Summary Water table response to rainfall was investigated at six sites in the Upper, Middleand Lower Chalk of southern England. Daily time series of rainfall and borehole water level

general, for cases when the unsaturated zone was greater than 18 m thick, the time lag for a

a critical value, which varied from site to site. For shallower water tables, a linear relationshipbetween the depth to the water table and the water-level response time was evident. The

ring. The majority of rapid responses were observed during the winter/spring recharge period,

c 2006 Elsevier B.V. All rights reserved.

KEYWORDSGroundwater flow;

Water resources

Introduction

The timing and quantity of recharge reaching the water ta-ble has significant consequences for water resources and for

0022-1694/$ - see front matter c 2006 Elsevier B.V. All rights reserved.doi:10.1016/j.jhydrol.2006.04.025

* Corresponding author. Tel.: +44 118 3786350; fax: +44 1189755865.

E-mail address: d.s.lawrence@reading.ac.uk (D.S.L. Lawrence).1 Present address: Water Management Consultants, 23 Swan Hill,

Shrewsbury, Shropshire, SY1 1NN, UK.

Journal of Hydrology (2006) 330, 604620

ava i lab le at www.sc iencedi rec t . com

journal homepage: www.elswhen the unsaturated zone is thinnest and the unsaturated zone moisture content is highest,and were more likely to occur when the rainfall intensity exceeded 5 mm/day. At some sites,a very rapid response within 24 h of rainfall was observed in addition to the longer termresponses even when the unsaturated zone was up to 64 m thick. This response was generallyassociated with the autumn period. The results of the cross-correlation analysis provide statis-tical support for the presence of fissure flow and for the contribution of multiple pathwaysthrough the unsaturated zone to groundwater recharge.observed variations in response time can only be partially accounted for using a diffusive modelfor propagation through the unsaturated matrix, suggesting that some fissure flow was occur-significant water-level response increased rapidly once the depth to the water table exceededwere cross-correlated to investigate seasonal variations in groundwater-level response times,based on periods of 3-month duration. The time lags (in days) yielding significant correlationswere compared with the average unsaturated zone thickness during each 3-month period. In

Aquifer recharge;Cross-correlation analysis;Chalk aquifer;Analysis of water-level response to rainfalland implications for recharge pathways in theChalk aquifer, SE England

L.J.E. Lee, D.S.L. Lawrence *, M. Price 1

School of Human and Environmental Sciences, University of Reading, P.O. Box 227, Whiteknights, Reading RG6 6AB, UK

Received 27 May 2005; received in revised form 10 April 2006; accepted 17 April 2006evier .com/ locate / jhydro l

fully detect response to rainfall. Historically, groundwater

Analysis of water-level response to rainfall and implications for recharge pathways in the Chalk aquifer 605levels have only been routinely recorded on a monthly basiswithin the UK, such that short-term responses of small mag-nitude are neglected and only the broadest annual trends inseasonal water levels are characterised (e.g., Headworth,1972). The work presented here takes advantage of recentlyavailable data for selected boreholes within the Chalkwhere water levels are recorded on a 6-h to daily basis,allowing the daily response to rainfall inputs to be evalu-ated. These data are used to analyse statistically the timelags associated with borehole water-level response to rain-fall using cross-correlation analysis. The time lags are fur-ther evaluated relative to the thickness of the unsaturatedzone associated with each response time at each site.Although the results only establish correlations betweentime series patterns and do not in isolation identify rechargepathways, the response times identified by the analysis maybe interpreted in light of previous field, laboratory and the-oretical investigations of recharge mechanisms in the Chalk.

The Chalk is the most significant aquifer in the UK, pro-viding 15% of the national water supply and 35% of suppliesin the southeast of England. It consists of a soft, fracturedlimestone with a matric porosity of 2540%, but a low mat-ric permeability, typically of the order of 0.110 millidarcys(about 16 mm/day) (Price et al., 1976). The fractures areof highly variable length and aperture and contribute only0.1% to 1% to the total porosity, although they significantlyenhance aquifer permeability (Price et al., 1982). The rela-tive importance of matric flow and fissure flow during therecharge process is not fully understood, although a rangeof geochemical and geophysical experiments and modellingstudies have been previously undertaken. The most impor-tant of these are reviewed in the following sections and pro-vide the physical context and basis for the interpretation ofthe cross-correlation results presented here.

Previous work

Recharge pathways and timing

Recharge pathways and rates of movement through theunsaturated zone of the Chalk have been considered usinggeochemical, isotopic and physical techniques. Smithet al. (1970) measured the natural tritium concentrationin groundwater at various depths in a vertical profile ofthe Upper Chalk. The pattern of the tritium profile in thematrix corresponded well with the pattern of tritium con-centration in rainfall since its introduction in the 1950s, sug-the movement of pollutants into groundwater. The combi-nation of low hydraulic gradients and large variations inChalk topography can create an unsaturated zone more than100 m thick in interfluve areas. These thicknesses can pro-duce significant time delays to and attenuation of rechargeresulting from rainfall, although the magnitude of these ef-fects is poorly understood and not well quantified. This is, inpart, due to the complexity of unsaturated flow in fissureddual-porosity systems such as the Chalk (Price, 1987) andto the relative difficulty of monitoring the matric potentialand water content throughout the thickness of the unsatu-rated zone. However, another major limitation is the pau-city of water-level data sampled at sufficient frequency togesting that the majority of recharge moved through theChalk matrix. However, tritium was also detected well be-low the depth predicted by assuming matric flow alone.Smith et al. (1970) concluded that flow in the Chalk wasdominated by matric seepage, but that approximately 15%of the recharge had bypassed the matrix in fissures. Wellings(1984b) estimated the rate of movement through the unsat-urated zone as 1 m/yr based on deuterium as a tracer inthe Upper Chalk (Hampshire), also demonstrating that ni-trate and chloride moved through this zone at similar rates.This flux rate is also supported by the cyclic variations inwater isotope ratios with depth found at the same site byWellings and Bell (1980). Gardner et al. (1990) found a sim-ilar rate of 0.8 m/yr for the Upper Chalk (Berkshire), alsobased on deuterium.

