Journal of Hydrology 393 (2010) 349–361

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Journal of Hydrology

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Variations in saturated and unsaturated water movement throughan upland floodplain wetland, mid-Wales, UK

Chris Bradley a,⇑, Andrew Clay a,1, Nicholas J. Clifford b, John Gerrard a, Angela M. Gurnell c

a School of Geography, Earth and Environmental Sciences, The University of Birmingham, Birmingham, B15 2TT, United Kingdomb School of Geography, University of Nottingham, Nottingham, NG7 2RD, United Kingdomc Dept. of Geography, Queen Mary, University of London, London, E1 4NS, United Kingdom

a r t i c l e i n f o s u m m a r y

Article history:Received 21 July 2009Received in revised form 2 August 2010Accepted 30 August 2010

This manuscript was handled by PhilippeBaveye, Editor-in-Chief

Keywords:Headwater wetlandTensiometerSoil–water Pressure

0022-1694/$ - see front matter � 2010 Elsevier B.V. Adoi:10.1016/j.jhydrol.2010.08.029

⇑ Corresponding author. Tel.: +44 121 414 8097; faE-mail address: C.Bradley@bham.ac.uk (C. Bradley

1 Currently at RPS Group, 2420 The Quadrant, Aztec WKingdom.

Variations in inferred water movement are examined during a two-year study of an upland floodplainwetland at Afon Llwyd, in mid-Wales, UK in 2001 and 2002. Soil–water pressures (w) were monitoredby six tensiometer nests, each comprising two sensors at depths of 30 and 60 cm below the surface.Detailed sedimentary sections were produced and changes in saturated and unsaturated hydraulic con-ductivity (and their relationship to w) were estimated. Seasonal trends in w were identified and verticalhydraulic gradients within different tensiometer nests were derived and interpreted in the context of thelocal channel and floodplain sedimentology, channel bedform morphology and hydro-climatic condi-tions. Although there is considerable variation in w, the results display characteristic trends at severallevels: (1) in relationship to precipitation; (2) reflecting changes in river stage (with downstream stageproviding base-level control through a riffle-pool couplet); and (3) marked spatial variations in the direc-tion and rate of water movement. These controls influence water redistribution through the wetland,which reflects the relative position of distinct sedimentary units within the floodplain. The results dem-onstrate that water movement is episodic and characterised by relatively rapid horizontal water fluxesduring and immediately following individual rain events, followed by residual seepage to the river andwater movement under a local soil moisture gradient. The implications of these results include elucidat-ing the role of upland floodplains in buffering sub-surface drainage, and attenuating patterns of river flowunder base flow conditions. This has the potential to advance the basis for future upland floodplain wet-land and channel restoration schemes.

� 2010 Elsevier B.V. All rights reserved.

1. Introduction of water movement and groundwater–river interaction (Winter,

Floodplain wetlands have an important function in regulatingcatchment-scale water, nutrient and sediment fluxes. Floodplainwetlands have the potential to attenuate peak river flows whilstduring low flows, seepage from wetlands sustains river base flow.Wetlands are also ecologically diverse, representing biogeochemi-cal hotspots (McClain et al., 2003) that are a potentially importantsource of organic carbon to fluvial systems (Bradley et al., 2007)which sequester carbon from the atmosphere whilst emittingmethane (Whiting and Chanton, 2001). As a result, there is growinginterest in conserving wetlands to protect and enhance their eco-system functions. This requires an understanding of wetlandhydrological dynamics (Acreman et al., 2007) but many wetlandsare characterised by marked heterogeneity and variable patterns

ll rights reserved.

x: +44 121 414 5528.).

est, Bristol, BS32 4AQ, United

2007) which contributes to significant spatial and temporal varia-tions in wetland water flux with implications for nutrient cycling(e.g. Carlyle and Hill, 2001; Clément et al., 2003).

Floodplain wetlands are characterised by a varying sedimentarycomposition of inorganic and organic sediments that differ mark-edly in their permeability and moisture characteristic (Andersen,2004; Bradley, 2002). Conceptual models have been derived whichemphasise the importance of river stage in determining floodplainhydrology (Jung et al., 2004), but there may be marked lateral (per-pendicular to the river) and longitudinal (along the valley axis) gra-dients in water movement reflecting floodplain geomorphology,and the composition and distribution of individual sedimentaryunits. Moreover, although the water table is often assumed to re-main close to the surface, unsaturated processes may be seasonallysignificant (Joris and Feyen, 2003; Bradley and Gilvear, 2000). Thishas implications for the effective permeability as the mode ofwater flow varies depending upon the level of saturation. Whenthe soil is close to saturation, significant preferential flow throughsoil macro-pores is likely (Bradley and van den Berg, 2005; Holdenand Burt, 2003), whilst water-flow pathways tend to be more tor-

350 C. Bradley et al. / Journal of Hydrology 393 (2010) 349–361

tuous at lower levels of saturation with progressive drainage fromthe largest soil pores. Permeability thus varies markedly with levelof saturation, and although characterised by hysteresis, it also re-flects pore interconnectivity and water–substrate relationships,the degree of hydro-phobicity and external controls on saturation(e.g. changes in river stage).

Most studies have focused on lowland floodplain wetlands(Bradley, 2002; Weng et al., 2003) although incipient, discontinu-ous floodplains extend into upland, headwater basins. At a catch-ment-scale, headwater floodplains may represent a significantproportion of the total drainage network. Benda et al., (2005) havesuggested that approximately 60–80% of the total drainage net-work length may lie within catchment headwaters. The floodplainwetlands therein are likely to possess distinctive characteristics incomparison with lowland wetlands downstream (the soligeneousheadwater wetlands studied inter alia by Hill (2000) and Roulet(1990)) and also upland blanket peatlands (Burt, 1995). Althoughlikely to be dominated by relatively coarse deposits, there may stillbe marked sedimentary contrasts within upland floodplains,reflecting alluvial and colluvial sedimentation and organic matteraccumulation. As a result, there is considerable potential for rapidsub-surface water fluxes, reflecting the interaction between pre-cipitation, lateral (valley-side) sub-surface drainage, and valley-bottom groundwater discharge. Flow pathways may also reflectthe presence of highly permeable alluvial sands and gravels inthe valley-bottom that enable preferential flows along the axis ofthe valley.

Fig. 1. Location of the study site on the floodplain of the Afon Llwyd, mid-Wales show(AB, CD and EF).

This paper investigates trends in water flow through an uplandfloodplain wetland and aims to resolve the different components ofsub-surface flow by identifying changes in the direction and mag-nitude of water flow within the wetland. The objectives are to:

� describe the spatial and temporal variability in soil–water pres-sures (w) across the riparian margin of the floodplain and theirrelationship to local sedimentology, channel morphology, pre-cipitation and river levels;� determine trends in the vertical hydraulic gradient (HG),

between individual monitoring points; and� estimate the rate and direction of vertical saturated and unsat-

urated water movement.

