Interstitial pore-water temperature dynamics across a pool-riffle-pool sequence

ECOHYDROLOGYEcohydrol. 4, 549–563 (2011)Published online 16 February 2011 in Wiley Online Library(wileyonlinelibrary.com) DOI: 10.1002/eco.199

Interstitial pore-water temperature dynamics acrossa pool-riffle-pool sequence

Stefan Krause,1* David M. Hannah2 and Theresa Blume3

1 Department of Earth Science and Geography, School of Physical and Geographical Sciences, Keele University, Keele, ST5 5BG, UK2 School of Geography, Earth and Environmental Sciences, University of Birmingham, Edgbaston, Birmingham, B15 2TT, UK

3 Helmholtz Centre Potsdam, GFZ German Research Centre for Geosciences, Section 5.4, Hydrology, Telegrafenberg, C4 2.25, D-14473 Potsdam,Germany

ABSTRACT

Hyporheic habitat conditions are controlled strongly by spatial and temporal dynamics of physicochemical processes at theaquifer–river interface. In particular, heat transport between groundwater and surface water has a great impact on streambedhabitats. This study uses high resolution observations of vertical hydraulic gradients (VHGs) and interstitial pore-watertemperatures to investigate space-time patterns of groundwater–surface water (GW–SW) exchange fluxes and streambedthermal conditions of a pool-riffle-pool sequence of a UK lowland river. The results indicate that, although groundwater isdominantly upwelling in the research area, exchange flow patterns are strongly influenced by the streambed geomorphology.Advective heat flux caused by groundwater upwelling is shown to have a moderating impact on interstitial temperaturepatterns and partly compensates the impact of conduction of diurnal surface water temperature fluctuations into the streambed.Consequently, diurnal temperature oscillations, which are clearly pronounced in the top 10 cm of the streambed (up to 2 °C)are reduced by >90% at depths below 20 cm. This study provides evidence that even in groundwater upwelling conditions,the spatially variable impact of heat conduction from the streambed surface may cause a spatially heterogeneous interstitialhabitat structure with thermal conditions differing significantly in a vertical (3 °C temperature gradient at a length scale of0Ð4 m) as well as longitudinal (0Ð75 °C at 16 m) domain. These results not only enhance the understanding of thermal patternsin lowland rivers but also have implications for interstitial habitat ecohydrology, community structures and stability. Copyright 2011 John Wiley & Sons, Ltd.

KEY WORDS hyporheic zone; interstitial; temperature; heat conduction

Received 17 August 2010; Accepted 24 December 2010

INTRODUCTION

Exchange fluxes of water, solutes (e.g. pollutants, nutri-ents) and heat between surface and groundwater envi-ronments create a physicochemical habitat template forecohydrological and biogeochemical processes in thehyporheic zone (HZ) (Datry et al., 2005; Boulton et al.,2008; Krause et al., 2011), and in turn influence ecolog-ical community structure and biodiversity (Dole-Olivieret al., 1997; Malard et al., 2003; Bencala et al., 2006).The HZ is a dynamic ecotone (i.e. a saturated inter-face between surface water and groundwater bodies)that is recognized as important in the context of widerriver ecosystem services and functioning (reviewed byBoulton, 2007; Krause et al., 2011). Here, the abioticconditions are often considered as intermediate systemswith interstitial flow velocities, amplitudes of diurnaland annual temperature fluctuations, physical and chem-ical gradients and substrate variability between thoseof groundwater and surface water (Brunke and Gonser,1997; Malcolm et al., 2002; Bencala, 2005; Brown et al.,2006). Because habitat conditions are transient between

* Correspondence to: Stefan Krause, Department of Earth Science andGeography, School of Physical and Geographical Sciences, Keele Uni-versity, Keele, ST5 5BG, UK.E-mail: [email protected]

surface water and groundwater (Gibert et al., 1994; Boul-ton and Hancock, 2006), the HZ operates as a refugiumfor benthic fauna from extremes of river flow and tem-perature (Robertson et al., 2009; Wood et al., 2010).

Interstitial pore-water temperature is a key state param-eter driving hyporheic biogeochemical and ecohydrolog-ical processes (reviewed by Webb et al., 2008). Heat andwater exchange across the hyporheic interface may mod-erate water column temperature (Hannah et al., 2004;Burkholder et al., 2008) and determine ecological poten-tial of the HZ for thermal refugia (Stubbington et al.,2009).

Riverbed pore-water habitats are highly dynamic inspace and time (Brunke and Gonser, 1997; Malard et al.,2003), and properties may converge towards either sur-face or groundwater end-members under different con-trols (Dole-Olivier et al., 1997; Wood et al., 2010).Research is needed to characterize the scales of eco-hydrological variability and explain these patterns interms of driving processes. The interdisciplinary reviewof hyporheic hydrology, biogeochemistry and ecohydrol-ogy by Krause et al. (2011) provides evidence for theexistence of ‘hot spots’ and ‘hot moments’ of hyporheicexchange, chemical reactivity and refugial functions.Although exchange fluxes, chemical reactivity and heattransport at aquifer–river interfaces have been subject

Copyright 2011 John Wiley & Sons, Ltd.

550 S. KRAUSE, D. M. HANNAH AND T. BLUME

to intensive investigation over the last two decades(reviewed by Krause et al., 2011), there is no holisticapproach for the assessment of HZ ecohydrological func-tioning in different river systems. There is a need toidentify the spatial patterns and temporal dynamics of HZphysical and chemical controls on habitat architecture andfunctions. This would in turn improve our understandingof scales of variability with respect to interactive driv-ing factors and mechanisms for instream and hyporheicecohydrological functioning and ecosystem services.

A number of studies have demonstrated spatio-tempo-ral complexity in riverbed energy transfer processes (asreviewed recently by Hannah et al., 2009); notably, heatadvection by loss or gain of hyporheic water may influ-ence pore-water thermal patterns significantly (Carde-nas and Wilson, 2007). Consequently, temperature maybe used as a tracer to infer groundwater–surface water(GW–SW) interactions (Malcolm et al., 2002; Schmidtet al., 2007; Westhoff et al., 2007; Anibas et al., 2009),with continuous temperature monitoring offering a meansto uncover new information on spatio-temporal dynam-ics in HZ processes and go beyond inferences basedon point sampling (Hannah et al., 2009). However, themajority of the studies investigating the highly spatiallyand temporally variable thermal regime of hyporheic usedheat as a tracer to understand HZ hydrological processesrather than focusing on the ecohydrological implicationsof streambed temperature patterns (Conant, 2004; Ander-son, 2005; Hatch et al., 2006, 2010; Constantz, 2008;Lautz et al., 2010). Furthermore, more research has beenfocused on the investigation of advective heat transport,often with the intention of identifying or even quantifyingGW–SW exchange fluxes (Keery et al., 2007; Schmidtet al., 2007; Anibas et al., 2009; Hatch et al., 2010).