Price et al. (2000) predicted that, in a uniform Chalk pro-file receiving steady recharge, fissure flow would be morelikely to be initiated near the water table, where pore suc-tions are lower, than higher up the unsaturated zone in thevicinity of the ground surface. This prediction was partlyconfirmed by more recent geochemical work (Johnsonet al., 2001), showing that recharge delivery by fissure flowcan be significant when the water table is shallow. Johnsonet al. studied soil samples, shallow cores and groundwaterquality on a weekly basis at an Upper Chalk site in Hamp-shire and found that peaks in isoproturon and chlorotoluronconcentrations during the winter recharge period could bedetected in groundwater samples where the water tablewas within 45 m of the ground surface. However, if thewater table was deeper (920 m below the surface), littleor no herbicide was detected; this can be interpreted asimplying that low suctions, and hence fissure flow, couldnot extend from this depth to the soil zone.

Field experiments using soil physical methods have pro-vided further insight into the relative contributions of ma-tric and fissure flow in the Chalk. Wellings and Bell (1982),Wellings (1984a,b), Gardner et al. (1990) and Cooperet al. (1990) monitored soil suction (pressure potential)and water content at several sites on the Upper and MiddleChalk in the UK at frequencies of up to twice weekly. Therelationship between hydraulic conductivity and pressurepotential was also determined at four of the sites. In gen-eral, hydraulic conductivity rose rapidly (>10 mm/day)when the pressure was above 5 kPa (0.5 m suction head)reaching a maximum value of over 100 mm/day. Below5 kPa, the conductivity was 16 mm/day and changedvery little as pressure potential decreased. The large in-crease in hydraulic conductivity at water potentials greaterthan 5 kPa is interpreted as resulting from fissure flow.The near constant value of hydraulic conductivity whenthe pore pressure drops below 5 kPa corresponds to flowin the matrix, with no drainage of matric pore space occur-ring at the pore-water suctions involved so that the hydrau-lic conductivity remains unchanged (Price, 1987). Therelationship between volumetric water content and matricpotential was also investigated in the field experiments.As pressure fell from 5 to 70 kPa the Chalk water contentshowed a negligible decrease below 1 m depth, supportingthe hypothesis of a non-draining matrix (Gardner et al.,1990).

The frequency of fissure flow appears to vary widely be-tween sites with different lithologies. At Fleam Dyke on the

606 L.J.E. Lee et al.Cambridgeshire Middle Chalk, fissure flow was observed tooccur at pore pressures greater than 5 kPa in the wintermonths (Jones and Cooper, 1998). Lysimeter and soil waterobservations indicate that approximately 30% of flow oc-curred through fissures throughout the year, and that duringthe winter months (when no soil moisture deficit was pres-ent) 50% of flow occurred through fissures. In contrast, atBridgets Farm in the Upper Chalk of Hampshire, the porepressure exceeded 5 kPa on only one occasion during thewinter, suggesting that fissure flow rarely occurs (Wellings,1984a). The Upper Chalk generally has a much higher matricpermeability than the Middle Chalk. However, measure-ments were generally made only weekly, so short periodsof fissure flow could easily have been missed.

In a more recent study, rapid response to rainfall in thesoil zone (i.e., the upper few metres of the unsaturatedzone) was observed by Hassan and Gregory (2002). Theyinvestigated the relationship between rainfall and half-hourly measurements of soil water content and matric po-tential based on a core sample from the Hampshire UpperChalk. Most hydraulic changes in the soil zone were ob-served a few hours after rainfall had started. During wetperiods, increases in water content following rainfall wereseen at a depth of 1 m after only 3 h, suggesting that bypassflow had occurred. Comparison with weekly measurementsof soil water and matric potential revealed that lower fre-quency measurements did not detect these responses.

Hourly water potentials, at a depth of up to 3 m, weremonitored by Haria et al. (2003) at two sites in the UpperChalk in Hampshire. The monitoring sites were on an inter-fluve where the water table has a depth of 18 m and in a dryvalley where the water table is within 4 m of the surface.They found evidence for both rapid preferential flow andmatric flow in the dry valley site, but found evidence formatric flow alone in the interfluve site. This differencewas attributed to the capillary fringe sustaining a highermoisture content in the unsaturated zone at the shallowgroundwater site. However, as the water-potential investi-gations did not extend through the full depth of the inter-fluve unsaturated zone, the possibility that fissure flow isinitiated beneath the interfluve at greater depth (i.e.,nearer to the water table, as suggested by Price et al.(2000)) cannot be discounted.

Evidence for substantial fissure flow in the Chalk alsocomes from observations of bacterial contamination ofChalk aquifers after heavy rainfall (Downing et al., 1979).Foster (1975) sought to explain the tritium concentrationprofile observed by Smith et al. (1970) as a result of diffu-sion processes between fissure and matrix. He assumed thatrecharge to the Chalk occurred predominantly through fis-sures and suggested that maximum diffusion into the matrix(corresponding to observed peaks in tritium concentration)would occur at any level where fluid movement was re-tarded by lower hydraulic conductivity values and smallerfissure apertures. Reeves (1979) also supported the domi-nance of fissure flow over matric flow in the Chalk, suggest-ing that the majority of flow occurs through microfissures(120 lm aperture) in the unsaturated zone and throughmacrofissures (210 mm aperture) in the saturated zone.He suggested that over 8090% of infiltration would occurat rates of less than 1 mm/day, so that the microfissure sys-tem will be saturated for most of the year and macrofissuresystems will become active only when infiltration exceedsthe ability of microfissures to transmit available water.However, Reeves was unable to produce any direct physicalevidence for the existence of microfissures.

Aperture size and the rate of effective rainfall shouldcontrol the flow of water in fissures (Price et al., 1993).The air-entry pressure for the matrix is very high andthe matric pores are normally filled with water close toground level. Since the maximum possible vertical hydrau-lic gradient in the unsaturated zone is less than unity, ifthe rate of effective rainfall supply to the ground surfaceis greater than the hydraulic conductivity of the matrix,then the fissures will become saturated and will conductwater.