2. Methods

2.1. Study area and site

The Afon Llwyd is a small upland headwater tributary of theAfon Clywedog, mid-Wales, UK (UK National Grid Reference:2874 2906; Fig. 1). Land use includes pasture and moorland grazedby livestock with a substantial proportion of the southern part ofthe catchment (45%) currently covered by a coniferous forestryplantation. The catchment is underlain by Ordovician and Siluriangrits and shales with soils that include peats, podzols, stagnopodz-ols and gleys (Neal et al., 1997a). Elevations vary from 620 m asl inthe southwest to 500 m asl to the north, with perennial streams

ing instrumentation across the site and the location of sedimentary cross-sections

C. Bradley et al. / Journal of Hydrology 393 (2010) 349–361 351

that extend headwards close to the catchment divide. Annual pre-cipitation of �2500 mm is evenly distributed through the yearwith a mean annual temperature of 7.3 �C.

Previous work in this and adjacent catchments has investigatedthe importance of water-flow pathways for stream acidification,highlighting the predominance of pre-event water in storm hydro-graphs. In the nearby Hafren catchment Haria and Shand (2004,2006) described the contribution of groundwater flow throughthe underlying bedrock to total streamflow. Groundwater andsoil–waters were found to be closely coupled in the vicinity ofstream channels with evidence of groundwater discharge withinthe valley-bottom. It was suggested that prolonged and intenseweathering had led to the development of a fractured flow systemcharacterised by hydrologically-active flow processes at shallowdepths (<1.0 m) that are comparable to those more normally ob-served in permeable catchments (Shand et al., 2005). Although thiswork focussed on an instrumented transect that extended up thevalley-side at a point with no floodplain, monitoring of soil–waterpressures indicated vertically downward water movement throughthe unsaturated zone at most sites, suggesting that water move-ment through deeper soils is controlled by rapid pressure wave re-sponses. For example, a deep valley bottom borehole was found tobe continually artesian, with shallow boreholes becoming artesianduring and/or after heavy precipitation (Shand et al., 2005).

These studies did not extend into the riparian zone per se, andthe significance of water flows through the floodplain needs tobe established given their potential importance with respect toboth the quantity and quality of upland river-waters. Where afloodplain has developed, it may be important in controlling lateralwater movement from the hill-slope to the river and also longitu-dinally along the axis of the valley (Clay, 2006). The latter may beparticularly important where the floodplain comprises contiguoussand and gravels deposits that effectively constitute an alluvialaquifer facilitating down-valley water flow. This provides anopportunity for topographically-induced flow exchange with theriver, potentially enabling seepage through the river-bed to bypasselements of the channel downstream and at the channel margins(Peterson and Sickbert, 2006; Wroblicky et al., 1998), but also toa certain extent, buffering the river from rapid sub-surface waterpulses from the valley-side. Flow exchange may also be enhancedin upland areas where past and current catchment managementinfluences sediment delivery to river systems, and hence riverbed sedimentology and structure, with implications for the magni-tude of groundwater–surface-water exchange.

As part of a wider investigation of the importance of riffle-poolsequences on riparian hydrology (Emery, 2003), a section of chan-nel and adjacent floodplain, approximately 5 km from the source ofthe Afon Llwyd was selected for study (Fig. 1). At this point(3�400W, 52�300N; 290 m asl.) the Afon Llwyd is a 3rd order stream,with an upstream catchment of �7.5 km2. Mean annual flow is�0.5 m3 s�1, but the regime is flashy with peak flows >9 m3 s�1,and consequently the cobble and pebble substrate (riffle D50:�5.45U; D95: �6.9U) is reworked continuously and the large un-vegetated point and lateral bars are regularly submerged. At thestudy site (Fig. 1), the channel of the Afon Llwyd has an averagebankfull width of 5.6 m, and contains two pool–riffle couplets (rel-ative relief >0.6 m). The channel lies on the northern margin of a�150 m wide floodplain dominated by purple moor grass (Moliniacaerulea) and soft rush (Juncus effusus).

2.2. Data collection

Instrumentation, both in-stream and across the floodplain, wasinstalled along an 80 m length of channel (Fig. 1) that included tworiffle–pool couplets. Six pressure transducers (PTs; Campbell Scien-tific: PDCR1830) were installed in plastic stilling wells 10 cm above

the river bed, to minimise sedimentation and determine in-chan-nel water levels and their relationship to bedforms along the studyreach. From upstream to downstream: PT1 was situated in a pool;PT2 was located immediately upstream of a riffle crest; PT3 waslocated towards the base of the downstream face of the same riffle;and PT4 in a pool. Two additional pressure transducers, PT5 andPT6, were positioned further downstream to measure watergradients across the channel bend and identify the potential forsub-surface flow through the floodplain (cf. Peterson and Sickbert,2006).

Twelve tensiometers (Eijkelkamp T4) were installed in six nestsalong three transects across the floodplain in June 2001. At eachnest shallow (S) and deep (D) tensiometers were installed todepths of 30 and 60 cm below the surface using a hand auger.The tensiometers consisted of a 6 cm long ceramic porous cup(OD 25 mm) filled with de-aerated water and with an integralpressure transducer that measures positive and negative soil–water pressures (w) relative to atmospheric pressure that was ob-served through a porous diaphragm in the sensor cable. Contactbetween the porous cups and the soil matrix was maximised dur-ing installation by imbedding the tensiometer cup within a soilslurry which was added immediately after inserting the ceramiccup. Bentonite pellets were packed around the tensiometer to pre-vent bypass flow along the shaft. The tensiometers were calibratedagainst a constant 10.6 V power supply provided via a voltage reg-ulator and 12 V battery (approximate accuracy: �100 cm water =�100 ± 3 mV). Pressure transducers and tensiometers werescanned at 10 s intervals by a data logger and 15 min mean valueswere derived. Shallow tensiometers were removed from the fieldin December 2001 to prevent damage to the porous cups and pres-sure transducers from sub-zero temperatures and were recalibrat-ed before re-installation in June 2002.

Instrumentation locations were designed to provide a well-con-strained vertical and lateral grid within which soil–water pressurescould be determined precisely and related to a common datum toidentify trends in water movement in three dimensions: laterally,longitudinally (i.e. along the valley axis) and vertically. Sensor ele-vations and topographic surfaces, including the channel bed, banksand floodplain, were determined by field survey with respect to alocal datum which indicated high in-channel gradients(0.0061 m/m along the channel axis). Precipitation was measuredby a tipping bucket rain gauge at Dolydd, 250 m to the southwestof the study site.