Although previous studies provide indirect evidencethat advective heat transport can have a high impacton the variability of streambed temperature patterns indownwelling conditions when highly dynamic surfacewater temperatures mix with usually rather homogeneousgroundwater temperatures (Hannah et al., 2009), thereis only limited understanding of streambed temperaturepatterns resulting from upwelling conditions as they arecharacteristic of many lowland rivers.

In many lowland environments the temporally andspatially homogeneous temperatures of the upwellinggroundwater are likely to attenuate the impact of highlyvariable surface water temperatures and thus, equili-brate interstitial streambed temperature patterns. How-ever, even in upwelling conditions, temporally highlyvariable surface water temperatures potentially impactinterstitial streambed temperatures by heat conduction.As the majority of previous studies have been focusingon upland rivers or surface water downwelling condi-tions, the current understanding of the impacts of heatconduction and advective heat transport on interstitialstreambed temperatures and consequently on hyporheicniche functions in lowland rivers is limited.

This paper focuses on these key research gaps and aimsto use a novel combination of high spatial and temporal

resolution field observations of hydraulic exchange fluxesand pore-water temperature:

1. To identify spatial patterns, temporal dynamics andscales of variability in these ecohydrologically impor-tant factors over a pool-riffle-pool sequence for a low-land river

2. To analyse the impact of seasonal and diurnal surfacewater temperature variability on the spatial streambedtemperature patterns in the largely groundwater domi-nated river section

3. To describe the hyporheic habitat structure of the inves-tigated pool-riffle-pool sequence with regard to inter-stitial pore-water temperature patterns and dynamics.

MATERIAL AND METHODS

Study area and field site

The field site (2°530W, 52°860N) is located in the catch-ment of the River Tern, an 852 km2 tributary of the RiverSevern in the UK (Figure 1(a)). The catchment is under-lain by the Permotriassic Sherwood Sandstone formation,which forms one of the UK’s major groundwater aquifers.The headwaters of the river are located within the undu-lating hills of the Woore moraine, a mainly forested ridgethat reaches elevations of up to 218 m above sea level(asl). However, the majority of the River Tern catchmentis situated in the Cheshire and Shropshire plains (low-lands), with elevations ranging between 55 and 70 m asl.Landuse around the field site is dominated by pasture withmoderate livestock densities. The area directly upstreamof the field site is arable, with cereals and root cropsgrown.

The investigated river reach forms part of a largerexperimental field site, which includes a 300 m meander-ing section of the River Tern and its immediate floodplain(Figure 1(b)). The focus site of this study is a ¾20-mlong pool-riffle-pool sequence (Figure 1(c)). This pool-riffle-pool reach provides a representative example forthe streambed topography of the lower River Tern. Thewider field area was the subject of previous intensiveinvestigation. It was selected by the UK Natural Envi-ronment Research Council (NERC) as a study area underthe Lowland Catchment Research Programme (LOCAR;Wheater and Peach, 2004) and it is monitored by theEnvironment Agency as part of the Shropshire groundwa-ter scheme (Streetly and Shepley, 2002). The field mon-itoring infrastructure installed for these major projectsprovides baseline data for this research (groundwaterobservation boreholes, borehole logs, flow gauge).

Mean annual precipitation close to the field site is583 mm with increased rainfall (¾740 mm) occurring inthe northern headwaters of the River Tern (Hannah et al.,2009). Mean daily air temperature varies from 3Ð7 °C inJanuary to 15Ð8 °C in July, with long-term (1957–2007)mean annual temperatures of 9Ð3 °C (Hannah et al.,2009). Mean river discharge at the Environment Agency-operated Tern Hill gauging station (basin area here

Copyright 2011 John Wiley & Sons, Ltd. Ecohydrol. 4, 549–563 (2011)DOI: 10.1002/eco

INTERSTITIAL PORE-WATER TEMPERATURE DYNAMICS 551

Figure 1. River Tern field site location within the UK (A), Experimental infrastructure and setup at the investigated river section (B), Longitudinalprofile of the streambed topography (C), Hatched lines in B and C indicate the location of the investigated pool-riffle-pool sequence.

Table I. Monitored variables, observation intervals and instrumentation.

Variable Observation interval Instrumentation Accuracy

Temperature—SW 5 min Solinst LT M5/F15 diver, combined level andtemperature logger

š0Ð05 °C

Temperature—GW 15 minš0Ð05 °C

Temperature—HZ 15 min Hobo—four-channel temperature logger andthermocouple sensors

š0Ð025 °C

Temperature—Air 1 h Keele, meteorological station š0Ð05 °CHydraulic head—SW 5 min Solinst LT M5/F15 diver, combined level and

temperature logger/Solinst BaroLoggerš0Ð3 cm

Hydraulic head—GW 15 min š0Ð3 cmBarometric head 5 min š0Ð3 cmPrecipitation 1 h Keele, meteorological station (18 km distance) š0Ð2 mm

Environment Agency’s gauging station Tern HillDischarge (Q) 1 h š5%

92 km2, elevation 62 m asl) is 0Ð9 m3 s�1 with a 95%exceedance (Q95) of 0Ð4 m3 s�1 and a 10% exceedance(Q10) of 13Ð9 m3 s�1 (data period 1961–1990; UKNational River Flow Archive, http://nwl.ac.uk/ih/nrfa).Summer baseflow conditions usually occur from May toOctober.

Data collection

Field data were collected over a nine-month period (June2009 to February 2010) across the study pool-riffle-poolsequence. Measurements and instrumentation are sum-marized in Table I. Variables recorded include ground-water, surface water, interstitial pore-water and air tem-perature, hydraulic heads in groundwater, surface water

and interstitial pore-water, precipitation and river dis-charge. Stream discharge data were provided by theEnvironment Agency from the nearby gauging stationat Tern Hill (2°5501200W, 52°8709200N). Meteorologicaldata were recorded at the nearby meteorological sta-tion.

Existing groundwater observation boreholes in thestudy area, reaching down to 30 m into the Permo–Triassic sandstone aquifer, were complimented by a net-work of ten 3-m-deep groundwater boreholes (Figure1(b)). These additional boreholes allow observation ofthe shallow riparian groundwater within the floodplaindrift deposits. Four of the groundwater boreholes (GW1,GW2, GW3 and GW7; Figure 1(b)) and two river stagegauging stations (SW1 and SW3; Figure 1(b)) were



instrumented with stage pressure transducers to monitorsurface water and groundwater head (i.e. water depth)at 5- to 15-min intervals. Multilevel mini piezometerswere installed in the streambed sediments at depths of20, 50 and 80 cm (Figures 1(b) and 4(c)) to observeinterstitial pore-water pressure head distributions. Moni-tored groundwater and surface water pressure heads werecorrected for barometric pressure fluctuations using anatmospheric pressure sensor located at groundwater bore-hole site GW7 (Figure 1(b)).

