Natural soil piping, water quality and catchment management in the British uplands

J A A Jones

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

Abstract Hydrological studies of natural soil piping in the Cambrian Mountains have shown pipeflow to be an important contributor to streamflow in headwater catchments, on occasions contributing as much as 70% of quickflow in the stream. Reconnaissance surveys of the distribution of piping in Britain have identified over 70 basins that possess pipe networks and analyses of the climatic, physiographic and edaphic properties of these basins suggest that up to 30% of Britain may be subject to pipe development. With minor exceptions, for example in pelosols with high shrinkage potential in East Anglia, these basins lie within the upland zone dominated by histosols and spodosols. In the Maesnant basin, Plynlimon, 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. These pipe networks can play an important role in determining water quality within the streams. Deficiencies in modelling the impact of acid rain on upland streams may be partly due to ignoring the role of soil pipes. Pipes may 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. The paper analyses field observations of water quantity and quality in piped catchments and considers the implications for basin management. Keywords: Catchment management, soil piping, hillslope hydrology, upland water quality 1. Introduction 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 recognised. Sheep-rearing and hill farming in general have 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, Tir Coed 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). These changes in landuse are likely to have many hydrological implications. Yet, despite some of the most detailed and informative experimental studies of the

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impact of conifer plantation and harvesting undertaken by the NERC Centre for Ecology and Hydrology 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 in 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% 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 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 OD, and with mainstream slopes of the order of 10o. A considerable amount of evidence has been published in recent years which 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 by a number of field monitoring experiments in a variety of climatic regimes, notably during the last decade. In comparison with the study of pipeflow regimes, 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 pipeflow on streamflow response and water quality, and considers the possible implications for the management of piped catchments. 2. 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 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 (Pumlumon) in the Cambrian mountains. The catchment ranges from 752 m O.D. at the peak of Plynlimon to 465 m O.D. 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 8 years during the 1980s and 1990s (e.g. Jones and Crane, 1984; Jones, 1987; Hyett, 1990; Richardson, 1992; Connelly, 1993; Jones, 1997b). This involved continuous data logging at 10 minute intervals at up to 17 pipeflow sites, 3 riparian

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seepage zones, 2 stream weirs and 2 tipping bucket raingauges. Initially, monitoring at the basin outfall was based on a Welsh Water weir that was enlarged for this research. This was subsequently replaced by an Institute of Hydrology weir. Water quality was monitored over a 3-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 in Figure 2 (Hyett, 1990). This was 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). 3. Effects of pipeflow yields on streamflow response 3.1. 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, 1987, 1988; Jones et al., 1991; Jones, 1997a, c, d; 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); 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, in press). 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. 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 (1977) 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 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

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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% of stormflow and 46% of baseflow to the stream, with the figure rising to over 50% and even 70% 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% 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 measurement 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. In a highly eroded peat bog in English Peak District, Gardiner (1983) and Burt et al. (1990) estimated that only about 1% 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 (in press) who monitored only a 10% contribution, plus about 0.5% from hand-sampled pipes (Holden, pers. comm., 2001). However, the percentage contributions from Holden and Burt’s pipes rose to 30% 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. Chapman (1994) and Chapman et al. (1997) also report only a 10% contribution from shallow ephemeral pipes in a 4 ha headwater study site within the Centre for Ecology and Hydrology’s Upper Wye catchment, adjacent to the Maesnant basin. In this case, the difference is that the Maesnant basin contains more large perennially-flowing pipes, and though these exist in parts of the Upper Wye, they have not been monitored and included as pipeflow. Interestingly, the direct contribution to streamflow from ephemeral pipes on Maesnant is very comparable 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