Floodplain sedimentology was determined by hand augering at72 points at 2 m intervals along transects perpendicular to the riv-er. Sub-samples from each core were taken for detailed analysis ofcolour, organic matter content, and particle size. Soil samples wereclassified on the basis of their sand, silt and clay compositionwhich was determined by laser diffraction (Malvern MasterSizerHydro 2000MU). Fig. 2 shows three sedimentary cross-sectionsacross the floodplain and river channel at points close to the posi-tion of individual tensiometers (transect locations shown in Fig. 1),indicating the relationship of the tensiometer nests to individualsedimentary units. The sediment characteristics adjacent to eachof the 12 tensiometers are summarised in Table 1 indicating thatsilt and fine-sand fractions dominate. All of the transects overliegravel deposits of unknown depth, although in their work in theadjacent catchment, Shand et al., (2005) found gravels extendedto 7 m below the surface in two valley bottom boreholes close toa stream confluence. The upstream transect (AB) is mainly com-posed of a sandy silt loam which overlies a slightly undulatinggravel surface that is marginally lower than the local river bed ele-vation. Coarser sediment (sandy loam) is found close to the riverchannel, with an area of finer sediments (silt loam) situated 9 malong the transect from the channel. The middle transect (CD)overlies a gravel surface with greater relief that varies by >1 m ver-

Fig. 2. Sedimentary profiles along cross-sections: AB, CD and EF, perpendicular to the river at locations indicated in Fig. 1 and showing the position of individual tensiometers.

352 C. Bradley et al. / Journal of Hydrology 393 (2010) 349–361

tically most probably reflecting the location of a palaeochannel(e.g. between 3 and 4 m along the transect in Fig. 2). The alluvialdeposits comprise laterally contiguous layers of sandy silt loamand silt loam while the river bank is composed of coarser, loamysand. These coarse deposits also in-fill a depression in the gravelsurface 3.5 m along the transect. The downstream transect (EF)has a relatively horizontal gravel surface, over which there is acoarsening upwards sequence: a silty loam which grades to a san-dy silt loam towards the surface.

Soil organic content, determined by loss on ignition, varied be-tween 6 and 24%. Organic content was low across transects AB andEF, but increased to �20% away from the river. Highest organic

contents were observed along transect CD, in a depression imme-diately above the gravel surface.

2.3. Data analysis

Output from tensiometers and in-channel pressure transducerswere downloaded at regular intervals, checked for consistency andoccasional spurious values removed. Hourly and daily mean w val-ues were determined, and hydraulic heads calculated with respectto a local datum (the minimum downstream river bed elevationmeasured during the field survey). Seasonal trends in w were iden-tified by summary statistics (mean, max, min, st. dev) for all tensi-

Table 1Sedimentary characteristics at the locations of individual tensiometers, indicated in Fig 1.

Colour Water content (%) Organic matter (%) Clay <2 mm Silt 2–60 mm Fine sand60–200 mm

Medium sand200–600 mm

Coarse sand600–2000 mm

1S 10YR3/3 49.8 14.4 3.6 70.0 20.8 5.6 0.11D 2.5Y 3/2 45.1 9.8 2.8 74.6 15.8 6.8 0.02S 10YR 3/3 49.7 13.8 2.5 58.1 24.9 13.7 0.82D 2.5Y 3/2 47.6 12.1 2.6 55.2 27.7 14.3 0.23S 10YR 3/3 47.7 11.2 4.6 68.8 21.2 5.4 0.03D 2.5Y 3/2 27.7 6.8 3.5 50.4 16.2 24.0 5.94S 10YR 3/3 51.4 12.6 4.1 76.6 17.4 2.0 0.04D 10YR 4/2 50.1 14.3 3.9 72.9 18.2 5.0 0.05S 10YR 4/3 30.9 10.7 2.7 57.0 27.4 12.6 0.35D 10YR 4/2 32.2 10.0 3.2 58.8 22.4 15.1 0.66S 2.5Y 4/2 54.2 14.0 4.4 71.8 16.9 5.3 1.66D 10YR 3/2 59.8 23.8 4.7 84.7 9.8 0.8 0.0

Table 2Parameters used to estimate unsaturated hydraulic conductivity.

hr cm3/cm3 hs cm3/cm3 Log (a) log (1/cm) Log (n) log10 Log Ks log (cm/day) L f

1S Silty loam 0.065 0.439 �2.296 0.221 1.261 0.365 1.31D Silty loam 0.065 0.439 �2.296 0.221 1.261 0.365 1.32S Loam 0.061 0.399 �1.954 0.168 1.081 �0.371 1.32D Loam 0.061 0.399 �1.954 0.168 1.081 �0.371 1.33S Sand 0.053 0.375 �1.453 0.502 2.808 �0.930 1.33D Sandy loam 0.039 0.387 �1.574 0.161 1.583 �0.861 1.34S Silty loam 0.065 0.439 �2.296 0.221 1.261 0.365 1.34D Silty loam 0.065 0.439 �2.296 0.221 1.261 0.365 1.35S Loam 0.061 0.399 �1.954 0.168 1.081 �0.371 1.35D Loam 0.061 0.399 �1.954 0.168 1.081 �0.371 1.36S Silty loam 0.065 0.439 �2.296 0.221 1.261 0.365 1.36D Silt 0.050 0.489 �2.182 0.225 1.641 0.624 1.3

Fig. 3. Estimated unsaturated hydraulic conductivities for the sediment typesidentified in Fig. 2 and their relationship to suction head (the �ve of pressure head).

C. Bradley et al. / Journal of Hydrology 393 (2010) 349–361 353

ometers for a selection of representative months across the periodof study.

Vertical hydraulic gradients between shallow and deep tensi-ometers were calculated to indicate variations in the directionand rate of water movement. Estimating actual vertical waterfluxes is more problematic since hydraulic conductivity variesmarkedly according to volumetric water content, falling as waterdrains progressively from the pore space with falling water table,but complicated by hysteresis and the potential for preferentialflow through soil macro-pores. Accordingly, unsaturated soilhydraulic properties were estimated using the method describedby Børgesen et al., (2006) who devised a scaling function (pm) torepresent the rapid increase in hydraulic conductivity near satura-tion, as a function of saturated hydraulic conductivity (Ks; cm/day),the level of saturation (Se) and the moisture characteristic:

KðSeÞ ¼ KsSLe 1� ½1� Sn=ðn�1Þ

e �n 1�1=n

�2

pm ð1Þ

where L is a dimensionless parameter describing pore tortuosityand connectivity, and n is a curve-shape parameter;

Se is defined by:

Se ¼hðwÞ � hr

hs � hr¼ ½1þ ðawÞn�1=n�1 ð2Þ

where h(w) is the water retention curve describing water content(h) as a function of w, hr and hs are residual and saturated water con-tents respectively (cm3/cm3), and a (dimensionless) is a curve-shape parameter; while pm is given by:

pm ¼ð 1jhjvþ1 Þ

f; h > hm

ð 1jhm jvþ1 Þ

f;h 6 hm

8<: ð3Þ

where v is unity, but with dimensions cm�1 so that pm is dimen-sionless, and f (dimensionless) is related to the moisturecharacteristic.