Hydraulic heads in streambed piezometers were mon-itored manually, on a fortnightly-to-monthly basis, bet-ween June 2009 and October 2009 (eight sampling dates)using a graduated electric contact meter (dip-meter).Manual dip-meter sampling also covered the network ofshallow riparian groundwater boreholes to provide qualityassurance for automatically logged pressure heads.

Temperature in groundwater and surface water weremonitored by thermistors integrated into the stage sensorsat 5- to 15-min intervals (Table I). For the observation ofinterstitial pore-water temperatures, a cluster of sensorswas installed in a nested set-up consisting of four ther-mistors in a vertical profile covering depths of 5, 10, 20and 40 cm at three locations (Figure 4(c)). For streambeddeployment the thermistors were installed at the end of5-, 10-, 20- and 40-cm long tubes of 12 mm diameter.The sensors were connected with four-channel Hobo log-gers (Table I), which were installed at the river bank.The tube walls protected the thermistor cable againstdamage by sediment sheering. To allow optimal con-tact with the streambed pore-water, the bottom of eachprobe was perforated at 3 cm length, corresponding withthe size of the thermistor sensor. To avoid preferen-tial heat flux, the entire tube was filled with low con-ductive silicon foam after sensor placement at the tubebottom. Isolators were deployed to prevent direct con-tact between the thermistor sensor and the tube walls.Cable damage on the riffle head 5 and 10 cm sensorscaused data loss at these locations from October 2009.Temperature was recorded at 15-min resolution alongthe vertical profiles at the head (stoss-side), at the crest(highest elevation) and on the tail (lee-side) of the riffle(Figure 4).

Topographical surveys of the riparian zone, channeland streambed were carried out by differential GPSmeasurement, leading to high-precision digital elevationmodels of streambed and floodplain topography with1 cm vertical resolution. Differential GPS was also usedfor measuring the exact heights of installed boreholes andpiezometers.

Data analysis

Vertical hydraulic gradients (VHGs), which indicate thestrength and direction of GW–SW exchange fluxes, weredetermined from hydraulic head measurements in thestreambed. VHGs were calculated by h/l, with hgiven by the elevation difference of the water tablesobserved at the inside and the outside of the piezome-ter and l given by the distance between the mid-screen

depth, surface water and sediment interface. The accu-racy of dip-meter-based hydraulic head observations wasapproximated as š3 mm head and accounts for uncer-tainties of the measurements introduced by turbulent flowconditions around the outsides of the piezometers, whichcould affect the outside head estimates.

Box and whisker plots (which summarize five sta-tistical measures: median, minimum, maximum, upperquartile and lower quartile) can be used for intersite com-parison of VHG depth profiles along the pool-riffle-poolsequence. To illustrate the spatial distribution of VHGs ina 2-D vertical longitudinal section through the pool-riffle-pool sequence, calculated VHGs were interpolated byInverse Distance Weighting of the 12 observation points.

Cross-correlation functions (CCFs) were calculated toassess the lag times (r(lag)) in the maximum correlationsbetween surface water and interstitial pore-water temper-atures at different depths and locations. Unless otherwisestated, all correlations are significant at p < 0Ð01 level.

To assess the potential of heat transport as a result ofpure conduction from the surface water into the sediment,the resulting modulation of temperature variation withdepth was estimated and compared to observed temper-ature amplitudes. Assuming pure heat conduction only,the amplitude of temperature variation at depth z is e�z/d

times smaller than the amplitude of surface temperaturefluctuation (Hillel, 1998). At the damping depth d theamplitude of temperature variation is reduced to 1/e. Itcan be calculated by (Hillel, 1998):

d D(

Dh�

�

)0Ð5�1�

where d is the damping depth, Dh is the thermal dif-fusivity and � is the period of the oscillation. Thermaldiffusivity is the ratio of thermal conductivity to vol-umetric heat capacity, where both thermal conductivityand volumetric heat capacity depend on water content,bulk density and composition of the substrate. In thisstudy, thermal diffusivity values representative for thefull range of saturated sediment ranging from 0Ð5 ð 10�6

to 1Ð5 ð 10�6 m2 s�1 were considered (Weight, 2008).

RESULTS

Hydroclimatological conditions

Although the annual precipitation regime in central Eng-land is usually dominated by winter precipitation, dur-ing the 2009–2010 observation period significant rainfallalso occurred during July and early August (Figure 2(b)),with total monthly rainfall reaching >100 mm for bothmonths. The wet summer was followed by a longer, rel-atively dry period until November, when early winterrainfall commenced. January and February were com-paratively dry, with total monthly precipitation of only51 mm in contrast to 106 mm during November andDecember (Figure 2(b)).

Air temperature exhibits clear seasonal dynamics withaverage temperatures of 9Ð2 °C and a range of more than



Figure 2. A: Timeseries of air temperature (2 m) and B: discharge (Q) at the Tern Hill gauging station and precipitation at the River Tern for theperiod 1 June 2009 to 1 March 2010.

Figure 3. Timeseries of surface water and groundwater levels for selected observation boreholes (GW1, GW2, GW3 and GW7 in Figure 1(B)) andat the River Tern (SW3 in Figure 1(B)) for the period 15 June 2009 to 15 February 2010.

30 °C (Tmax D 26Ð3 °C in July; Tmin D �7Ð6 °C in Jan-uary). Diurnal air temperature amplitudes vary markedlyover the annual cycle (Figure 2(a)). While day versusnight temperature differences reach values of up to 14 °Cin June and July, and even 6–8 °C in November, the diur-nal temperature range in winter (December–February) issmall (2–3 °C) with this variability subsumed by day-to-day meteorological changes.

Mean daily river discharge for Tern Hill (Q) overthe observation period averaged 9Ð1 m3 s�1 but exhibitsstrong seasonal patterns, ranging between summer min-imum baseflow discharges of 5Ð1 m3 s�1 and maxi-mum winter discharges of 31Ð2 m3 s�1 (Figure 2(b)). Thesummer baseflow period, with daily discharge rangingbetween 7 and 8 m3 s�1, is punctuated by two majordischarge events with Q > 15 m3 s�1. These episodesof increased summer discharge coincided with precipi-tation events of different intensities and duration. Whilethe first, 2-day-long peak flow episode in June 2009resulted from a single storm event (Figure 2), the Julyhigh flow was the result of prolonged wet conditionsand was proceeded by a >20 days with discharges>10 m3 s�1 (precipitation for June D 60 mm cf. JulyD 158 mm).