More recent studies of fluid flow in fissures suggest thatflow may not be controlled by the size of the fissure aper-ture alone. Tokunaga and Wan (1997) investigated filmflow on the surfaces of fractures in a block of Bishop Tuffover a range of near-zero matric potentials, i.e., when thematrix of the block was saturated. The block had a matricporosity of 33% and a saturated hydraulic conductivity ofabout 0.003 m/day, similar to the properties of chalk.Average velocities of the water film were 240 m/day or103 times greater than matric flow under a unit gradientwhen matric suctions were below about 250 Pa (25 mm).The high velocities suggest that film flow could be a prin-cipal factor contributing to fast flow in unsaturated Chalkfractures. However, suctions as low as 25 mm are rarelyobserved in the unsaturated Chalk (Price et al., 2000),and if they occurred, fully saturated fissure flow wouldtake place. Price et al. (2000) carried out laboratoryexperiments using acoustic techniques and resin impregna-tion methods to investigate whether microfissures andmacropores are present within Chalk blocks and serve aspossible sites for short-term water storage. Results indi-cated that microfissures are absent, and although macrop-ores are present, they would not drain under normalsuctions. The findings also suggest that drainage occursthrough fissure walls and that the irregular surfaces ofthe fissures provide extra storage previously noted in Chalkcatchment water balance studies (e.g., Lewis et al., 1993).Tokunaga and Wan (2001) subsequently derived a similarmechanism for the initiation of fissure flow to that pro-posed by Price et al. (2000).

In summary, the present evidence seems to suggest that:

matric flow is the normal mode of recharge through theunsaturated zone of much of the Chalk aquifer inEngland;

reasonably rapid response of the water table can occurwhen matric flow is occurring, as a result of the pistondisplacement process;

fissure flow will be initiated at those times and in thoseareas and depths of the unsaturated zone when andwhere the pore-water suction falls to a level where nar-rower fissures can fill with water (suction typicallyaround 0.5 m of water). This will happen, in essence,when the recharge rate locally approaches or exceedsthe hydraulic conductivity of the matrix, so that thematrix is unable to conduct water at a sufficiently highrate without the hydraulic gradient approaching unity(Price et al., 1993, 2000)

interpretation of isotopic and geochemical data is com-plicated by the potential for molecular diffusion betweenmatric pore water and water in fissures.

Recharge estimation based on field hydrographdata

Rushton and Ward (1979) proposed a method for estimatingrecharge that has been widely adopted in Chalk groundwa-ter modelling. This method permits recharge to occur evenin periods of soil moisture deficit, in line with field observa-tions from lysimeters and observation boreholes (Head-worth, 1972). They modelled Chalk recharge by calibratingthe percentage of effective and actual precipitation thatentered the aquifer to match observed field hydrographs.They concluded that a by-pass flow of 15% of the actual pre-cipitation (>5 mm/day) plus 15% of the effective precipita-

using a matric diffusion model to highlight those timeswhen fissure flow must be contributing to the observedresponse, as it is too rapid to be accounted for by matricdiffusion alone.

Study sites

This study focused on sites in the Chalk of southern England(Fig. 1) where daily, and in some cases 6-h, observations ofgroundwater levels were available. The sites cover a largearea of the Chalk geographically and sample a range of dif-ferent Chalk lithologies. Water-level response to rainfallwas investigated at six sites where the Chalk is unconfinedand extends to the ground surface. A summary of the bore-hole site locations, the rain gauges associated with eachborehole and the length of the available time series is givenin Table 1 and further site details are provided in the follow-ing paragraphs. Fig. 2 shows part of the records of rainfall

iveon,

Analysis of water-level response to rainfall and implications for recharge pathways in the Chalk aquifer 607tion provided the best fit.A few previous investigations of delay in the unsatu-

rated zone have compared rainfall and groundwater-levelresponse time series. Calver (1997), when studying theRhee and Cam Chalk catchments in southern England,found the recharge pulse to be distributed over a numberof months, with the most significant response occurring inthe first month of effective rainfall input. Studies byOakes (1981), using a transfer function to model unsatu-rated zone behaviour, also showed that at the majorityof sites the greatest correlation occurred at a time lagof 1 month. These studies relied on monthly or weeklygroundwater-level records. Studies using more frequentdata have shown more rapid water-level responses. Head-worth (1972) examined Chalk well recorder charts fromHampshire in the 1960s to detect water-level responsesto rainfall. He found that wells with deep water tables(>80 m) have the longest response interval (1522 days)and shallow wells (

Table 1 Details of study sites

Borehole grid reference Rain gauge gridreference

Chalk lithology Ground surfaceelevation(m AOD)

Monitoring perioda Dominantlanduse

Depth to averagewater table (m)

Average waterlevel fluctuation(m)b

1. Preston Candover,HampshireSU60714186

Preston CandoverSU60764221 @0.35 km

Seaford Chalk(Upper Chalk)

100.20 March 00August 02n = 914

Ruraldevelopment

3.42 3

2. BroadhalfpennyDown, HampshireSU67671663

CowplainSU69101140 @5.42 km

Seaford Chalk(Upper Chalk)

116.50 March 00August 02n = 914

Arable 66.61 34

3. Chilgrove, SussexSU83501440

WaldertonSU78611034 @6.35 km

Seaford & LewesChalk (Upper/Middle Chalk)

77.18 March 00January 02n = 686

Rough grazing 22.71 26

4. Houndean, SussexTQ39301020

HousedeanTQ36900930 @2.56 km

Upper/MiddleChalk

40.01 March 00August 02n = 914

Arable 26.52 15

5. Wolverton, KentTR26804290

Temple EwellTR28404430 @2.12 km

Middle Chalk 47.09 December 95August02 n = 2349

Grazing/ruraldevelopment

9.74 8

6. Ogbourne, BerkshireSU18427646

Ogbourne St.GeorgeSU19107620 @0.72 km

Lower Chalk 156.00 September 91November 94 n = 996

Grass/linseed/wheat

15.72 14.5

n, number of data points.a Daily data.b Calculated peak to trough for monitoring period only.

608L.J.E

.Le

eetal.

Analysis of water-level response to rainfall and implications for recharge pathways in the Chalk aquifer 60930

40

50

60

ll (m

m)Broadhalfpenny Down is located above a dry valley nearthe crest of the South Downs. It is an interfluve site and isapproximately 10 km from the headwaters of the WallingtonRiver which rises from springs at the boundary between theChalk and Tertiary clays. The Broadhalfpenny Down bore-hole is situated on a grass verge between the main roadand arable land.