Børgesen et al., (2006) developed this scaling method using datafrom 32 Danish soil profiles where the organic matter content islikely to be low, and consequently the technique should be usedwith caution in organic soils. However, the mean organic mattercontent for the samples at Afon Llwyd listed in Table 1 was12.8%, suggesting this site can be considered an inorganic wetland,and the scaling function was applied using the unsaturatedhydraulic parameters in Table 2. These are approximate values

354 C. Bradley et al. / Journal of Hydrology 393 (2010) 349–361

derived from Schaap et al. (2001) for soil types equivalent to thosefound in the study at Afon Llwyd. Estimated changes in hydraulicconductivity according to the level of saturation for the actual sed-iment types found on the study transects are shown in Fig. 3.Hydraulic conductivity (K) varies over two orders of magnitudefor all soil types, increasing steadily as the suction head falls untilclose to saturation, at �5 cm suction, when K increases rapidly asthe soil macro-pores become water-filled and account for anincreasing proportion of total flow until K becomes equal to Ksat

at a suction of 0 cm. The combined effects of the soil moisture char-

Fig. 4. Seasonal trends in precipitation (i); pressure head (w) at tensiometer nests: 1S andstage at PT6 plotted with respect to a common datum (iv) from 1st July 2001 to 31st D

acteristic, soil structure and texture, on K is evident as the perme-ability of the most conductive soil near saturation (sand: �630 cm/day at saturation) falls rapidly as the pore spaces drains, so thatfrom a suction head of 50 cm onwards, silt becomes the most con-ductive soil. Evidently, given the rapid decline in K as the watercontent falls, rates of water movement can be expected to be mostrapid at or near saturation, although this will depend also upon thelocal hydraulic head gradient.

Accordingly, vertical water fluxes at selected tensiometer nestswere estimated for short periods of time, using mean K derived

1D (ii) and 3S and 3D (iii); hydraulic head (h) at 3S and 3D (w + elevation) and riverecember 2002. Annotations are described in the text.

C. Bradley et al. / Journal of Hydrology 393 (2010) 349–361 355

from individual K(Se) determined at 5 cm intervals between theshallow and deep tensiometer cups (situated at depths of 30 and60 cm below the surface), assuming that w varied linearly betweenthe two points. K was estimated hourly as the mean of the individ-ual K(Se) for each 5 cm increment and water flux (q) was estimatedusing the Darcy–Buckingham Law (Narasimhan, 2004).

3. Results

3.1. Seasonal trends in pressure head

Seasonal variations in precipitation, pressure head (w), hydrau-lic head (h) and river stage from 1st July 2001 to 31st December2002 are given in Fig. 4. Over this period precipitation totalled3186 mm, equivalent to a mean monthly precipitation of177 mm. Precipitation exceeded 50 mm on 5 days, with periodsof concentrated rainfall from 15th January to 26th February 2002(740 mm) and from 10th October to 4th December 2002(598 mm) and relatively little precipitation between 21st Marchand 16th April 2002 (18 mm) and from 11th July to 4th September2002 (76 mm). Pressure heads at both tensiometer nests (Fig. 4 iiand iii) are largely negative and slightly lower near the surface(mean w at 1D: �18.62 cm; st. dev: 18.04 cm; cf. mean w at 1S:�28 cm; st. dev. 25.15 cm). Typically, tensiometers respond rapidlyduring individual precipitation events with conditions oscillatingaround saturation, but with w lower (more �ve) at nest 3 com-pared with nest 1 (mean w at 1D: �53.17 cm; st. dev: 25.15; meanw at 1S: �59.93 cm; st. dev. 22.31 cm). Initially, from July 2001, wthe behaviour of 1S is distinctly different: remaining above 0 cm(saturated) and failing to respond to precipitation (period ‘a’ inFig. 4ii.) until a moderate rainfall event of 26 mm on 12th Septem-ber probably reflecting local soil compaction when the tensiometerwas installed. In contrast, at nest 3, there are periods in late Octo-ber and November 2001 and 2002 when w at the shallow and deep

Fig. 5. Seasonal variations in pressure head: plots give range (max: min), standard dev(monthly precipitation totals were 175 mm, 252 mm, 490 mm and 145 mm respectively

tensiometers are almost identical. This indicates consistent mois-ture contents with depth when precipitation coincides with lowevapotranspiration.

Hydraulic heads (h) of 3S and 3D are plotted in Fig. 4iv, togetherwith river stage at PT6, against the local datum: the minimum riverbed elevation in the reach downstream. Comparison of h at 3S and3D indicates the direction of water movement (water moving fromhigh to low h). Typically the direction of water movement is verti-cally downwards during summer rain events (i.e. July and August2002 when h at 3S is greater than at 3D), but becomes verticallyupwards at certain times (e.g. September 2001 and 2002) due toevapotranspiration. Both river levels and h respond rapidly to localrainfall: peak river level coincide with most rainfall events moni-tored over the period, with the exception of a succession of smallrainfall events in early July 2002 when fluctuations of h at 3S werematched at neither 3D nor PT6 (‘b’ in Fig. 4iii and iv). During thistime, w at 3S and 3D remained <0, and precipitation appears onlyto replenish soil–water storage, indicating that summer daily rain-fall totalling less than 15 mm produces little change in river flows.River levels are consistently higher than h at 3S and 3D, indicatingthat the direction of water movement at this point is from the riverto the floodplain (i.e. influent seepage). Seasonal trends in w aresummarised in Fig. 5 which gives comparative statistics for eachtensiometer for four periods: August 2001, October 2001, February2002 and April 2002. In August 2001, most tensiometers havemean w below 0 cm, but with maximum w > 0. Smallest variationsin w were at 1S and 2D, whilst the range in w, and standard devi-ation, were greatest at 6D in the downstream tensiometer nest.Generally, the range in w was lower in autumn than summer, withmean pressure heads close to saturation, with the exception of 2Dwhere maximum w exceeded 200 cm. This appears to be a true re-sponse to one precipitation event (8th October 2001; precip.>51 mm). Although less data are available over the winter, thereis a considerable range in w between individual tensiometers withw at 3D remaining <0 and w at 2D remaining consistently higher

iation and mean w for August 2001, October 2001, February 2002 and April 2002).

356 C. Bradley et al. / Journal of Hydrology 393 (2010) 349–361

than the other tensiometers in winter and spring. Patterns of satu-ration thus vary markedly across the site, with most consistentconditions found in summer. In winter and spring, the responseof individual tensiometers to precipitation varies, possibly reflect-ing local variations in the pattern of water movement as a result ofthe proximity of the undulating gravel surface.