Riparian GW–SW head patterns

Timeseries of surface water and groundwater levelsobserved at stream gauges and groundwater observa-tion boreholes (Figure 1) were characterized by similarpatterns of variability (Figure 3). Groundwater (GW1,GW2, GW3 and GW7, Figure 3) and river levels (SW3,Figure 3) were generally low during the 2009 base-flow period from June to November (Figure 2(b)) withminimum water levels occurring towards the end ofOctober 2009 (Figure 3). However, the summer base-flow period was interrupted by a 3-week episode ofincreased groundwater and surface water levels in July2010 (Figure 3), caused by the precipitation events dis-cussed above (Figure 2(a)).

With increasing precipitation in November 2009,groundwater and river water levels rose (Figure 3). Peri-ods of low groundwater and surface water levels havebeen observed during winter 2009–2010 (Figure 3).Constant groundwater levels of 59Ð5 m at GW7 inNovember/December 2009 and January/February 2010were the result of the submersion of the observation bore-hole due to the ponding of rain water in the ultimatevicinity of the borehole.



Figure 4. Distributions of vertical hydraulic gradients (VHGs) observed at 20, 50 and 80 cm at the riffle front (A), the riffle centre (B) and the riffletail (C) based on eight observation dates from 21 May 2009 to 30 September 2009. (D) Longitudinal profile of the investigated pool-riffle-poolsequence and 2-D section of VHGs based on inverse distance weighted interpolation of VHGs at nine streambed piezometers, streambed piezometer

and thermistor locations.

Groundwater levels were higher than surface waterlevels throughout most of the monitoring period, indicat-ing a general flow direction from riparian groundwatertowards the stream. These conditions were only reversedduring storm events (Figure 2), when surface water lev-els rose faster and higher than associated groundwaterlevels (Figure 3), leading to loosing conditions with sur-face water infiltrating into the riparian groundwater. Suchperiods of inverse pressure head distributions, with sur-face water stages being higher than groundwater levels,were limited to peak flow events (Figure 2) and did notexceed 36 h in length.

VHGs and hyporheic water exchange

Figure 4(a), (b) and (c) illustrates that the VHGs wereobserved in the streambed sediments at three depths(20, 50 and 80 cm) at the riffle head, crest and tail.VHGs were positive throughout the observation periodat all depths and locations, indicating constant ground-water upwelling into the riffle bedform (Figure 4(a),(b) and (c)). Negative VHGs, indicating surface waterdownwelling, were not observed at any time during thestudy. Observed VHG values range from 0Ð78 (suggestingstrong groundwater upwelling) to 0 (indicating hydraulicheads at the piezometer equal the hydrostatic pressure ofthe stream).

Figure 4(d) shows the distribution of VHGs in twodimensions across the pool-riffle-pool sequence for 30June 2009. These date were chosen because they arerepresentative of characteristic summer baseflow condi-tions. The inverse distance weighted spatial interpolationof VHG demonstrates that pressure head distributions

broadly mirror the streambed topography (Figure 4). Thesteepest pressure gradients were observed at lowest ele-vations on the head and tail of the riffle.

A comparison of the statistics of observed VHGs atdifferent locations and depth shows that VHGs generallydecreases along the groundwater upwelling flow pathwith reduced streambed depth (Figure 4(a), (b) and (c)).At all three locations, VHGs at 80 cm depth are higherthan the VHGs observed at 20 cm, again indicating anupwelling flow direction.

The reduction of VHGs from deep (80 cm) to shallow(20 cm) piezometers appears to be more intensive in theriffle head and crest than at the riffle tail (Figure 4). Whileat 80 cm depth, average VHGs are relatively similaracross the riffle (head D 0Ð40; crest D 0Ð44; tail D 0Ð41),the differences in VHGs along the riffle section becomemore pronounced at shallower depths. At the riffle head,the VHG is reduced to 0Ð20 at 50 cm and 0Ð10 at 20 cmdepths; while at the riffle crest, VHG declined to 0Ð08 at50 cm and 0Ð16 at 20 cm. At the riffle tail, VHG valueswere higher, being 0Ð31 at 50 cm and 0Ð21 at 20 cmdepths.

Groundwater and surface water temperature patterns

Interstitial pore-water temperatures at depths from 5 to40 cm (Figure 4) were compared with groundwater andsurface water temperatures at the riffle head (Figure 5),crest (Figure 6) and tail (Figure 7). By comparing thegroundwater and surface water temperature at these threelocations, it is possible to identify substantial spatio-temporal differences in thermal patterns. Seasonal surfacewater variability was >20 °C (min D 0Ð1 °C; max D



Figure 5. Stream (SW3), groundwater (GW1) and interstitial pore-water temperatures at depths from 5 to 40 cm at the riffle head between 15 June2009 and 15 February 2010 (C) with zoom-in view of summer conditions from 20 July 2009 to 20 August 2009 (A) and winter conditions from 1

to 31 January 2010 (B).

22Ð2 °C), far exceeding the range of groundwater temper-ature of <2Ð5 °C (min D 9Ð8 °C, max D 12Ð2 °C). Whilethe seasonal dynamics of stream temperature mirroredair temperature (Figure 2), with maximum in June–July2009, groundwater temperature was highest in October2009, indicating a 2–3 month time lag in response to sur-face water/atmospheric conditions (Figures 5–7). Mini-mum surface water temperature was observed in earlyJanuary 2010, whereas minimum groundwater temper-atures may not be covered by the observational win-dow, with the lowest groundwater temperature recordedat the end of the observation period in February 2010(Figures 5–7). These differences in seasonal tempera-ture patterns yield strong temperature gradients betweengroundwater and surface water (Figures 5–7). Duringsummer (June to September), surface water temperaturewas up to 9Ð0 °C (average 3Ð1 °C) higher than ground-water temperature. Conversely, during winter, an inversetemperature gradient occurred and groundwater was upto 10Ð0 °C warmer than surface water, with an aver-age groundwater/surface water temperature difference of4Ð4 °C from November to February. From September toOctober 2009, temperature differences between ground-water and surface water became less pronounced andthe direction of temperature gradients changed on severaloccasions (Figures 5–7).

The daily amplitude of surface water temperature var-ied between 2Ð4 °C in June and 0Ð2 °C in January andFebruary (Figures 5–7), again tracking air temperature

patterns (Figure 2). In contrast, no clear diurnal peri-odicity of groundwater temperatures was observed withmaximum daily temperature variability <0Ð2 °C. Short-term (at the scale of several days) fluctuations in surfacewater temperature coincided with meteorological vari-ability (Figure 2) with up to 9Ð8 °C temperature shiftsbetween antecedent days between June and July 2009while groundwater temperature variability did not exceed0Ð4 °C.

Interstitial pore-water temperatures

Vertical patterns. In addition to groundwater and sur-face water temperature, Figures 5–7 show the tempera-ture observed in interstitial pore-water at 5–40 cm depthbelow the streambed surface. All three riffle locations(head, crest and tail; Figure 1) showed very clear verticalthermal gradients, with temperatures decreasing typicallywith depth in summer and inverse conditions in win-ter when temperatures increased with depth. There wasepisodic temperature inversion (deeper locations exhibit-ing higher temperatures) during summer (Figures 5–7)lasting no longer than 2 days and coinciding with stormevents (i.e. increased discharge and surface water levels,reduced surface water and air temperatures; Figure 2).