0

10

20

Jan-00 Apr-00 Jun-00 Sep-00 Dec-00 Mar-01 JDate

Ra

infa

0

10

20

30

40

50

60

70

80

90

100

Feb-00 May-00 Aug-00 Nov-00 FeDat

Rai

nfal

l (mm)

0

10

20

30

40

50

60

70

80

90

100

110

120

Jan-00 Apr-00 Jun-00 Sep-00 Dec-00 Mar-01Dat

Rai

nfal

l (mm)

Figure 2 Rainfall and groundwater-level data for Broadhalfpen50

60

70

80

90

100

leve

l (m A

OD)

RainfallGroundwater levelThe Houndean borehole is located in the Brighton Chalkblock, approximately 2.4 km from the River Ouse. Studiesof the Brighton Block have shown transmissivity values rang-ing from 50 m2/day to 2000 m2/day and a storage coeffi-cient of approximately 0.01 (Robins et al., 2001). Pumpingtests in the Winterbourne Valley at Houndean Farm closeto the Houndean borehole indicate transmissivity values

un-01 Sep-01 Dec-01 Mar-02 Jun-020

10

20

30

40

Gro

un

dwat

er

b-01 May-01 Aug-01 Nov-01e

0

10

20

30

40

50

60

70

80

90

Gro

un

dwat

er le

vel (m

AO

D)

Jun-01 Sep-01 Dec-01 Mar-02 Jun-02e

0

10

20

30

40

Gro

undw

ater

leve

l (m A

OD)

ny Down (top), Chilgrove (middle) and Houndean (bottom).

1ays

610 L.J.E. Lee et al.0

2

4

6

8

10

12

1 2 3 4 5 6 7 8 9 10

D

Rai

nfal

l (mm/

day)

Rainfall

0.4

0.5greater than 1000 m2/day (Sussex River Authority, 1974).The Houndean site is in a dry valley approximately 2 m froman unsurfaced track. The land use is mainly arable, withsome deciduous woodland in the valley. Some lateral flowmay occur from surrounding hills towards the borehole atthis site.

The Chilgrove borehole is situated in a dry valley con-nected to the River Lavant. The borehole occasionally over-flows and the bourne very occasionally as in 1994 and 2000 rises as high up the valley as Chilgrove. Water level datafrom Chilgrove have been interpreted by Allen et al. (1997)as showing various mechanisms for storage in the Chalk. Forexample, a period of heavy rainfall during 19891990 (fol-lowing a severe episode of drought) caused a sharp increasein water levels in the well, followed by an equally rapid de-cline. Allen et al. (1997) suggest that this type of responseto recharge would be expected if only the large fractureshad been replenished and subsequently drained during therecharge period.

The Ogbourne borehole lies about 2 km to the north-westof the source of the River Og in the Lower Chalk. The River

-0.2

-0.1

0

0.1

0.2

0.3

1 2 3 4 5 6 7 8 9 10 1

Days

Corr

elat

ion

Figure 3 Example of cross-correlation results (bottom) for syntheThe significance level for correlations between the two data sets is dthe number of values.1 12 13 14 15 16 17 18 19 2014.6

14.8

15

15.2

15.4

15.6

15.8

16

16.2

Wat

er le

vel (m

)

Water levelOg lies in the Kennet Catchment, which has transmissivityvalues ranging from 50 to 2000 m2/day; the lower valuescorrespond to interfluves and the highest to dry valleys (Al-len et al., 1997). Owen and Robinson (1978) note that trans-missivity also varies with depth, with most large fracturesand the highest transmissivity being located in the upper60 m of the aquifer below ground level. A pumping stationat Ogbourne St. George is approximately 750 m from theborehole.

The Wolverton borehole is situated in a flat grassy pad-dock on the side of a winterbourne valley. A winterbournestream which is tributary to the River Dour occasionallyflows along the valley floor approximately 25 m from thesite. Deciduous trees separate the paddock from a mainroad approximately 100 m further up the valley.

Rainfall and water-level data for Chilgrove, Preston Can-dover, Houndean and Broadhalfpenny Down were availablefor the 2-year period January 2000 to August 2002. Recordsfor Ogbourne were available for September 1991 to May1995 and for Wolverton from December 1995 to August2002. Water-level data were obtained from borehole loggers

1 12 13 14 15 16 17 18 19 20

Significance level

tic rainfall and water-level response to one rainfall event (top).etermined by the length of the data series as 2/pN, where N is

16

e

Analysis of water-level response to rainfall and implications for recharge pathways in the Chalk aquifer 611-0.4

-0.2

0

0.2

0.4

0.6

0.8

0 2 4 6 8 10 12 14

Tim

Corr

elat

ion

0.8or from the UK Environment Agency regional telemetry net-work. Rainfall data were taken from the nearest storage ortipping-bucket rain gauge maintained by the EnvironmentAgency, as indicated in Table 1.

Methodology

Cross-correlation is a time series technique which can beused to evaluate the statistical correlation between twosets of data at different time lags. Before the data areanalysed, any daily trend in the water-level time series isremoved by differencing consecutive values. The cross-cor-relation of daily rainfall and change in groundwater levelcan reveal the significance of the water-table response torainfall after a given number of days, and can also allowthe time taken for the first water-table response to rainfallto be calculated. Cross-correlations were calculated usingthe following relationship:

qyk Ext lxytk ly

rxry1

-0.4

-0.2

0

0.2

0.4

0.6

0 2 4 6 8 10 12 14

Time

Corr

elat

ion

Figure 4 Cross-correlograms for Houndean, win18 20 22 24 26 28 30 32 34 36

lag (days)

Significance Levelwhere qy(k) = cross-correlation at time lag k,k = 0, 1, 2 . . . n time lag between the two series (days),xt = observed rainfall at time t, yt = observed water level attime t, lx = mean of rainfall series, ly = mean of water-levelseries, rx = standard deviation of rainfall series, ry = stan-dard deviation of water-level series.

Significant correlations at the 95% confidence level aretaken to be those greater than the standard error 2/pN(Diggle, 1990), where N is the number of values in the dataset. This is effectively testing the hypothesis of no correla-tion and assumes that the variance is finite and normally dis-tributed about a mean of zero. Autocorrelations were firstcalculated to identify any significant correlations occurringwithin the individual rainfall and water-level response datasets. Fig. 3 shows an example of a cross-correlation be-tween synthetic rainfall and water-level data sets. Rainfalloccurs on Day 1 only and the water level declines untilDay 4 when the pulse reaches the water table. During thisperiod the correlation between rainfall and water-level re-sponse is negative for the first 3 days and becomes positiveon Day 4. On Days 57 the water level continues to rise and

16 18 20 22 24 26 28 30

lag (days)

Significance Level

ter 2000/1 (top) and autumn 2001 (bottom).

