6). The damping may be partly explained by adsorption onto the soil

cation exchange sites; Neal and Kirchner (2000) found that adsorption increases as sodium concentrations increase

above the norm. It also suggests that the discharge is to some extent diluted by older groundwater. But, again,

the short lag time in the chemical signal between rainfall and pipeflow shows that there is no significant decoupling

of timescales between fluctuations in the hydraulic head that drives pipeflow and the movement of the water itself,

in contrast to the decoupling found in streamflow in the CEH catchments by Kirchner et al. (2000).

The responses of aluminium and dissolved organic carbon (DOC) to storm events in Figure 5 also point to a

significant portion of pipeflow deriving from shallow soil water. This is similar to streamflow in the Upper Severn

research catchment, where Neal et al. (1997a) have shown how base cations associated with deeper groundwater

decrease in concentration as streamflow increases, whilst DOC and aluminium associated with soil water increase.

Hyett (1990) concluded that the main controls on Maesnant pipewater quality are the amount, intensity and

quality of storm rainfall, followed by antecedent soil moisture levels.

Short residence times are also supported by the purely hydrological observations of: (1) the large proportion

(averaging 68 per cent) of total storm rainfall in the catchment that seems to appear as stream quickflow (Jones,

1997b); (2) the high levels of volumetric flow contributions to stream quickflow (averaging 49 per cent) from the

pipes; (3) the close correlation between flow patterns and rainfall patterns shown throughout the ephemeral



network (Figure 7); and (4) the successful modelling of pipeflow assuming only surface and shallow groundwater

sources (Jones and Connelly, 2002).

Sklash et al. (1996) suggested that the rapid response of their ephemeral pipes might be explained by piston

flow. Piston flow might well be an element in the hydrological response. However, the hydrochemical evidence

collected by Hyett (1990) seems to marry well with the purely hydrometric data and together they point to a

substantial proportion of quickflow being provided by ‘new’ water from the current storm on Maesnant in both the

ephemeral and perennial pipes. Baseflow in the perennial pipes is, of course, a different case.

Overall, it seems reasonable to conclude that the proportions of old and new water vary from catchment to

catchment, from storm to storm, even between pipes and within a stormflow event. This creates problems for

modelling water-quality effects that have not yet been tackled.

Figure 6. Sodium concentrations in rainfall and pipeflow in perennial Pipe 4. Based on data collected by C. G. Hyett 1990.

338 J. A. A. JONES


Fig

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EFFECTS OF PIPING ON THE HILLSLOPE ENVIRONMENT

Effects on Soils and Vegetation

The Maesnant pipes have a number of important effects on the hillslopes as well as the stream water. Surveys

initiated by Richardson (1992) have shown effects on soil-profile development, the distribution and diversity of

moorland plant communities and the lateral redistribution of plant nutrients.

Measurements of electrical conductivity in the top 150 mm of soil suggest that the ephemeral pipes have a

marked effect on the distribution of plant nutrients (Figure 8). The highest levels of electrical conductivity in

topsoil extracts were found below the outfalls of the ephemeral pipes, where the pipes issue onto the surface and

create a zone of overland flow during heavy storms before re-entering the pipe network at the head of the perennial

pipes. This is counterbalanced by a zone of low electrical conductivity around the head of the ephemeral pipes,

which suggests that the pipes are pathways for a nutrient depletion and enrichment process running down the

hillslope, depleting the nutrient status of the surface soil over the main upslope area of piping and enriching the

area downslope of the pipe resurgences (Richardson, 1992; Jones, 1997b). Richardson (unpublished data) has also

found evidence indicating nutrient enrichment at depth around the lower reaches of the perennial pipes. This might

suggest that the pattern seen in the surface soils around the shallow, ephemeral pipes, that run within the depth of

the 150 mm ‘surface’ soil samples, is repeated around the perennial pipes, but at depths of around 500 mm and

hence not apparent at the surface. On the other hand, Chapman et al. (1993, 1997) suggest that base-cation

enrichment on the lower slopes might be due to adsorption from groundwater. Indeed, the distinction between pipe

transport and groundwater solute transport may be blurred, because automatic records from banks of dipwells set

perpendicular to Pipe 4 on Maesnant suggest that waves of throughflow run downslope parallel to the pipes during

storms (Jones, 1997b).

The pipes also drain and aerate sections of hillslope. This affects the plant communities and soil devel-

opment, so that linear patterns are found in both following the lines of the pipe networks down the hillside,

as illustrated in Figure 9 (Jones et al., 1991; Richardson, 1992; Jones, 1997b). Drainage and aeration around

the pipes accelerates the decomposition of the peat, creating oligo-amorphous peat soils around the perennial

pipes leaving the deep peat on the micro-interfluves. These bands of oligo-amorphous peat along the perennial

pipe are up to 500 mm lower than the deep peat and mixed grassland heath interfluves between the pipes (Jones,

1997b).