Differences in the moisture regime between tensiometers areshown in Fig. 6 by frequency duration plots giving the time a par-ticular soil–water pressure head is exceeded. The distributioncurves should reflect the local sedimentology and soil moisturecharacteristic to a certain extent, but potentially could also suggestpoints where there might be other influences on the hydrology.Thus, at 3S and 3D where the surrounding matrix is sandy and san-dy loam respectively, there is an approximately linear durationcurve over a relatively large range in w, whilst the range in w is sig-nificantly less where there are loamy deposits as at 2S and 2D. Inthese cases, and at tensiometer nests 5 and 6, the moisture regimeapproximates an inverse ‘S’. Generally the moisture regime is sim-ilar at both the shallow and deep tensiometers for each nest, withthe exception of nest 1 where there appears to be some de-cou-pling between the shallow and deep tensiometers, possibly indi-cating an external control on w at this point (which is �1.5 mfrom the river bank and situated immediately above the gravellayer).

Fig. 6. Frequency duration curves showing the percentage of time

3.2. Vertical hydraulic gradients and water flux estimates

The hydraulic gradient (HG) between deep and shallow tensi-ometers in each nest indicates the direction of water movement,and vertical water fluxes can be estimated where the permeabilityis known. Hydraulic gradients were calculated for all tensiometernests, and a sub-set are described here to highlight different trendsacross the floodplain, and indicate how the gradients change dur-ing individual hydrological events. Hydraulic gradients for two30-day periods are given in Fig. 7i) for tensiometer nests 2 and 3(15th July–14th August 2001) and ii) for tensiometer nests 4 and6 (5th July to 4th August 2002). The former period includes twodays of substantial precipitation, which totalled >100 mm overthe period, whilst the second period is of low rainfall (i.e. 25 mmbetween 12th July and 4th August).

The direction of water movement vertically is highly variableacross the site. During heavy precipitation (Fig. 7i) water continuesto move vertically upwards at tensiometer nest 2, suggesting thatthis point is consistently characterised by groundwater discharge,and that groundwater continuing to upwell during precipitationevents (as on 11th August 2001) albeit to a smaller extent. Duringperiods of low precipitation (i.e. from 23rd July to 1st August) thehydraulic gradient becomes increasingly negative at this tensiom-eter next, with a clear diurnal trend as the hydraulic gradient in-

a given pressure head (w) is exceeded for each tensiometer.

C. Bradley et al. / Journal of Hydrology 393 (2010) 349–361 357

creases during the day in response to evapotranspiration. In con-trast, at tensiometer nest 3, the HG fluctuates around 0 for mostof the 30-day period, but becomes slightly positive during rainfallevents, reflecting infiltration. Again, there is a diurnal trend in HGof ±10 cm in w at 3S as a result of evapotranspiration; however, thevertical HG remains relatively small here, reflecting the high localpermeability due to the sand and sandy-loam substrate at 3S and3D.

During drier conditions from 5th July to 4th August 2002, (Fig7ii.) water movement at nest 4 oscillates close to, but slightlyabove 0, indicating slight downward water movement. At this ten-siometer nest, diurnal trends in the HG are not as pronounced as atnest 3, although the gradient increases significantly following twodays of rainfall (22 and 23rd July) as a result of a rapid change in wat 4S. For the remainder of the period w at 4S tends to track 4D. Incontrast, at nest 6, there are rapid fluctuations in HG with consid-erable differences in w between the shallow and deep tensiometernests. From the 5th to the 13th July, 6D displays short-term in-creases in w (more +ve) during individual rainfall events whichare not matched at 6S and are thus associated with a reversal inthe direction of water movement (to vertically upwards) suggest-ing upwelling groundwater in down-valley areas of the wetland.Vertical hydraulic gradients between 6D and 6S are less markedfor the remainder of the period, with drainage mainly verticallydownwards, but becoming slightly negative towards the 20–21stJuly due to evapotranspiration and seven successive days withoutprecipitation. Subsequently, rainfall on the 22nd and 23rd Julyleads to a marked reduction in w at 6S and to a strong positivehydraulic gradient.

The vertical hydraulic gradients, summarised in Fig. 7 revealsignificant variations in the direction of water movement over

Fig. 7. Trends in w and vertical hydraulic gradients at selected tensiometer nests duringprecipitation (5th July to 5th August 2002). A positive hydraulic gradient indicates thattensiometer).

relatively short time periods. Rates of water movement are likelyto exhibit similar variability, with fluxes that are proportional toK(Se) and hence reflecting changes in moisture content and thewater-filled porosity. Fig. 8 shows estimated changed in K for thesame time periods and tensiometer nests at Fig. 7, together withcalculated water fluxes (+ve equals vertically downwards). Alsoshown is ‘particle movement’ that describes the movement of ahypothetical water particle from an initial height of 0 cm overthe 15-day period as a result of the imposed water flux. K variessignificantly over three of the four periods simulated, but at notime does K remain consistently high, as periods of saturation areof relatively short duration.

At nest 2 in July 2001 (Fig. 8i), K generally lies between 1 and10 cm/day with peaks that correspond with precipitation (shownin Fig. 7). Estimated water fluxes show considerable short-termvariability, fluctuating by ±2 cm over a 24 h period. However,fluxes remain consistently negative indicating upward watermovement even during a period of high precipitation. This equatesto a particle movement of 150 cm vertically in the month from the15th July. However, a different trend is seen over the same periodat nest 3 (Fig. 8ii), where K increases rapidly during precipitationevents to 100 cm/day, but then falls rapidly with the drainage ofwater-filled pores. The estimated water flux suggests that at thispoint water movement is negligible, oscillating around 0, withslight positive peaks during rainfall as a result of infiltration. Thesepeaks are of limited duration, and hence the particle movementindicates that at this point water moved <5 cm in total over themonth from 15th July.

Similarly at nest 4 in July 2002 (Fig. 8iii), water fluxes and par-ticle movement are very low during a period of low precipitation.Evapotranspiration seems to have no discernible effect on water

two 30-day periods of (i) high precipitation (15th July–15th August 2001); (ii) lowthe direction of water movement is downwards (i.e. from the shallow to the deep

Fig. 8. Estimated mean hydraulic conductivity and water fluxes for the periods summarised in Fig. 7: 15th July–15th August 2001 (i. 2D and 2S; ii. 3D and 3S) and 5th July–5th August 2002 (iii. 4S and 4D; iv. 6S and 6D) and the particle movement that this would produce at a depth of between 30 and 60 cm below the surface.

Table 3Soil hydraulic properties used to characterise unsaturated flow.