14

e

ign

612 L.J.E. Lee et al.-0.4

-0.3

-0.2

-0.1

0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0 2 4 6 8 10 12

Tim

noitalerr

oC

S

0.7

0.8the positive correlation becomes statistically significant.As the rate of water-level change starts to decrease onDay 8, the values fall below the significance level for thedata set, and as the water level starts to recess, the corre-lation again becomes negative. The negative lags are, how-ever, generally ignored in cross-correlation analysis as theydo not provide any additional information: they simply rep-resent those times when the two series are out of phase,rather than in phase.

Results

Cross-correlation of rainfall and water-levelresponse data

Cross-correlation analyses of rainfall and water-level re-sponse were carried out using the daily time series, whichwere subdivided into seasonal data sets of 3-month dura-tion. Sets of 3-month duration were chosen so that seasonaldifferences in water-level response could be identified. Theshape of the resulting correlograms varies significantly be-

-0.4

-0.3

-0.2

-0.1

0

0.1

0.2

0.3

0.4

0.5

0.6

0 2 4 6 8 10 12 14

Time

noitalerr

oC

Sign

Figure 5 Cross-correlograms for Chilgrove, win16 18 20 22 24 26 28 30

lag (days)

ificance Leveltween sites. For example, in Sussex, the Houndean cross-correlogram tends to have a few very distinct correlations,particularly at short time lags (Fig. 4), whereas at Chilgrovethe most significant correlations are weaker, but are persis-tent over several consecutive time lags (Fig. 5).

For the data sets considered (i.e., daily series over a 3-month period) correlations >0.2 were taken to be statisti-cally significant. As part of the study, cross-correlationswere also carried out using data sets 1 month in length.However, these short periods were not sufficient to identifysignificant correlations at a time lag greater than approxi-mately 1 week, should they be occurring. The ability ofthe cross-correlation technique to detect significant corre-lations at larger time lags is limited by short data sets. Asummary of significant time lags between rainfall and waterlevel is shown in Table 2. Only results for spring 2000 tosummer 2002 are shown for Wolverton to ensure consistencywith the monitoring period at four of the other sites.

An important point to note is that the time lag for signif-icant correlations is heavily dependent on the season, withmuch shorter time lags (some

Candover.

Table

2Statisticallysignificantcross-correlations(indays)

betw

eendaily

rainfallan

dwater-table

response

timeseries,

calculatedseasonally

forSouthern

Chalksites

1.Preston

2.Broad

halfpennyDown

3.Chilgrove

4.Houndean

5.Wolverton

6.Ogb

ourne

Spring2000

4,5,

6,7,8,16,18,19

8,9,

10,17,

18,19,20

10,11,18,19,20,21

30,

6,7,

8,9,

10Autumn1991

7,8,

9,10

Summer2000

Nocorrelations

Nocorrelations

Nocorrelations

0Nocorrelations

Winter1991/2

6,7,8

Autumn2000

0,1,2,

3,4,

5,6,

7,8,

90,26,27

0,1,

20,

1,2,

190,

1,2,3,

4,5,

21,22,23,24

Spring1992

2,5,9,

16

Winter2000/1

0,1,2,3,

4,5,

6,7,

80,1,2,

3,4,

51,

2,3,

4,5,6,

71,2,

360,1

Summer1992

10Spring2001

0,4,

5,6,8,

11Nocorrelations

0,1,

2,3,

4,5,

6,7,

8,9

1,2,3,

4,5

0,1,

2,3,

4,5,

6,9,

10,11,12,

14,16,17,18

Autumn1992

1,2

Summer2001

2118

2,6,8

4,21,36

12Summer1993

Nocorrelations

Autumn2001

5,6,8,11,15,18

14,

17,29

18,19,20,21,22,23,24,

25,26,27,28,29,30,31

12,25

17Autumn1993

2,3

Winter2001/2

0,1,

2,3,4,

5,6,

7,8,

9,10,

11,12,13,14,15,16

1,2,

9,10,11

Nodata

1,2,

3,4,

131,

2,3,

4,5,6,

7,8,

9,10,11,12,13,

14,15,16,17,18

Winter1993/4

0,1,2,

3

Spring2002

Nocorrelations

Nocorrelations

Nodata

Nocorrelations

Nocorrelations

Spring1994

1,2,3,

5,6

Summer2002

015,22

Nocorrelations

0,7

Nocorrelations

Summer1994

2,15

Autumn1994

Nocorrelations

Maxim

um

cross-correlationisindicatedwithbold

typeface.ResultsforWolvertonareshownforSpring2000

toSummer2002

only.

Analysis of water-level response to rainfall and implications for recharge pathways in the Chalk aquifer 613During some seasons, a delayed secondary water re-sponse is observed. The maximum time period betweensignificant cross-correlations for each borehole is shownin Table 3. This period varies from 1 day (i.e., cross-cor-relations occur on consecutive days) to 34 days. Duringautumn 2000, a delay greater than 2 weeks betweenpulses is seen at Houndean, Broadhalfpenny Down andWolverton. For example, during autumn 2000, the waterlevel responds at lags of zero and of 26 days at Broadhalf-penny Down (Fig. 6). During the same period, at Houn-dean, the water level responds at a lag of zero and 19days after rainfall (Fig. 6). The remaining two sites, Chil-grove and Preston Candover, show a maximum time delayof 1 day. During autumn 2001 a time delay of nearly 2weeks is seen at two sites, Houndean and BroadhalfpennyDown.

Unsaturated zone thickness and water-levelresponse

The first statistically significant correlation at each site isplotted as a function of the average unsaturated zone thick-ness for each season in Figs. 7 and 8, in order to illustratethe change in time lag with change in unsaturated zonethickness. At Chilgrove, Houndean, Broadhalfpenny Downand Ogbourne, the unsaturated zone thicknesses vary from3 to 73 m (Fig. 7). The majority of correlation points suggesta major increase in time lag when the water table is below acritical depth, which varies from site to site. This trend canbe fitted with a bilinear function, as illustrated in the fig-ures. A very rapid water-table response is indicated at thesefour sites with the first significant correlations betweenrainfall and water level occurring within 24 h of a rainfallevent. For example, at Broadhalfpenny Down, a water-levelresponse is observed at a depth of 64 m within 1 day of arainfall event, during autumn 2000. However, these shorttime lag correlations are followed by a rapidly increasingtime lag once the unsaturated zone has exceeded a criticalseasons than in dry seasons. Dry seasons are generallysummer and autumn, but they also include periods in winterand spring where there has been generally low rainfall. Forexample, spring of 2000 was wet, but followed a relativelydry winter when rainfall from October to March in southernEngland was less than 85% of long-tem average (NERC,2000a). Similarly, autumn of 2000 was exceptionally wet,with rainfall in southern England in September and Octoberapproaching 23 times the long-term average (NERC,2000b).