The direction of causality is perhaps less clearcut with the ephemeral pipes, which are associated with areas of

stagno-podsol within a ‘sea’ of blanket peat—was the peat always thinner there and desiccation cracking and pipe

development has caused a positive feedback? Attempts to improve the upland grazing by ploughing and reseeding

in the Upper Wye basin have resulted in waterlogging caused by the destruction of this natural pipe drainage

(Gilman and Newson, 1980).

Vegetationally, both types of pipe create belts of dry grassland associations surrounded by mixed heath of

heather and bilberry (Figure 9).

Pipe Location, Intensity and Erosional Impact

The location of piping within upland basins has been studied in detail by Jones et al. (1997). This demonstrated the

importance of different soil types, depths and exposure to desiccation cracking in causing pipe networks to develop

in areas away from topographic hollows, which are commonly regarded as streamflow source areas. There are

therefore two groups of controlling factors: topographic and edaphic.

Data on the intensity of piping are still very limited. However, there is a clear need to try to predict the size and

density of pipes in unsurveyed catchments. Jones (1997c) measured the average cross-sectional area of pipes,

in square metres, per kilometre of streambank or contour length in nine intensively mapped basins, and compared

this with annual rainfall, annual water surplus and mean sideslope gradient. The latter was the best predictor of

pipe intensity, with r¼ 0�67 (�¼ 0�05). Interestingly, despite the evidence from Holden and Burt (2002) that the

hydrological role of piping may be different in deep peats compared with peaty podzols, the ‘intensity’ of piping

along the LDPS stream fits the relationships established by Jones (1997c) with annual water balance surplus and

340 J. A. A. JONES


Figure 8. The distribution of plant nutrients in the surface soil on Maesnant in relation to the pipe networks, as indicated by the electricalconductivity of the soil extract. Based on data collected by J. M. Richardson, 1992.



Fig

ure

9.

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342 J. A. A. JONES


sideslope gradient very well. Adding LDPS now gives a correlation of r¼ 0�68 with sideslope gradient and

improves the correlation with water surplus from r¼ 0�42 to r¼ 0�49 (Figure 10).

In terms of erosional effects, there are very few measurements of pipe sediment yields from the British uplands.

In part, this seems to reflect the relative importance of piping as an agent of erosion here compared with the results

of flow monitoring quoted above. Nevertheless, measurements taken from sediment traps located behind pipeflow

weirs set up at the bankside outfalls of the four perennial pipe systems feeding Maesnant produced a total sediment

load of 430 kg over an 18-month period. The bulk of this sediment was transported during the winter and in high

flows, and comes from the pipe beds.

Taking the average dynamic contributing areas calculated for these pipes (Jones, 1986: Table 1), suggests an

erosion rate of 50�2 kg ha�1 a�1 for the area of the basin drained by piping. Assuming a similar rate of erosion for

the two overflow pipes (numbered 6 and 8 on Figure 2), suggests a total sediment yield of 358 kg a�1. Interestingly,

Lewin et al. (1974) estimated an annual bedload yield of 2 tonnes at the lower stream weir on Maesnant, which

suggests an overall erosion rate of 37 kg ha�1 a�1 from the 54 ha catchment as a whole. Although this estimate

relates to different years, it does suggest that the areas of piping are yielding more sediment than the areas of

hillslope with no piping. Since the average pipeflow contributing areas cover approximately one-third of the basin,

this would suggest that the piped areas yield at least 70 per cent more sediment per hectare than the unpiped

hillslopes. In fact, they contribute far more, because Lewin et al. (1974) identified the river channel as the main

source of the bedload, especially channel scars in the river terrace.

Both the bedload trap in the river channel and the traps on the perennial pipes naturally collect mainly inorganic

sediment. An unknown volume of organic material is also eroded from the walls of the pipes and passes out of the

system as suspended sediment. This partly explains why Jones’s (1987a) calculation that, at the measured rates of

erosion, the volume of the shallow linear depressions (less than 0�5 m deep) formed over the perennial pipes would

have taken longer to erode than the time available since the last glaciation. Erosion rates may well also have been

higher in the past, e.g. during the ‘Little Ice Age’. Another factor in the formation of the depressions is greater

oxidation and decomposition of the peat above the pipes due to aeration by the pipes.

Despite the high sediment yields from the pipes, two decades of observations have shown no significant

extension of the pipe networks, and we conclude that they are in quasi-equilibrium with the annual water balance

and storm frequency in the basin. Most of the sediment in the perennial pipes appears to come from the pipe beds.