Soil type Depth (cm) hr (cm3/cm3) hs (cm3/cm3) a (cm�1) n (–) Ks (cm/day) Ss (cm�1)

Nest 2Sandy silt loam 0–90 0.015 0.45 0.011 2.2 30 4 � 10�7

Nest 3Loamy sand 0–15 0.035 0.437 0.008 1.8 146 4 � 10�7

Sandy loam 15–70 0.041 0.453 0.118 2.7 62 1 � 10�7

Sandy silt loam 70–100 0.015 0.45 0.011 2.2 30 4 � 10�7

358 C. Bradley et al. / Journal of Hydrology 393 (2010) 349–361

movement and K remains close to 1 cm/day throughout. As a re-sult, during this period there is no apparent vertical water move-ment at nest 6 (Fig. 8iv), water fluxes are initially highly variableand generally positive, indicating downward water movement. Kis initially relatively high (between 10 and 60 cm/day), but fallsrapidly to <1 cm/day on the 14th July as water content falls inthe absence of precipitation. K occasionally rises to 10 cm/day asa result of rainfall, but water fluxes are low. Consequently, themajority of water movement occurred during periods when Kwas high, from the 5th to the 12th July, giving a total water move-ment of �50 cm in the month from 5th July.

3.3. Vertical moisture gradients during infiltration

The results described above indicate that water movement isepisodic and concentrated during periods when the level of satura-tion is relatively high. However, these conclusions are based uponinterpolations from point measurements of soil–water pressure atdepths of 30 and 60 cm below the surface. To confirm these obser-vations, unsaturated water contents were modelled using UNSAT1,a one-dimensional Hermitian finite-element model (van Genuch-ten, 1978). The model is governed by the Richards equation, withthe soil hydraulic properties expressed in terms of the residualand saturated moisture contents (hr and hs), the saturated hydrau-lic conductivity (Ks), the specific storage coefficient (Ss) and empir-ical coefficients describing the shape of the moisture characteristic(a and n; Bradley and Gilvear, 2000). The distribution of soil types

and hydraulic parameters used to describe each layer are summa-rised in Table 3. Moisture contents were simulated for 14 daysfrom 15 July 2001 (the period described in Figs. 7i and 8ii) fortwo tensiometer nests: 2 and 3, as water infiltrates into the soilprofile following 32 mm of precipitation on the 18th July and issubsequently redistributed with daily evapotranspiration of4 mm/day, and drainage of 4.5 mm/day.

Simulated moisture profiles over this period are given in Fig. 9iand ii for nest 2 and 3. Nest 2 has a uniform soil profile, comprising90 cm of a sandy silt loam, whilst nest 3 consists of a loamy sand,above a sandy loam overlying a sandy silt loam (Table 3). The mois-ture profiles differ significantly between the two nests, with a rel-atively consistent trend in moisture content with depth at nest 2,and with the soil remaining unsaturated to a depth of 40 cmthroughout the period. In contrast, moisture contents in nest 3 dis-play a more varied pattern reflecting greater saturated hydraulicconductivities near the surface (i.e. 146 cm/day for the loamysand), and differences in the moisture characteristic and saturatedmoisture content. These results emphasise the importance of de-tailed determinations of wetland soil profiles (Fig. 2), and whenpoint-data from individual tensiometers need to be interpretedwithin the wider stratigraphic context.

4. Discussion

The floodplain and in-stream data collected during the monitor-ing programme on the Afon Llwyd, which included continuous and

Fig. 9. Modelled moisture profiles from the 15th to 29th July 2001 for: (i) nest 2; (ii) nest 3.

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intensive observations of soil–water pressure heads (w) in six ten-siometer nests, reveal significant variations in the rate and direc-tion of water movement within a small headwater floodplain.Tensiometers have previously been used in work in adjacent catch-ments in mid-Wales (e.g. Roberts and Crane, 1997), and in uplandwetlands elsewhere (e.g. in New Zealand by Bowden et al. (2001))where they have provided detailed information on the location andtiming of sub-surface flow, but tensiometers have not been widelyused in upland floodplains. However, the results at Afon Llwyddemonstrate that although climate is relatively cool and wet, withprecipitation well-distributed through the year, unsaturated zoneprocesses are important in determining the rate of water move-ment at certain times of the year. The trends in w shown inFig. 4 indicate clear seasonal trends in the level of saturation, withw typically lower in the deeper tensiometers 60 cm below the sur-face. Although much of the variation in w is related to the timingand magnitude of precipitation events, there are periods whenthe relationship between individual tensiometers were not as ex-pected. There are, for example, occasional periods of congruent re-sponse when w at depths of 30 and 60 cm were the same andvaried together for a short period (‘c’ in Fig. 4iii and iv). Whensoil–water pressures are corrected for elevation, h is higher at 3Sthan 3D at these times, indicating that the direction of watermovement is vertically downwards at this point, whilst peak h at3D is also identical to river stage. The results indicate that river lev-

els provide a significant local control in w at depth (discussed inmore detail below); whilst near the surface the shallow tensiome-ters reflect local precipitation.

The variations in soil moisture content observed across thefloodplain and through time, have implications for the location,frequency and intensity of redox exchange processes, and henceammonification, nitrification and denitrification. Although flood-plain water tables may regulate denitrification to a certain extent(e.g. Burt et al., 1999), actual water-flow pathways are likely tovary considerably across riparian areas. Although only a sub-setof the total data collected are described here, analysis of the com-plete tensiometry data-set indicate that the direction of watermovement across the floodplain at Afon Llwyd is highly variableas illustrated schematically in Fig. 10. The results indicate that atthree points (tensiometer nests 1, 2 and 6) the direction of watermovement is vertically upwards reflecting groundwater dischargein the valley bottom and/or the effects of local variations in thetopography of the underlying gravels. At tensiometer nests 1 and6, the direction of water movement is reversed during rain events,whilst at nest 2, water continued to flow to the surface. At nest 3,variations in w were closely related to changes in river levels, butwith evidence of infiltration during rain events. However, therewas very little water movement vertically at nest 4. While thefloodplain sedimentology (Fig. 2), and particularly the continuitybetween individual high permeability units, is important, broader

Fig. 10. Schematic diagram of the headwater wetland indicating observed trends in vertical water movement during the period of study.

360 C. Bradley et al. / Journal of Hydrology 393 (2010) 349–361

scale trends in water movement will reflect proximity to the river(particularly for tensiometer nests 1, 3 and 5 situated adjacent tothe channel), and the topography of the underlying sands and grav-els. The latter represents a highly permeable unit, orientated in adown-valley direction, but with significant variations in surfacetopography across the site as a result of changes in the locationof the channel across the floodplain (e.g. transect CD in Fig. 2where the elevation of the gravel surface varies by >1.0 m).Hydraulic heads in the gravel underlying the study site will reflectchanges in river stage in adjacent reaches, both upstream anddownstream, with potential interaction from the discharge of shal-low groundwater from the wider catchment reflecting watermovement through fractured bedrock (e.g. Shand et al., 2005).