These effects can be observed at Houndean (Fig. 4)where the first water-level response to rainfall is observedat a time lag of 1 day during winter 2000/1. In contrast,the first water-level response at Houndean during autumn2001, a much drier period, is observed at a time lag of 12days. At Chilgrove (Fig. 5) a similar response can be seen;during winter 2000/1, water-level response occurs within1 day, whereas the first significant response takes 18 daysduring the following autumn. The time lag for water-levelresponse can also vary greatly between sites; in the summerof 2001, a water-level response to rainfall is observed at alag of 2 days at Chilgrove and at 21 days at Preston

thickness. The trend lines fitted for each site are unique; nosingle relationship between unsaturated zone thickness andtime lag is present at all Chalk sites.

At Wolverton and Preston Candover (Fig. 8), where theunsaturated zone is always less than 18 m thick, the corre-lations display two distinct trends. The first type of response

Table 3 Maximum time period between significant cross-correlations for all boreholes

1. PrestonCandover

2. BroadhalfpennyDown

3. Chilgrove 4. Houndean 5. Wolverton 6. Ogbourne

Spring 2000 8 7 7 6 Autumn 1991 1Summer 2000 Winter 1991/2 1Autumn 2000 1 26 1 17 16 Spring 1992 7Winter 2000/1 1 1 1 34 1 Summer 1992 Spring 2001 4 1 1 3 Autumn 1992 1Summer 2001 4 17 Summer 1993 Autumn 2001 4 12 1 13 Autumn 1993 1Winter 2001/2 1 7 No data 9 1 Winter 1993/4 1Spring 2002 No data Spring 1994 2Summer 2002 7 7 Summer 1994 13

Autumn 1994

Results for Wolverton are shown for Spring 2000 to Summer 2002 only.

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0

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Time lag (days)

noitalerr

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Significance Level

614 L.J.E. Lee et al.0.5

0.6-0.2

-0.1

0

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0.3

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Time l

noitalerr

oC

S

Figure 6 Cross-correlograms showing delayed secondary responsautumn 2000 (bottom).16 18 20 22 24 26 28 30

ag (days)

ignificance Level

e at Houndean, autumn 2000 (top), and Broadhalfpenny Down,

oz d

Analysis of water-level response to rainfall and implications for recharge pathways in the Chalk aquifer 6150

5

01

51

02

52

0302010

etarutasnU

Tim

e de

lay

(day

s)occurs within 2 days of a recharge event, and the time lagappears to be independent of the thickness of the unsatu-rated zone. The second type of response occurs some timeafter this rapid response and as the unsaturated zone in-creases in thickness, the time taken for the first water-tableresponse also increases. The relationship between unsatu-rated zone thickness and the time delay for water-level re-sponse is approximately linear at these sites. A statisticaloutlier occurs during autumn 1996, following a very dry epi-sode when water-level response to rainfall took 21 daysthrough an unsaturated zone thickness of 18 m. At PrestonCandover, the unsaturated zone thickness is always lessthan 11 m, and the majority of water-level responses occurwithin 1 day of a rainfall event.

In Figs. 7 and 8, the first significant correlation was se-lected for plotting against unsaturated zone thickness. Thisvariable was chosen over the maximum significant correla-

ynnepflahdaorB enruobgO

Figure 7 Time lag for first statistically significant water-level resOgbourne, Houndean and Chilgrove.

0

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01

51

02

52

03

86420

oz detarutasnU

Tim

e de

lay

(day

s)

notserP

esnopser fo epyt dn2)srh 84 retfa(

Figure 8 Time lag for first statistically significant water-level res(responses for Wolverton shown for entire monitoring period).0807060504

)m( ssenkciht en

a ta gal emit ni esaercnI detarutas lacitirc

ssenkcihttion because in some cases the maximum correlation occurswithin the initial response and in some cases within the sec-ondary response. Using the first significant correlation en-sures that the same information is plotted for eachcorrelogram.

Discussion

Unsaturated flow

Flow through the unsaturated zone may occur through thematrix, fissures, or a combination of both pathways. In prin-ciple, matric response may take the form of flow throughpores or may occur as a matric pulse, which displaces waterby a piston-type movement. Matric pore flow is extremelyslow as the saturated hydraulic conductivity of the matrixis only 35 mm/day and the porosity is high (Price et al.,

naednuoH evorglihC

ponse vs. unsaturated zone thickness at Broadhalfpenny Down,

028161412101

)m( ssenkciht ennotrevloW

esnopser fo epyt ts1)srh 84 nihtiw(

ponse vs. unsaturated zone thickness at Preston and Wolverton

1976). Movement of water in a matric pulse (i.e., diffusiveflow as suggested by Barker, 1993) produces a faster re-sponse, but the speed of this response depends on the watercontent of the unsaturated zone matrix. Fissure flow can oc-cur at rates of 0.1100 m/day but is thought to occur in theChalk only when rainfall intensity is greater than approxi-mately 5 mm/day, i.e., when rainfall intensity is greaterthan the hydraulic conductivity of the matrix.

Some of the responses observed at many of the studysites are much too rapid to be attributed to matric poreflow. The most rapid response to rainfall is observed atBroadhalfpenny Down where a water-level response oc-curred at 64 m depth within 1 day of a rainfall event.The unsaturated hydraulic conductivity Ku is approximately35 mm/day; so, if matric pore flow were the only route forwater flow through the system, the expected response timewould be many years. However, the speed of propagation ofa matric pulse (piston displacement) through the Chalk ismuch more rapid. The time (t) for a significant responseover a distance (z) in a one-dimensional diffusive system(e.g., Barker, 1993; Price et al., 2000) is given as:

t z2C

2Ku2

plotted in Fig. 9 together with the unsaturated zone thick-ness (z) and response time (t) for the six boreholes in thesouthern Chalk. The fastest possible matric response isgiven by the (Ku = 0.01 m/day, C = 0.0001 m

1) curve inFig. 9, obtained using the highest possible value of matricKu and the lowest possible value of C. This curve shows thatall the responses at Broadhalfpenny Down and several of theresponses at Houndean, Wolverton and Chilgrove cannot beattributed to a matric response. Water-level responses atthese sites during several periods are too rapid for a matricpulse and can therefore only result from fissure flow. Themaximum daily rainfall intensity was greater than 5 mm/day (i.e., greater than the assumed hydraulic conductivityof the saturated matrix) during all these periods of likely fis-sure flow.