Most of the ephemeral pipes, however, run only within the peaty O horizon of the podzols. In the similar ephemeral

pipes of the Upper Wye basin, Newson (1976) measured 0�5 kg from one pipe over a winter period. Small though

Figure 10. The relationship between the intensity of piping within the soil profile (m2 km�1) and the mean gradient of hillslopes in tenintensively surveyed basins, showing the position of the deep-peat LDPS basin. The exponential regression curve is fitted with R2¼ 0�52 and

�¼ 0�02.



this sounds, if we assume this adds up to c.1 kg a�1 and multiply this up on the basis of the number of larger pipes

on the hillslope mapped by Gilman and Newson (1980), we get a very rough estimate of around 3�3 kg ha�1 a�1 of

mainly organic material for this ephemeral pipe system, and again Gilman and Newson did not monitor the finer

suspended sediment yields.

IMPLICATIONS FOR CATCHMENT MANAGEMENT

Sensitive management of piped catchments should recognize both the hydrological and the ecological role of

pipeflow. This may well involve value judgements, since the pipes have both positive and negative effects on their

environment.

Recognizing the Extent of the Stormflow Contributing Area

Accurate delimitation of the dynamic contributing area can be important for many purposes. It may be needed to

determine the optimal area for liming in order to neutralize runoff (see above). It could also help in planning

afforestation (see above).

The Maesnant pipes carry the contributing area well beyond what would be recognized by topographic indices

such as a/s (area drained per unit contour length divided by slope angle) used in TOPMODEL (Jones, 1986; Beven

and Kirkby, 1979). They approximately double the quickflow contributing area. Moreover, this extension occurs in

an irregular way rather than a simple expansion of the riparian contributing area. The longest network of combined

ephemeral and perennial pipes extends 750 m from the stream. The longest single ephemeral pipe feeding directly

into the stream extends some 300 m away from the stream. When the ephemeral pipes ‘switch’ into the flow

network, they can link remote contributing areas to the stream that may be regarded as disjunct from the stream if

looked at simply from the point of view of overland connections.

Overland flow plays a relatively small part in quickflow generation. Sets of crest stage gauges installed in the

most likely places to develop overland flow recorded very little, and visual observations confirm the general lack of

overland flow contributions. This is despite the fact that surveys of infiltration capacities across the basin suggest

that overland flow should in fact occur over large areas of the Maesnant basin. Comparing the frequency

distribution of rainfall intensities in the basin with infiltration measurements made with a standard double-ring

infiltrometer and a Guelph permeameter suggests that overland flow should occur on 75 per cent of hillslopes in

one in every two rainstorms. Nothing like this occurs in practice. This is because the standard methods of

measuring infiltration capacity ignore the cracks and the blowholes and inlets in the roofs of the pipes, which have

near infinite capacity to drain depression storage and overland flow. However, pipes do have a limited capacity to

transmit flow and some clearly reached that capacity during monitoring, whereas overland flow only increases in

efficiency as flow increases.

Controlling Acidification and Stream Aluminium Levels

The pipes are a significant source of acid waters and aluminium for the stream, especially during stormflow.

Gee (2002, pers. comm.) has admitted that one of the failings in the Welsh Acid Waters Programme was a lack of

understanding of the role of pipes and instead identifying the sources of streamflow solely on the basis of the a/s

index in TOPMODEL (cf. Edwards et al., 1990). Jones (1986, 1997a) found only limited correlation between

either a/s or ln (a/s) and pipe location and discharge. Ephemeral pipes begin at an a/s of only 0�5 in the region with

the densest piping, compared with an a/s index up to 3�0 towards the head of the catchment, suggesting that the

area with the densest piping owes its initiation to some extra factor(s). This extra factor seems to be soil properties

that favour pipe development, particularly a propensity to desiccation cracking and a shallow soil over an

impermeable bedrock base.

Bearing this in mind, liming of the source areas for the pipes, as identified in Jones and Connelly’s (2002) recent

physically-based model, should be more cost-efficient than liming whole catchments and more effective than

simply relying on ‘traditional’ methods of identifying source areas. It is worth noting that basins are likely to be

more sensitive to acidification where more ephemeral pipes feed the stream directly, as opposed to issuing onto

344 J. A. A. JONES


the hillside or into perennial pipes. The monitoring of an ephemeral pipe, which issues onto the lower hillslope

in the Upper Wye catchment, by Chapman et al. (1993, 1997) shows how pipe water can pick up base cations

(Ca and Mg) as it flows overland to the stream. They also showed how this buffering varies and is weaker in wetter

antecedent conditions and heavier storms. They speculate that the base cations are brought to the surface by

groundwater during inter-storm periods.

Implications for Forestry, Biodiversity and Moorland Management

As illustrated above, piping has developed its own environment of moorland soils and plant associations on

Maesnant, adding to the biodiversity and developing an increasingly marked micro-topography towards the lower

end of the network.