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stream, it was estimated that an average of 34% of rainfall drained through the ephemeral pipes. It is also notable that contributions from these ephemeral pipes rise markedly during storm runoff and Chapman (1994) and Chapman et al. (1997) reported levels reaching 32% of streamflow in peak flow, or 38% in the table presented in Chapman et al. (1993). 3.2. Comparative measurements from elsewhere Field monitoring experiments on pipeflow in a wide variety of environments broadly reveal a similar range in pipeflow contributions, strongly weighted towards the higher end. 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 India (Putty and Prasad, 1999). The highest percentage contributions, over 75% of basin runoff, come from small headwaters in the Tama Hills near Tokyo (Yasuhara, 1980; Tsukamoto et al., 1982). In the same area, Tanaka (1982) produced estimates almost identical to Maesnant. The monitoring programmes in China, Canada and India have all produced estimates in the range 20-35%. Again, there is abundant evidence of wide differences between mean and peak contributions, from highs of 76% during snowmelt in Quebec (Roberge and Plamondon, 1987) and 59% in India (Putty and Prasad, 1999) compared with 78% on Maesnant. Other aspects of pipeflow are reviewed in Jones (1990, 1994, 1997b) and Bryan and Jones (1997). 3.3 Conclusions on the significance of pipeflow contributions 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%. 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, b, c). 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 basin to basin (cp. Jones, 1997c). Nevertheless, the fact that nearly 30% of the land area of Britain is susceptible to piping (Jones et al., 1997) does suggest that current physically-based models of basin hydrology that are used in upland catchments and do not attempt to model pipeflow are ignoring a potentially important process. 4. Effects of pipeflow on streamwater quality At the last Celtic Hydrology conference, Colin Neal gave us an excellent analysis of the inscrutability of hydrochemistry in the CEH Plynlimon catchments (Neal, 2000). Among the major conclusions of his review, he 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

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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. Studies of the hydrochemistry of pipeflow, streamflow, rainfall and soil moisture in the Maesnant basin largely corroborate Neal’s conclusions (e.g. Jones and Hyett, 1987; Hyett, 1990; Richardson, 1992; Jones, 1997b). Water quality is highly variable, but it is possible to make a number of generalisations. 4.1. 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 allow rapid infiltration of rainwater into the pipe network. Although recent modelling simulations suggest that the 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.

Yields at the outfalls of the ephemeral pipes are also more acid than the discharges at the outfalls of the perennial pipes (Table 1), 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 (Table1). This groundwater is the source of most baseflow in the pipes

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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 (unpub.) 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 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 neutralising capacity, which is due to the 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% 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. 4.2. Aluminium concentrations The pipes are also a significant source of aluminium in the streamwater. Table 1 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 2 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 (Figure 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

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following a larger storm can result in a disproportionately high response once the hillslope system is thoroughly wetted. 4.3. 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 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% of streamflow. 4.4 The old or new water controversy 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 Upper Wye are draining predominantly ‘old’ water from previous storms. The fact that the Maesnant pipes respond to short-term, within storm variations in rainfall quality and that a significant proportion of acidity is derived from the peaty surfaces and the deeper peat layers suggests otherwise: they suggest short residence times and limited contact with the mineral layers (Jones, 1997b). Hyett (1990) concluded that the main controls on Maesnant pipewater quality are primarily the amount, intensity and quality of storm rainfall, plus antecedent soil moisture. Short residence times are also supported by the purely hydrological observations of (1) the large proportion (averaging 68%) 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%) from the pipes, (3) the close correlation between flow patterns and rainfall patterns shown throughout the ephemeral network (Figure 6) 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. 5. Effects of piping on the hillslope environment 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. The highest levels of electrical conductivity in topsoil extracts were found below the

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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). The pipes also drain and aerate sections of hillslope. This affects the plant communities and soil development, so that linear patterns are found in both following the lines of the pipe networks down the hillside, as illustrated in Figure 7 (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 through 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 7). 6. Implications for catchment management Sensitive management of piped catchments should recognise 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. 6.1. Recognising 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 neutralise runoff (cf. section 6.2). It could also help in planning afforestation (cf. section 6.3).