Unsaturated zone thickness

The plot of water-level response time vs. unsaturated zonethickness (Fig. 9) shows that the time lag for the significantresponse to rainfall is dependent on the thickness of theunsaturated zone. The four sites with the deepest unsatu-rated zone show a rapid increase in time lag when the unsat-urated zone reaches a critical thickness. One would expect

4oz

ed

ho

616 L.J.E. Lee et al.where C is the specific moisture capacity (dh/dw), repre-senting the drainage of bulk volume h per metre of watersuction w. Eq. (2) represents the solution of a one-dimen-sional diffusion equation, with the diffusivity D = K(w)/C(w). For typical suctions in the Chalk, in the range 10150 kPa, laboratory values of C lie in the range 0.00010.0007 m1 (Price et al., 2000). Inserting these C values intoEq. (2), along with the z and t values for the most rapid re-sponse at Broadhalfpenny Down, gives a range of requiredKu values of 0.21.4 m/day. This value is much greater thanthe saturated hydraulic conductivity of the Chalk matrix.Therefore, this response cannot be attributed to propaga-tion through the matrix. Solutions for the matric pulse equa-tion (Eq. (2)) for a range of feasible values of Ku and C are

0

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0302010detarutasnU

Tim

e de

lay

(day

s)

ynnepflahdaorB enruobgO nuoH

d/m 1000.0=uKm 1000.0=C 1-

d/m 10.0=uKm 100.0=C 1-

Figure 9 Time delay vs. unsaturated zone thickness for all boredifferent values of Ku and C, as indicated.the time lag to be dependent not only on the unsaturatedzone thickness, but also on the degree of saturation of thiszone. The majority of rapid responses are observed duringthe winter/spring recharge period when the unsaturatedzone is not only at its thinnest, but when the water contentof the unsaturated zone is also likely to be at its highest.The majority of slower responses are observed duringsummer/autumn periods when the unsaturated zone is atits thickest and the water content is also likely to be lowest.Given that specific moisture capacity (C) decreasesand unsaturated hydraulic conductivity increases withincreasing water content, the apparent change in time lagwith unsaturated zone thickness may actually reflect thisco-variation with water content. The findings support those

080706050)m( ssenkciht en

na evorglihC notserP notrevloW

d/m 10.0=uKm 1000.0=C 1-

les. Solutions of the matric pulse equation (Eq. (2)) are given for

Analysis of water-level response to rainfall and implications for recharge pathways in the Chalk aquifer 617of Lewis et al. (1993) and Price et al. (2000) which indicatethat there can be significant storage in the unsaturated zoneof the Chalk.

The abrupt change in slope in the curves suggests a pos-sible change in the dominant flow pathway. This change inmechanism may be from fissure to matric flow, as the waterpotential decreases, or from fissure flow to partial fissurethin film flow. Previous work by Gardner et al. (1990) indi-cates that as water potential increases in the Chalk, thehydraulic conductivity becomes too great to be attributedto matric flow and this indicates that fissure flow has beeninitiated. Their graph of conductivity as a function of matricpotential exhibits a sharp break in slope when flow changesfrom matric to fissure flow. The transition from matric tofissure dominated flow may also explain the change in slopeobserved in the unsaturated zone vs. time-lag graph. Theresponses occurring above the change in slope can be attrib-uted to matric flow but they do not lie on a quadratic curve(which would be expected from Eq. (2), when t is plottedagainst z). This is because each matric pulse curve is plottedfor constant Ku, whereas the value of Ku and the value of Care changing seasonally, and so are different for each pointon the curve.

The response line plotted for Broadhalfpenny Down,which is too rapid for any point to be attributed to pistonflow through the matrix, shows a clear change in slope. Inthis case, it is worth noting that the solution for the matricequation giving the fastest response in Fig. 9 is dependanton the values for Ku and C being at the known extremesfor the Chalk. Given the variation in Chalk hydraulic proper-ties at differing lithologies and location, values for theseparameters may cover a wider range than can be measuredfrom a limited number of sample locations. In this case,some of the longer time lags could be attributed to a matricresponse and the transition from matric to fissure flow mayaccount for the change in slope.

As no field measurements of water potential are avail-able for the study sites during the monitoring period, it isnot possible to ascertain whether the actual potential isof the correct magnitude to initiate fissure or matric flowduring different seasons. Without these field measure-ments, it cannot be discounted that the responses detectedthrough cross-correlation are all actually a result of fissureflow in the Chalk. Recently, however, Mathias et al.(2005) have shown that a fissure-diffusion model such asthat proposed by Barker and Foster (1981) cannot replicateobserved solute profiles unless an unrealistically close frac-ture spacing is assumed. They conclude that matric flow is asignificant process in the unsaturated zone of the Chalk.

No general trend linking the statistically significant timedelay to unsaturated zone thickness is observed for theChalk, nor would it be expected. The different trend lineshapes and transition points between rapid and delayedresponse will be influenced by local Chalk properties suchas chalk lithology, proximity to rivers and dry valleys Oxfordcomma and Quaternary weathering. At two of the bore-holes, Wolverton and Preston Candover, the water tableusually responds rapidly to rainfall. This response can beattributed to fissure flow. However, the response time isoccasionally greater than 2 days. This type of response lieson an approximately straight line from the origin. Compari-son of this line with values obtained using a low value for Ku(0.0001 m/day) and a low value for C (0.0001 m1) (Fig. 9)suggests that these values show matric response behaviourunder fairly dry conditions.

Delayed secondary responses

In several boreholes an initial rapid water-level response torainfall is followed by a second delayed response. It is pos-sible that the two distinct responses indicate separate fis-sure flow and matric pulse responses. Two responses areclearly seen during autumn 2000 at Houndean and at Broad-halfpenny Down (Fig. 10). If we assume that the correlationsobserved at a lag of one and 2 days are fissure responses,the matric flow would be represented by the response seenat 19 days at Houndean and 26 days at BroadhalfpennyDown. Inserting these times for secondary response intoEq. (2), along with the range of C values given previously,results in a range of Ku values of 0.21.7 mm/day at Houn-dean and 17 mm/day at Broadhalfpenny. These values cor-respond well with the measured Ku value of the Chalkmatrix.