Meteorologically driven multi-day surface water tem-perature variability during summer was associated withthermal responses at all four observed depths (Figures5–7). Diurnal temperature oscillations of up to 1Ð8 °Cwere observed during summer at 5 and 10 cm depth but



Figure 6. Stream (SW3), groundwater (GW1) and interstitial pore-water temperatures at depths from 5 to 40 cm at the riffle crest between 15 June2009 and 15 February 2010 (C) with zoom-in on summer conditions from 20 July 2009 to 20 August 2009 (A) and winter conditions from 1 to 31

January 2010 (B).

Figure 7. Stream (SW3), groundwater (GW1) and interstitial pore-water temperatures at depths from 5 to 40 cm at the riffle tail between 15 June2009 and 15 February 2010 (C) with zoom-in on summer conditions from 20 July 2009 to 20 August 2009 (A) and winter conditions from 1 to 31

January 2010 (B).



Table II. Cross-correlation functions (CCFs) and calculated lag times (r(lag)) of surface water to interstitial pore-water temperaturetimeseries at depths between 5 and 40 cm at the riffle head, crest and tail for conditions in summer (7 July 2009–21 September

2009), autumn (1 October 2009–1 November 2009) and winter (15 November 2009–11 February 2010).

Summer(7 July 2009–

21 September 2009)

Autumn(1 October 2009–1 November 2009)

Winter(15 November 2009–

11 February 2010)

r(lag) (min) CCF r(lag) (min) CCF r(lag) (min) CCF

Head 5 cm �75 0Ð990 No data No data No data No dataHead 10 cm �255 0Ð935 �270 0Ð911 �225 0Ð903Head 20 cm �1905 0Ð784 No data No data No data No dataHead 40 cm �2925 0Ð702 �2670 0Ð502 �2385 0Ð799Crest 5 cm �90 0Ð985 �45 0Ð991 0 0Ð922Crest 10 cm �285 0Ð923 �270 0Ð907 �180 0Ð908Crest 20 cm �1905 0Ð802 �930 0Ð736 �1095 0Ð873Crest 40 cm �3120 0Ð668 �3465 0Ð488 �3060 0Ð797Tail 5 cm �180 0Ð971 �180 0Ð955 �90 0Ð915Tail 10 cm �315 0Ð911 �345 0Ð876 �270 0Ð902Tail 20 cm �705 0Ð824 �915 0Ð761 �735 0Ð877Tail 40 cm �2565 0Ð716 �2475 0Ð553 �2025 0Ð816

Negative lag times indicate a delayed signal transduction.

were less evident at 20 and 40 cm depth (Figures 5–7).However, as for surface water and air, diurnal fluctua-tions in interstitial pore-water temperature were less pro-nounced in winter, when pore-water temperature variabil-ity was dominated by fluctuations in the range of severaldays that coincided with meteorological conditions.

In addition to a vertical dampening of temperaturewith increasing depth, interstitial pore-water tempera-tures exhibited a temporal shift in the surface watertemperature signal with lag times increasing with depth(Figures 5–7(a and b)). To analyse the interrelationshipof interstitial pore-water temperature and surface watertemperature, CCF and time lags (r(lag)) for the propaga-tion and temporal offset of the surface water signal aregiven in Table II for summer (7 July 2009–21 September2009), the autumn transition period (1 October 2009–1November 2009) and winter (15 November 2009–11February 2010). The strength of correlation decreasedwith depth for all three periods (from 0Ð915–0Ð990at 5 cm depth to 0Ð488–0Ð816 at 40 cm depth). Lagtime indicated that the surface water signals were moredelayed with increasing depth, ranging between 45 and180 min at 5 cm depth and 2025–3465 min at 40 cmdepth (Table II). Together, the weaker correlation andobserved lag times highlighted progressive dampening ofsurface water temperature signal transduction with depth.No substantial temporal variability in CCF and lag timewas found within the observation period (Table II).

To test if heat conduction alone can explain observedtemperature patterns, Equation (1) was solved to calcu-late the depth-related dampening of surface water temper-ature oscillation to 40 cm streambed depth. Calculationsfor the full range of thermal diffusivity values in sat-urated sediments varying from 0Ð5 ð 10�6 to 1Ð5 ð10�6 m2 s�1 (Weight, 2008) indicated that a diurnal sur-face water temperature oscillation of 2Ð5 °C (as observed

in Summer 2009) would cause purely heat conduction-induced temperature oscillations of 0Ð09° –0Ð34 °C at40 cm streambed depth.

For thermal diffusivities values of the sandy and finegravely material within the research area (0Ð75–1Ð25 ð10�6 m2 s�1), the calculated temperature oscillationdampening indicated that a diurnal temperature oscilla-tion of 2Ð5 °C of the surface water would, in the caseof pure heat conduction, produce an average oscillationof 0Ð24 °C at a depth of 0Ð4 m. However, temperatureobservations at this depth, did not reveal any diurnaltemperature oscillation at all. Maximum recorded dailytemperature variation at this depth was <0Ð2 °C and wasnot related to diurnal oscillation but rather long-term sea-sonal trends.

Longitudinal patterns. Marked longitudinal differencesin interstitial pore-water temperature became evident bycomparing thermal patterns at the same depths across theriffle head, crest and tail (Figures 8 and 9). The differencebetween surface water temperature and interstitial pore-water temperature at 5 cm depth was smallest (averageof 0Ð2 °C for the maximum daily temperature and 0Ð8 °Cfor the minimum daily temperature) at the head duringsummer (Figure 8(a)) and winter (Figure 8(b)). Towardsthe tail of the riffle, differences between 5 cm interstitialpore-water and surface water temperatures were higher(average of 0Ð4 °C for the maximum daily temperatureand 1Ð1 °C for the minimum daily temperature) than atthe riffle head, in both summer and winter (Figure 8).

Temperature at 40 cm depth appeared to be largelyunaffected by the diurnal oscillation of surface watertemperatures. However, the deviation between 40 cminterstitial pore-water temperatures and surface watertemperatures was constantly 0Ð3–0Ð6 °C larger at the riffletail than at the riffle head (Figure 9).

At 5 and 40 cm depth, temperature was cooler at thetail than at the head during summer and warmer during



Figure 8. Interstitial pore-water temperatures at 5 cm depth at the head, crest and tail of the riffle in comparison to stream (SW3) and groundwater(GW1) temperatures for selected summer conditions from 20 July 2009 to 20 August 2009 (A) and winter conditions from 1 to 31 January 2010 (B).