The Maesnant pipes carry the contributing area well beyond what would be recognised by topographic indices such as a/s (area drained per unit contour length divided by slope angle) used in TOPMODEL (Jones, 1986). 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.

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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% of hillslopes in 1 in every 2 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. 6.2. Controlling acidification and stream aluminium levels The pipes are a significant source of acid waters and aluminium for the stream, especially during stormflow. Gee (pers.comm., 2002) has admitted that one of failings in the Welsh Acid Waters Programme was lack of understanding of the role of pipes and identifying the sources of streamflow solely on the basis of the a/s index in TOPMODEL (cp. Edwards et al., 1990). Jones (1986; 1997a) found only limited correlation between pipe location and discharge and a/s.

With this in mind, liming of pipeflow source areas, as identified by Jones and Connelly (2002), should be more cost-efficient than liming whole catchments and more effective than simply relying on ‘traditional’ 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 the hillside or into perennial pipes. 6.3. Implications for forestry, biodiversity and moorland management As illustrated in section 5, 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, 1997). 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, which is a possible solution for the ‘dirty’ water problem as well. 7. Conclusions

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Pipeflow can be a substantial contributor to catchment quickflow in upland moorland basins in Britain, as demonstrated in the results of monitoring by a number of research teams. On Maesnant, pipes are equally important as sources of baseflow. Pipes increase acidification of surface waters, especially where ephemeral pipes issue directly into stream courses. Yet they also encourage diversity in plant communities and soils within moorland habitats.

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

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15

Table 1 Mean water quality parameters for rainfall, streamflow and pipeflow sites on Maesnant

Rainfall

Stream

Perennial pipe outfalls

Heads of perennial

pipes

Ephemeral

pipe outfalls

Pipe mean (n=63)

pipe 2

pipe 3

pipe 4

pipe 5

pipe 9

pipe 13

pipe 14

pH

4.84

5.16

4.90

4.48

4.52

5.20

5.50

4.26

4.10

4.58

conductivity μS cm-1 at 20oC

39.5 34.1 35.8 41.9 40.0 39.9 33.4 57.4 47.0 41.5

dissolved aluminium mg l-1

- 0.211 0.162 0.238 0.295 0.104 0.208 0.524 0.37 0.271

dissolved organic carbon mg l-1

- 2.69

4.11 4.95 3.19 1.60 1.40 2.20 15.6 3.81

Pipe reference numbers as in Figure 2.

16

Table 2 Contemporaneous water quality samples from 6 drainage flowpaths, Maesnant, during storm runoff Determinand

Stream

2

Pipes 3

4

Matrix

Overland flow

pH

5.5

5.3

4.7

4.7

4.3

4.3

Conductivity μS cm-1 20C

32.0 32.0 34.0 38.0 39.0 36.0

Total hardness mg l-1 CaCO3

4.7 4.4 3.7 4.9 2.4 2.0

Chloride mg l-1

5.0 6.0 6.0 7.0 6.0 5.0

Dissolved silicate mg l-1

1.5 1.8 1.9 1.3 0.3 0.3

Dissolved potassium mg l-1

0.18 0.19 0.16 0.19 0.83 0.35

Dissolved aluminium mg l-1

0.077 0.118 0.167 0.257 0.109 0.122

Dissolved organic carbon mg l-1

1.7 3.6 3.3 5.4 10.6 8.8

17

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

18

Figure 2. The main area of piping in the Maesnant catchment, showing gauging and sampling sites.

19

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

20

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

Figure 5. Rainfall, pipeflow and pipewater quality in a sequence of storms. Note the rise in aluminium levels during storms and the larger response to the second storm when the hillslope system is wetter.

21

Figure 6. Close correlation between rainfall and pipeflow response in two ephemeral pipes (pipes 14 and 15, Figure 2).

22

Figure 7. The pattern of major plant associations around piping on Maesnant.

23