Variations in river stage are likely to be important in a numberof respects, and the effects will be accentuated by the relativelycoarse valley bottom sands and gravels that extend beneath thewetland. River bed permeabilities are likely to be high, particularlygiven that the fine sediment composition of the river bed is limited(Emery, 2003). The in-stream survey also indicated markedchanges in topography, with a rapid reduction in river bed eleva-tions downstream, upon which are superimposed local changesin the river bed profile reflecting the distribution of pools and rif-fles along the reach. Although pool: riffle couplets can influencepatterns of hyporheic flow in upland channels (Harvey and Bencal-a, 1993; Kasahara and Wondzell, 2003), in mid-Wales the rifflesupstream appear to be important in determining the local mini-mum river stage that provides a base-level control on floodplainwater movement. Given the strong down-valley gradient(0.006 m/m), influent flow (from the channel to the wetland)may also occur occasionally along the upstream margins of thewetland during times of high river stage.

Generally the results emphasise the importance of the widergeomorphological context, within which the floodplain has devel-oped through the Holocene. Local-scale variation, particularlyrelating to the distribution of individual stratigraphic units, ap-pears to explain many of the trends in w across the different tensi-ometer nests, demonstrating the importance of understanding themanner in which the upland floodplain has evolved over time.While fluvial processes appear to be most important here, in someareas colluvial deposits will be locally important in contributing tothe heterogeneity of upland floodplains and potentially affecting

water movement from the valley-side. However, the results pre-sented here suggest that given the time-dependent nature of w,the relationship between w, river stage and hydraulic heads inthe alluvial aquifer could be quantified and related to floodplainstratigraphy, thereby enabling determination of lateral waterfluxes across the floodplain. Given the importance of episodicwater movement between points close to saturation, this wouldrequire the application of a saturated–unsaturated model, butwould confirm the role of upland riparian areas, and their impor-tance within the context of the wider catchment.

The results therefore have a number of implications for catch-ment management plans and could also guide the format of suchschemes to restore channel/riparian reaches in upland areas. Valleybottom sand and gravel deposits can have an important bufferingrole in attenuating water fluxes between hillslope and groundwa-ter flow systems and upland channels. Within upland floodplainsgenerally, the topography of the underlying gravels may vary sig-nificantly, affecting patterns of water movement and potentiallyleading to considerable variations in water availability (in the rootzone) to vegetation communities, both spatially and seasonally. Itappears that the down-valley gradients in the gravel surface andthe continuity of the underlying gravels with the river are particu-larly significant in determining the pattern of wetland water move-ment. Where possible, restoration schemes should therefore seekto preserve the form and integrity of the valley-bottom alluvialdeposits and, as suggested by Boswell and Olyphant (2007), hydro-logical modelling can be used to determine the implications ofindividual restoration programmes. In this way, the effects of rela-tively small changes in the sedimentary composition of the flood-plain can be investigated, and the significant changes inpermeability that occur according to the level of saturation canbe taken into account.

5. Conclusions

The significance of the results described here are that they indi-cate marked differences in the rate and direction of water move-ment within a relatively small upland floodplain wetland. This isimportant in the context of the increasing recognition of catch-ment headwaters which have been relatively neglected (e.g. Bishop

C. Bradley et al. / Journal of Hydrology 393 (2010) 349–361 361

et al., 2008). Significantly, Lischeid (2008) has called for more workspecifically on the riparian zone in headwater areas with a changein emphasis from hillslope hydrology to micro-scale riparian zonestudies, given the importance of near-stream, riparian areas onstream-water chemistry and runoff generation. In particular,changes in water-flow pathways across upland floodplains, wherethere are marked differences in gradient between hillslopes andthe stream network, have the potential to buffer rivers down-stream from the effects of preferential flow in catchment headwa-ters (e.g. Jones, 2004; Haria and Shand, 2004, 2006; Wenningeret al., 2004) whilst differences in the relative contribution ofsoil–water and groundwater processes to runoff influence down-stream trends in pH and alkalinity (Neal et al., 1997b).

Acknowledgements

This research was enabled by a NERC research grant awarded toNC and AMG and a NERC research studentship held by AC. Jo Em-ery, Richard Johnson and Ian Morrissey assisted with the fieldwork.We are very grateful for the constructive comments of the threereviewers, and Anne Ankcorn and Kevin Burkhill for cartography.

References

Acreman, M.C., Fisher, J., Stratford, C.J., Mould, D.J., Mountford, J.O., 2007.Hydrological science and wetland restoration: some case studies from Europe.Hydrology and Earth System Science 11 (1), 158–169.

Andersen, H.E., 2004. Hydrology and nutrient balance of a seasonally inundatedDanish floodplain wetland. Hydrological Processes 18, 415–434.

Benda, L., Hassan, M.A., Church, M., May, C.L., 2005. Geomorphology of steeplandheadwaters: the transition from hillslopes to channels. Journal of the AmericanWater Resources Association 41 (4), 835–851.

Bishop, K., Buffam, I., Erlandsson, M., Fšlster, J., Laudon, H., Seibert, J., Temnerud, J.,2008. Aqua Incognita: the unknown headwaters. Hydrological Processes 22,1239–1242.

Børgesen, C.D., Jacobsen, O.H., Hansen, S., Schaap, M.G., 2006. Soil hydraulicproperties near saturation, an improved conductivity model. Journal ofHydrology 324, 40–50.

Boswell, J.S., Olyphant, G.A., 2007. Modeling the hydrologic response ofgroundwater dominated wetlands to transient boundary conditions:implications for wetland restoration. Journal of Hydrology 332, 467–476.

Bowden, W.B., Fahey, B.D., Ekanayake, J., Murray, D.L., 2001. Hillslope and wetlandhydrodynamics in a tussock grassland, South Island, New Zealand. HydrologicalProcesses 15, 1707–1730.

Bradley, C., 2002. Simulation of the annual water table dynamics of a floodplainwetland, Narborough Bog, UK. Journal of Hydrology 261, 150–172.

Bradley, C., Gilvear, D.J., 2000. Saturated and unsaturated flow dynamics in afloodplain wetland. Hydrological Processes 14, 2945–2958.

Bradley, C., van den Berg, J.A., 2005. Infiltration mechanisms in a herbaceous peat:results of a controlled infiltration experiment. Hydrological Sciences Journal 50,713–725.

Bradley, C., Baker, A., Cumberland, S., Boomer, I., Morrissey, I.P., 2007. Dynamics ofwater movement and trends in dissolved carbon in a headwater wetland in apermeable catchment. Wetlands 27 (4), 1066–1080.

Burt, T.P., 1995. The role of wetlands in runoff generation from headwatercatchments. In: Heathwaite, J.M.R., Hughes, A.L. (Eds.), Hydrology andhydrochemistry of British Wetlands. John Wiley and Sons Ltd., Chichester, pp.21–38.