Figure 9. Interstitial pore-water temperatures at 40 cm depth at the head, crest and tail of the riffle section in comparison to stream (SW3) andgroundwater (GW1) temperatures for selected summer conditions from 20 July 2009 to 20 August 2009 (A) and winter conditions from 1 to 31

January 2010 (B).

winter (Figures 8 and 9). Winter data for 5 cm depth wereunavailable due to the loss of a thermistor. A comparisonof CCFs between surface water and interstitial pore-watershowed that lag times at similar depths increased fromhead to tail (e.g. between 7 July 2009 and 21 September2009 at 5 cm depth from 75 min (head) to 90 min (crest)to 180 min at the tail) and the correlation coefficientweakened from the riffle head to tail (e.g. from 0Ð990(head) to 0Ð985 (crest) to 0Ð971 at the tail for the sameperiod and depth), suggesting a progressive delay inthe transduction of the surface water temperature signal.Similar patterns for correlation strength and lag timeswere found at 10 cm depths and does occur throughoutthe autumn and winter period (Table II).

Calculated lag-time values at all depths at riffle head,crest and tail declined from summer over autumn towinter. Relative lag time decreases were found to behigher at 5 and 10 cm depth than in depths of 20 and40 cm.

Vertical and longitudinal interstitial temperature changes

In order to analyse the spatial patterns and variancein interstitial thermal habitat conditions, the horizon-tal and vertical variability of streambed temperatureswas compared. Therefore, differences between riffle headand tail temperatures at similar depths of 5 and 40 cmwere calculated to quantify the horizontal variability ofstreambed temperatures (Figure 10). Horizontal temper-ature differences were calculated as T D Triffle tail �

Triffle head. Therefore, negative values indicate conditionswere Triffle tail < Triffle head and positive values indicateTriffle tail > Triffle head. For comparison, the vertical vari-ability of streambed temperatures along the upwellinggroundwater flow path were calculated as differencebetween the 40 and 5 cm temperatures (T D T40 cm �T5 cm) observed at the riffle head, crest and tail, respec-tively (Figure 11).

Averaged horizontal head to tail temperature differ-ences at 40 cm ranged from minus 0Ð74 °C in July toplus 0Ð75 °C in January and were greater than aver-age temperature differences at 5 cm depth, which wereonly 0Ð4 °C (Tmin D �1Ð1 °C; Tmax D 0Ð25 °C) in July(Figure 10). However, temperature differences betweenriffle head and riffle tail at 5 cm depth were character-ized by higher temporal variability than at 40 cm depth.While at 40 cm depth, temperature differences betweenriffle tail and riffle head varied on a rather seasonal scalewith colder riffle tails in summer and warmer riffle tailsin winter, at 5 cm depth, temperature differences werefluctuating on a daily scale (Figure 10). At 5 cm depth,temperature differences between riffle head and riffle tailchanged by up to 0Ð7 °C within 24 h, whereas at 40 cmdepth daily variability of temperature differences betweenriffle head and riffle tail did not exceed 0Ð1 °C.

Vertical temperature differences in the upwellinggroundwater between 40 and 5 cm depth within thestreambed (T40 cm � T5 cm) were mostly negative in July



Figure 10. Interstitial pore-water temperature differences between riffle head and riffle tail at 5 and 40 cm depth of the streambed for the period 1July 2009 to 15 February 2010, T D Triffle tail � Triffle head.

Figure 11. Comparison of interstitial pore-water temperature differences between 40 and 5 cm depths at the riffle head and riffle tail for the periodfrom 1 July 2009 to 30 September 2009 (T D T40 cm � T5 cm).

and August 2009 (Figure 11), indicating that intersti-tial pore-water temperatures during this time were gen-erally smaller at the streambed bottom (40 cm) andincreased towards the streambed surface (5 cm). Ver-tically, streambed temperatures between 40 and 5 cmdepths at the same location varied by up to 3 °C(Figure 11). Positive values for T40 cm � T5 cm dif-ferences in September 2009 (Figure 11) indicate that5 cm streambed temperatures were smaller than 40 cmstreambed temperatures. At the riffle head, the verticaltemperature variation in upwelling groundwater was onaverage 0Ð4 °C (in some cases even 0Ð6 °C) smaller thanat the riffle tail (Figure 11).

DISCUSSION

Hydraulic heads

The strong correlation of temporal dynamics in observedgroundwater and surface water heads (Figure 3) sug-gests close interactions between the riparian groundwaterand the stream. Groundwater levels are predominantlyhigher than river water levels throughout the observationperiod, which indicates that the wider study site is expe-riencing groundwater upwelling and surface water accre-tion. The occurrence of brief episodes with inverse flowconditions (stream towards groundwater), when observedgroundwater heads were smaller than surface water heads(Figure 3), was limited to storm events (Figure 2).

Streambed piezometer observations found positive val-ues of VHGs (Figure 4) throughout the observation

period, indicating that GW–SW exchange fluxes wereindeed dominated by groundwater upwelling at the studysite. No direct evidence of surface water downwelling todepth ½20 cm was found. However, as the window ofVHG observations is limited to depths between 20 and80 cm, it cannot be excluded that shallow surface waterinfiltration still might occur at depths <20 cm.

The observed VHG patterns (Figure 4), which indi-cated that VHGs along the upwelling flow path decreasedfaster at the riffle head than at the tail, can be inter-preted as the result of greater hydrostatic pressure ofthe surface water at the riffle head, which acted as aflow obstacle. Streambed morphological features suchas pool-riffle-pool sequences have been found to sup-port advective pumping (Packman et al., 2004; Cardenasand Wilson, 2007; Bottacin-Busolin and Marion, 2010).Under these conditions, increased hydrostatic pressure atflow obstacles causes surface water to infiltrate into thepore space upstream of the flow obstacle and re-exfiltrateinto the stream further downstream. The fact that theobserved reduction of VHGs from 80 to 20 cm depthswas significantly smaller at the tail than at the head,and 20 cm VHG at the riffle tail at all times exceeded20 cm VHG at the riffle head (Figure 4), supports thehypothesis that surface water influence was higher atthe riffle head than at the tail. Similar flow patternswith increased upwelling hyporheic flow in low pressureregions of the riffle tail have been observed in previ-ous studies by Kasahara and Wondzell (2003) as wellas Storey et al. (2003). It cannot be dismissed that sur-face water infiltration may have occurred at streambed



depths above the shallowest observation piezometers at20 cm, with infiltration of stream water at the head, fol-lowed by subsurface flow parallel to the river and exfil-tration at the tail, as hydraulic head observations did notcover streambed depths smaller than 20 cm. Such flowpatterns are common for pool-riffle-pool sequences andhave been frequently described in literature (Kasaharaand Wondzell, 2003; Storey et al., 2003; Kasahara andHill, 2008). A strong influence of spatial variability instreambed hydraulic conductivities on the observed VHGpatterns can be excluded as core profiles confirmed veryhomogeneous porosities (0Ð28–0Ð32) in the medium tocoarse sands of the investigated pool-riffle-pool sequence.