The pipes also have implications for afforestation. Conifer afforestation has been an important accelerator for

stream acidification in Wales (Edwards et al., 1990; Neal, 1997a and b). Planting coniferous trees on piped areas

of hillslope is likely to aggravate the effect by speeding runoff, even without prior ploughing and ditching.

This suggests that piped areas should be mapped as ‘no-go’ zones for planting. Less information is available on the

effects of deciduous trees. However, although the effects may be less due to the reduced interception of acid

aerosols during the leafless period and to the lower release of organic acids from the trees, they are still likely to be

efficient interceptors during foliage. This clearly has implications for the newly proposed tree planting schemes.

Alternatively, if piping is not to be conserved, then the most acid pipes could be ‘switched out’ of the drainage

system, for example, by blocking and diverting to overland flow and a ‘buffer’ bog. Sphagnum moss is known to be

an efficient, short-term adsorber of acid anions. This reduces the acidity of the stormflow entering the stream.

Although the ions are subsequently released during recession or baseflow, it is the concentration of acidity in

so-called ‘acid flushes’ during stormflow that seems to be responsible for most ecological damage (Edwards et al.,

1990; Ormerod and Jenkins, 1994). This is also a possible solution for the ‘dirty’ water problem.

CONCLUDING REMARKS

Pipeflow can be a substantial contributor to catchment quickflow in upland moorland basins throughout Britain.

In the Maesnant basin, soil pipes have been found to double the average partial contributing area. Moreover,

this additional contributing area can lie well beyond the riparian zone source area, which is commonly seen as the

main contributing area. On Maesnant, the longest stretch of pipe network extends 750 m from the stream, the

pattern of contributing area is highly irregular, and in many areas the location of the piping bears little relationship

to standard topographic indices, like a/s and its variants, which have been used in physically-based runoff

simulation models, like TOPMODEL, to define contributing areas. Equally, standard algorithms for calculating

flood discharges ignore soil piping, and may significantly underestimate concentration times, or else approximate

them for the wrong reasons, e.g. where pipeflow is an effective substitute for overland flow. The Maesnant pipes

are equally important as sources of baseflow.

Pipes increase the acidification of surface waters in the British uplands, especially where ephemeral pipes issue

directly into stream courses. Deficiencies in modelling the impact of acid rain on upland streams may be partly due

to ignoring the role of soil pipes. Pipes may also transmit faecal pathogens, heavy metals and other pollutants from

extensive areas of hillslope ‘classically’ regarded as non-contributing areas. They therefore have important

implications for landuse planning. Afforestation on a piped hillslope may cause greater surface water acidification

than expected from standard theory. Ploughing and reseeding of upland pasture can produce a negative result if

pipe networks are destroyed and waterlogging results.

Landuse planning and management strategies should therefore be sensitive to the possible role of piping

processes within moorland basins and not rely solely upon delimiting the surface drainage networks and surface

topography. The role of piping in encouraging diversity in plant communities and soils within moorland habitats

also needs to be recognized.

The general presence of piping may often be indicated by linear depressions or vegetation patterns and

dye tracing can be used to establish linkages (Jones and Crane, 1984; Smart and Wilson, 1984). However,



ground-penetrating radar techniques are now beginning to offer the prospect of more accurate and rapid

delineation of the pipe networks, although as yet Holden et al. (2002) report that they could not identify pipes

with diameters less than 10 cm or running at depths of less than 10–20 cm, typically shallow, ephemeral piping.

Management strategies should also consider the probable effects of global warming. Future climate change may

well enhance the influence of piping on the moorland environment. Desiccation cracking is the main initiator of

the ephemeral pipe networks on Maesnant. Gilman and Newson (1980) noted the increase in the density of piping

in the nearby Nant Gerig catchment following the severe drought of 1976. Evans et al. (1999) note that the

acrotelm layers in blanket peats become more hydrophobic in a drought, as do many mineral soils after dry periods

in otherwise generally wet upland regions in Western Europe (Doerr et al., 2000) and British Columbia (Barrett

and Slaymaker, 1991). Increased and more persistent cracking and hydrophobicity could well be reinforcing in

creating more ephemeral pipeflow, whilst the predicted increase in winter rainfall in upland Britain (Pilling and

Jones, 1999, 2002) is likely to increase flow in perennial pipes. These effects could well out-balance the reduction

in water-table levels during the summer, as the evidence shows that most pipeflow occurs during the winter half-

year. Potential consequences of this include more peat erosion, gullying and water discoloration (Evans et al.,

1999), higher flood discharges and greater streamwater acidification.

acknowledgements

I would like to thank former research assistants and students Francis Crane, Glyn Hyett, Mark Richardson and Liam Connelly.I would also like to thank the Natural Environment Research Council (Grants GR/3683, GR3/6792 and a studentship) and alsothe University of Wales (studentship) for supporting this research.

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