Burt, T.P., Matchett, L.S., Goulding, K.W.T., Webster, C.P., Haycock, N.E., 1999.Denitrification in riparian buffer zones: the role of floodplain hydrology.Hydrological Processes 13, 1451–1463.

Carlyle, G.C., Hill, A.R., 2001. Groundwater phosphate dynamics in a river riparianzone: effects of hydrologic flowpaths, lithology and redox chemistry. Journal ofHydrology 247, 151–168.

Clay, A., 2006. The Hydrodynamics of Floodplain Wetlands: Investigations on theRiver Tern (North Shropshire) and Afon Llwyd (Plynlimon), United Kingdom.Unpublished PhD, the University of Birmingham, UK.

Clément, J.-C., Aquilina, L., Bour, O., Plaine, K., Burt, T.P., Pinay, G., 2003. Hydrologicalflowpaths and nitrate removal rates within a riparian floodplain along a fourth-order stream in Brittany (France). Hydrological Processes 17, 1177–1195.

Emery, J.C., 2003. PhD. Characteristics and Controls of Gravel-bed Riffle-poolSequences for Habitat Assessment and River Rehabilitation Design.Unpublished PhD, the University of Birmingham.

Haria, A.H., Shand, P., 2004. Evidence for deep sub-surface flow routing in forestedupland Wales: implications for contaminant transport and stream flowgeneration. Hydrology and Earth System Science 8 (3), 334–344.

Haria, A.H., Shand, P., 2006. Near stream soil–water–groundwater coupling in theheadwaters of the Afon Hafren, Wales: implications for surface water quality.Journal of Hydrology 331, 567–579.

Harvey, J.W., Bencala, K.E., 1993. The effect of streambed topography on surface–subsurface water exchange in mountain catchments. Water Resources Research29 (1), 89–98.

Hill, A.R., 2000. Stream chemistry and riparian zones:. In: Jones, J.B., Mulholland, P.J.(Eds.), Streams and Ground Waters. Academic Press, San Diego, pp. 83–110.

Holden, J., Burt, T.P., 2003. Hydraulic conductivity in upland blanket peat:measurement and variability. Hydrological Processes 17, 1227–1237.

Jones, J.A.A., 2004. Implications of natural soil piping for basin management inupland Britain. Land Degradation & Development 15, 325–349.

Joris, I., Feyen, J., 2003. Modelling water flow and seasonal moisture dynamics in analluvial groundwater-fed wetland. Hydrology and Earth System Science 7 (1),57–66.

Jung, M., Burt, T.P., Bates, P.D., 2004. Toward a conceptual model of floodplain watertable response. Water Resources Research 40, W12409. doi:10.1029/2003WR002619.

Kasahara, T., Wondzell, S.M., 2003. Geomorphic controls on hyporheic exchangeflow in mountain streams. Water Resources Research 39 (1). doi:10.1029/2002WR001386.

Lischeid, G., 2008. Combining hydrometric and hydrochemical data sets forinvestigating runoff generation processes: tautologies, inconsistencies andpossible explanations. Geography Compass 2 (1), 255–280.

McClain, M.E., Boyer, E.W., Dent, C.L., Gergel, S.E., Grimm, N.B., Groffman, P.M., Hart,S.C., Harvey, J.W., Johnston, C.A., Mayorga, E., McDowell, W.H., Pinay, G., 2003.Biogeochemical hot spots and hot moments at the interface of terrestrial andaquatic ecosystems. Ecosystems 6, 301–312.

Narasimhan, T.N., 2004. Darcy’s law and unsaturated flow. Vadose Zone Journal 3,1059.

Neal, C., Robson, A.J., Shand, P., Edmunds, W.M., Dixon, A.J., Buckley, D.K., Hill, S.,Harrow, M., Neal, M., Wilkinson, J., Reynolds, B., 1997a. The occurrence ofgroundwater in the Lower Palaeozoic rocks of upland Central Wales. Hydrologyand Earth System Sciences 1, 3–18.

Neal, C., Hill, T., Hill, S., Reynolds, B., 1997b. Acid neutralization capacitymeasurements in surface and ground waters in the Upper River Severn,Plynlimon: from hydrograph splitting to water flow pathways. Hydrology andEarth System Sciences 3, 687–696.

Peterson, E.W., Sickbert, T.B., 2006. Stream water bypass through a meander neck,laterally extending the hyporheic zone. Hydrogeology Journal 14, 1443–1451.

Roberts, G., Crane, S.B., 1997. Temporal variations in near surface soil moisture attwo contrasting sites in the Wye catchment and their control on stormstreamflow generation. Hydrology and Earth System Sciences 1 (3), 453–461.

Roulet, N.T., 1990. Hydrology of a headwater basin wetland: groundwater dischargeand wetland maintenance. Hydrological Processes 4, 387–400.

Schaap, M.G., Leij, F.J., van Genuchten, M.Th., 2001. Rosetta: a computer program forestimating soil hydraulic parameters with hierarchical pedotransfer functions.Journal of Hydrology 251, 163–176.

Shand, P., Haria, A.H., Neal, C., Griffiths, K.J., Gooddy, D.C., Dixon, A.J., Hill, T.,Buckley, D.K., Cunningham, J.E., 2005. Hydrochemical heterogeneity in anupland catchment: further characterisation of the spatial, temporal and depthvariations in soils, streams and groundwaters of the Plynlimon forestedcatchment, Wales. Hydrology and Earth System Sciences 9 (6), 621–644.

Van Genuchten, M.Th., 1978. Numerical Solutions of the One-DimensionalSaturated–Unsaturated Flow Equation. Research Report 78-WR-09, WaterResources Program, Dept. of Civil Engineering, Princeton University.

Weng, P., Sánchez-Pérez, J.M., Sauvage, S., Vervier, P., Giraud, F., 2003. Assessment ofthe quantitative and qualitative buffer function of an alluvial wetland:hydrological modelling of a large floodplain (Garonne River, France).Hydrological Processes 17, 2375–2392.

Wenninger, J., Uhlenbrook, S., Tilch, N., Leibundgut, C., 2004. Experimental evidenceof fast groundwater responses in a hillslope/floodplain area in the Black ForestMountains, Germany. Hydrological Processes 17 (17), 3305–3322.

Whiting, G.J., Chanton, J.P., 2001. Greenhouse carbon balance of wetlands: methaneemission versus carbon sequestration. Tellus 53B, 521–528.

Winter, T.C., 2007. The role of ground water in generating streamflow in headwaterareas and in maintaining base flow. Journal of the American Water ResourcesAssociation 43 (1), 15–25.

Wroblicky, G.J., Campana, M.E., Valett, H.M., Dahm, C.N., 1998. Seasonal variation insurface-subsurface water exchange and lateral hyporheic area of two stream-aquifer systems. Water Resources Research 34, 317–328.