It is acknowledged that water upwelling into theresearch area at 40 cm depth in the streambed may notbe pure groundwater but may contain a proportion oflarger scale hyporheic flow that may have infiltratedfurther upstream of the research area. However, positiveVHGs observed at 20, 50 and 80 cm depth (Figure 4)clearly indicate that the pore-water (which also differs intemperature from the surface water) cannot result fromsmall-scale hyporheic flow originating within the studyreach.

GW–SW temperatures

Seasonal temperature patterns in groundwater and surfacewater differed substantially. Surface water temperatureexhibited a clear seasonal pattern, whereas groundwa-ter temperatures remained relatively constant throughoutthe year. Seasonal and diurnal surface water temperaturedynamics mirrored air temperature patterns, suggestingthat surface water temperature was determined primar-ily by meteorological conditions. In contrast, ripariangroundwater temperatures at 3 m depth showed signif-icantly less seasonal variability than air or surface watertemperatures and did not exhibit any diurnal periodicityat all. The by one order of magnitude lower amplitudein annual temperature oscillation and the compared to airand surface water temperatures 2–3 month offset in max-imum groundwater temperature suggests a well bufferedresponse to surface water temperature and atmosphericconditions.

As a result of different seasonal temperature patternsfor groundwater and surface water, the direction of thethermal gradients between groundwater and surface watervaried over the year. Groundwater temperatures werelower than surface water temperatures during summerbut warmer during winter, so that groundwater dischargecaused either cooling (summer) or warming (winter) ofthe stream. The difference between groundwater andsurface water temperatures indicates clear mixing patternsof both water sources in the streambed (Conant, 2004;Anderson, 2005; Cardenas and Wilson, 2007; Schmidtet al., 2007; Constantz, 2008; Hannah et al., 2009),except for October and November when temperaturedifferences between groundwater and surface water areless pronounced.

Interstitial pore-water dynamics

Observations of interstitial pore-water temperature indi-cated clearly that the streambed thermal regime is con-trolled by GW–SW interactions. It is possible to inter-pret streambed temperature dynamics by linking themwith hydraulic head information to yield understanding ofhyporheic processes across this pool-riffle-pool sequence.

Interstitial pore-water temperatures were strongly influ-enced by surface water temperatures with high temporalvariability and diurnal oscillations and by the groundwa-ter with fairly constant temperature. During summer, withgroundwater cooling the warmer surface water, inter-stitial pore-water thermal profiles decreased with depth.During winter, when the relatively constant groundwatertemperatures were higher and more stable than fluctuat-ing surface water temperature, groundwater contributionswarmed the surface water, resulting in interstitial pore-water temperature profiles that were the reverse of sum-mer conditions with temperatures increasing with depth.These results are similar to findings of previous stud-ies (Hannah et al., 2009; Hatch et al., 2010; Lautz et al.,2010) However, in contrast to these studies, where thesurface water influence mainly resulted from SW down-welling, advective heat flux from the surface can beexcluded at depth below 20 cm in the research area. Atransduction of surface water temperature dynamics intothe streambed, as most clearly observed during the sum-mer months, cannot result from advective heat transportfrom the surface but only from heat conduction.

Shallow pore-water temperatures (5 and 10 cm) exhib-ited a similar daily temperature oscillation as the surfacewater. The attenuation of the surface water signal atdepths >20 cm, highlighted the impact of advective heattransport by upwelling groundwater which at these depthsoutweighs the heat conduction from surface water intothe bed. In shallower depths of 5 and 10 cm, however,the attenuation of the surface water signal is compara-bly small, indicating a reduced impact of advective heattransport by upwelling groundwater on interstitial pore-water temperatures. The propagation of surface watertemperature oscillations into these depths provides strongevidence for the impact of heat conduction from thestreambed surface. It cannot be excluded that advec-tive heat transport by surface water infiltration is alsoresponsible for the observed temperature patterns at 5and 10 cm depths (where VHGs could not be observed).However, this is extremely unlikely because VHGs at20 cm depth were indicating strong upwelling for allthree locations and the dampening of the temperatureoscillation is slightly lower than estimated for pure con-duction.

The transduction of the surface water heat signalthrough the streambed is characterised by a clear dampen-ing effect and a temporal offset of the surface water tem-perature signal within the interstitial pore-water, whichincreased with depth. With streambed depth increasing,lag times and decreasing temperature oscillations high-light the progressive delay of signal transmission fromthe streambed surface and the moderating effect of the



upwelling groundwater. The calculation of the temper-ature oscillation dampening depth to test whether theinterstitial temperature dynamics observed at 40 cm depthcan be explained by heat conduction from the surface(as SW downwelling at this depth has been excluded)confirmed that observed temperature oscillations can beexplained by heat conduction through the streambed. Thecalculated value of 0Ð24 °C oscillation at a depth of 0Ð4 mexceeds the observed daily temperature variability of<0Ð2 °C (more related to seasonal dynamics than dailyoscillation), which is to be expected as the calculationdid not account for the attenuating effect of the upwellinggroundwater.

The occurrence of inversed vertical pore-water tem-perature profiles in summer coincided with storm events(associated with increased surface water levels andreduced air and surface water temperature). Similarresponses in depth-related thermal profiles have beenfound elsewhere due to increases in river stage and chang-ing hydraulic gradient in gaining river reaches (Mal-colm et al., 2002; Fritz and Arntzen, 2007). During theseepisodes, temperature inversion occurred mainly duringthe night, caused by a reduced diurnal variability of upperpore-water temperature, while temperatures in the deeperpore-water (with permanently small diurnal oscillation)remained relatively constant (Figures 5–7(a)).

The comparison of temperature patterns at the rifflehead, crest and tail showed that interstitial pore-watertemperatures at the tail differed more from surface watertemperatures than at the head or the crest. This maybe interpreted as a consequence of higher groundwaterupwelling rates at the tail than at the riffle head and crest,causing streambed temperature at the tail to be cooler thanat the head during summer and warmer than the headduring winter. Smaller groundwater upwelling rates atthe head support a more instant and deeper transmissionof the diurnal cycles in surface water temperature intothe streambed when the conducted surface water heatsignal is less disturbed by heat advection from theupwelling groundwater. This inference is supported bythe observation of increased lag times of maximumcorrelation between surface water and interstitial pore-water temperatures from the head to the tail, highlightingthe dampening effect groundwater upwelling has onvertical transmission of the diurnal oscillation of surfacewater. Increased upwelling at the riffle crest and tail is inagreement with model predictions and field observationspublished for gaining reaches (Malcolm et al., 2002;Storey et al., 2003; Cardenas and Wilson, 2006, 2007).

The observed reduction of lag times of maximumcorrelation between surface water and interstitial pore-water temperatures from summer to winter conditions(Table II) may be interpreted as a result of reducedgroundwater upwelling rates during winter flow periodswith increased surface water levels and reduced hydraulichead gradients between groundwater and surface water(Figure 3). A lag time of zero as observed in 5 cm depthat the riffle crest for winter conditions indicates that thesurface water heat signal was directly conducted into the

top 5 cm of the streambed without any apparent signaldampening by groundwater upwelling.

The fact that vertical temperature changes in upwellinggroundwater were more pronounced at the riffle tail thanat the riffle head (Figure 11) highlights the strong impactof streambed geomorphologic structures as the investi-gated pool-riffle-pool sequence on small-scale patternsof groundwater upwelling and subsequent temperaturedistribution. However, in contrast to previous investiga-tions, where streambed morphology controlled advectiveheat transport has been shown to have a direct impact oninterstitial pore-water temperature patterns, in the investi-gated lowland river section it rather modulates/attenuatesthe heat signal conducted from the surface. In the inves-tigated lowland setting, streambed morphology controlsinterstitial pore-water temperature patterns rather indi-rectly by causing spatial heterogeneity in groundwaterupwelling-related advective heat transport which affectsthe attenuation intensity of the surface water temperaturesignal.

Observed inversion of vertical temperature changesin upwelling groundwater, in particular, at the rifflehead (Figure 11) with several episodes in September2009 of positive T in contrast to negative T asobserved most of the time could be interpreted as anindication of surface water influx into the streambed.However, episodes of positive T were not correlatedto any major high flow events (Figure 2) which wouldmake an episodic flow inversion more likely. In contrast,during high flow events in July, when surface waterlevels were increased for a prolonged period (Figure 3),T were negative throughout the high flow period.More frequent positive T towards September 2009were therefore interpreted as the result of the generaldecreasing trend in surface water temperatures, whichbecame more similar to groundwater temperatures duringthis period (Figure 2).

The observed interstitial temperature patterns are ofgreat ecohydrological importance because they providedetailed information on space and time dynamics of thehabitat template. The interstitial habitat structure of theinvestigated lowland river section is characterized byspatially more heterogeneous temperature patterns thanexpected for strongly groundwater upwelling conditions.Groundwater contributions proved to have a major con-trol on interstitial habitat conditions by its moderatingeffect on the streambed temperatures during summer(cooling) and winter (warming). However, spatial vari-ability in groundwater upwelling controlled advectiveheat transport and in heat conduction resulted in complexspatial patterns and temporal fluctuations of interstitialtemperatures (Figure 12) with potentially great impli-cations for hyporheic habitat conditions. With its pro-nounced daily temperature oscillations in summer, habitatconditions at the top 10 cm of the streambed differ signif-icantly from the conditions at depths below 20 cm whereinterstitial temperatures are more stable due to the mod-erating effect of the upwelling groundwater (Figure 12).This is of particular interest as previous research has



Figure 12. Interstitial thermal habitat conditions with longitudinal andvertical temperature variability along the investigated the pool-riffle-poolsequence indicating higher bottom to top temperature differences at theriffle tail (3Ð0 °C) than at the riffle head (2Ð4 °C) as well as vertical patternsin diurnal temperature oscillations (max diurnal T change of <0Ð2 °C at40 cm and diurnal oscillations of 1Ð8 °C at 5 cm) for summer conditions.

demonstrated that the HZ provides a refugium for benthicfauna from extremes of river flow or river temperature(Robertson et al., 2009; Stubbington et al., 2009; Woodet al., 2010). Habitat temperature patterns do not onlyvary in a vertical domain but also exhibit significantlongitudinal variability. At the riffle tail, where VHGobservations proved higher groundwater upwelling, thetemperature moderating effect of groundwater is morepronounced than at the head, with the effect of reducedshort-term temperature oscillation. These results supportprevious findings about hyporheic exchange across pool-riffle sequences but also extend the understanding oftemporal dynamics in heat transport as identified in pre-vious studies (Malcolm et al., 2006; Fritz and Arntzen,2007; Nyberg et al., 2008; Hannah et al., 2009).

Hyporheic habitat conditions are known to often bediverse with distributions of invertebrates being highlypatchy (Boulton et al., 1998). Much of the variability indistribution has been attempted to be explained by theexistence of transient physicochemical gradients (Frankenet al., 2001; Olsen and Townsend, 2003). The results ofthis study highlight the scale of temperature variabilityin a groundwater upwelling-dominated lowland streamsection. Further research will be required to attempt aquantification of the impact of interstitial temperaturepatterns on the distribution of invertebrates in the HZas this variability is likely to impact on habitat qualityand has potential implications for local species diversity(Ward and Tockner, 2001).

CONCLUSIONS

This study combines high spatial and temporal resolutionfield observations of hydraulic exchange fluxes andinterstitial pore-water temperature (1) to identify scalesof variability in these ecohydrologically important factorsover a pool-riffle-pool sequence for a lowland riverunder generally groundwater upwelling conditions, (2) toexplain dynamics by inferring driving HZ processesand (3) to describe the hyporheic habitat structure withregard to patterns and dynamics in interstitial pore-watertemperatures.

Our work illustrates that hydraulic head and temper-ature timeseries can be used to improve knowledge ofHZ processes and GW–SW interactions at the spatialscale of bedforms and over daily to seasonal timescales.Patterns in these data indicate, for the investigated

groundwater upwelling-dominated lowland river section,that the key controls on streambed temperature patternsare: (1) hydroclimatologically-driven variability in sur-face (channel) water thermal conditions and (2) localstreambed geomorphology.

The results of this study highlight the importance ofheat conduction from the surface water into the streambedand its spatially variable modulation by heterogeneousgroundwater upwelling patterns as major reasons for thespatially highly variable and temporally dynamic inter-stitial temperature patterns that define the heterogeneoushyporheic habitat conditions. Upwelling groundwater hasa moderating effect on streambed temperature patterns,with the potential to compensate the impact of short-term (daily) surface water temperature fluctuations. Con-sequently, the propagation of daily surface water tem-perature oscillations was limited to interstitial sedimentsof the top 10 cm of the streambed, creating habitat con-ditions that significantly differ from greater streambeddepths of 20 cm and below.

The observed spatial and temporal variability instreambed temperature patterns has potential implicationsfor the hyporheic community structure, function and bio-diversity as the stability, variation, range and extremes ofthis important ecohydrological state parameter stronglycontrol interstitial habitat conditions.

ACKNOWLEDGEMENTS

The authors gratefully acknowledge the field support byE Naden (Keele University), L Angermann (Institute forFreshwater Ecology Berlin), C Tecklenburg and M Munz(Potsdam University). We Extend our thanks to K Voyce(Environment Agency) for his support and the provisionof hydrometric field data

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Interstitial pore-water temperature dynamics across a pool-riffle-pool sequence