A delayed secondary response to rainfall is most commonduring wetter periods, e.g., during the winter floods of2000. The lower rainfall intensities during drier periodsmay not produce such intense pulses through the matrixand may not, therefore, produce distinctive peaks, whichare detectable in the cross-correlation above backgroundnoise. Additionally, the analysis is based on time series of3 months length and this may not cover a long enough timespan to detect the longer time lags expected when Ku islower.

Limitations

Cross-correlation analysis merely establishes whether or nota significant correlation exists between patterns of variationin rainfall and changes in borehole water level. It does noton its own indicate what causes the response. There areother factors, which could possibly contribute to thewater-level response, e.g., changes in atmospheric pres-sure, entrapped air in the unsaturated zone, or lateral flowto a borehole from an up-valley location. However, most ofthese can probably be discounted. Significant barometric ef-fects can occur in unconfined aquifers (Healy and Cook,2002) but are not widely reported from the Chalk. The sizeof the fluctuations is generally much larger than changes inatmospheric pressure (Fig. 2), making it impossible to ex-plain them by this mechanism. Furthermore, rainfall insouthern England is usually associated with low barometricpressure, which means that the rise in the potentiometricsurface would occur before or during the precipitation,not after it. Entrapped air during infiltration is most likelyto be associated with finely textured soils as they becomeimpermeable to air when saturated (Healy and Cook,2002). As soils tend to be very thin on the Chalk, it is unli-kely that they will form a thick enough barrier to preventair movement. It is even more unlikely that air could beentrapped in an unsaturated zone that is dissected by awell-developed fissure network that would be at least partlyair-filled; it is also unlikely that this effect would takeseveral weeks to manifest and would control the long-termresponses observed here.

14

e

l

618 L.J.E. Lee et al.-0.4

-0.2

0

0.2

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noitalerr

oCSignificance Leve

Fissure

0.8FissureLateral subsurface flow, (e.g., along highly-permeablelayers like that seen in the Candover Valley) cannot be dis-counted, but since the water involved would have had toinfiltrate the unsaturated zone anyway and would then havetravelled relatively quickly through the saturated zone, itspresence or absence does not seriously affect the argumentsof this paper. Furthermore, at many interfluve sites, Chalkpermeabilities are very low, limiting the capacity for lateralmigration. A limitation of the current data set is that it isdifficult to estimate the variance in the data as the monitor-ing period is short and relatively few data points are avail-able. The current findings will be enhanced considerablyonce daily data sets of longer length are available for thesites.

Pumping from nearby boreholes could affect water lev-els. There are few borehole sites in southern England thatare not affected by pumping from other boreholes. Datawere collected on all licensed abstractions within 5 km ofthe study sites. Several of them have large public-supplyor industrial licensed abstractions close enough to cause a

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Figure 10 Cross-correlograms showing delayed secondary respoHoundean, autumn 2000 (top), and Broadhalfpenny Down, autumn16 18 20 22 24 26 28 30

lag (days)Matrixtheoretical effect. However, such abstractions tend tooperate at similar rates each day (except on weekends),so it is unlikely that recovery from such an abstraction couldcause an effect that would be mistaken for a rise of waterlevel in response to rainfall such as those illustrated inFig. 2. If pumping patterns were dominating the responseillustrated in Fig. 2, one would expect a weekly pattern ofwater table response that was largely independent ofrainfall.

Conclusions

This study has considered the correlations between rainfallevents and rises in the water table at six sites on the Chalkof southern England. The time for the water table to re-spond to rainfall has been found to vary from less than 1day to more than 4 weeks. The slower responses occur dur-ing or at the end of dry periods, when both storage in theunsaturated zone and its matric hydraulic conductivity are

16 18 20 22 24 26 28 30

lag (days)

nificance Level

Matrix

nses with possible fissure and matric responses indicated for2000 (bottom).

Richard Marks of the British Geological Survey kindly pro-vided water-level data for the Ogbourne borehole, which

Analysis of water-level response to rainfall and implications for recharge pathways in the Chalk aquifer 619are also gratefully acknowledged. The comments of twoanonymous reviewers significantly improved the manuscriptand are gratefully acknowledged.

References

Allen, D.J., Brewerton, L.J., Coleby, L.M., Gibbs, B.R., Lewis, M.A.,MacDonald, A.M., Wagstaff, S.J., Williams, A.T., 1997. Thephysical properties of major aquifers in England and Wales. BGSTechnical Report WD/97/34, British Geological Survey,Keyworth.

Barker, J.A., 1993. Modelling groundwater flow and transport in theChalk. In: Downing, R.A., Price, M., Jones, G.P. (Eds.), TheHydrogeology of the Chalk of North-West Europe. Clarendon,Oxford, pp. 5966.

Barker, J.A., Foster, S.S.D., 1981. A diffusion exchange model forsolute movement in fissured porous rock. Quarterly Journal ofEngineering Geology 14, 1724.

Calver, A., 1997. Recharge response functions. Hydrology and EarthSystem Sciences 1, 4753.

Cooper, J.D., Gardner, C.M.K., Mackenzie, N., 1990. Soilcontrols on recharge to aquifers. Journal of Soil Science41, 613630.

Diggle, P.D., 1990. Time Series: A Biostatistical Introduction.Oxford Statistical Science Series, 5. Oxford Science Publications,Oxford.at a minimum. The rapid responses occur during or after wetperiods, when these conditions are reversed; the autumnand winter of 20002001 were especially notable for rapidresponses. Although many of the responses can be explainedas the result of a piston-displacement mechanism throughthe matrix, some of the most rapid responses of deeperwater tables can be explained only as the result of fissureflow through at least a large part of the unsaturated zone.These events occurred when the rainfall intensity exceededthe saturated hydraulic conductivity of the matrix. As fur-ther and longer high-resolution time series of boreholewater levels become available, the methodology demon-strated here could potentially be very useful in developingmodels of aquifer recharge, which reflect location responsepatterns under a range of conditions and climatic fluctua-tions. When combined with numerical modelling of matricand fissure flow, it represents a feasible technique for cali-brating such models, without the need for detailed monitor-ing of matric potential in the unsaturated zone. Moregenerally, the results presented here demonstrate the rangeand consistency of water-level responses to rainfall eventsin the Chalk, both of which can be explained by invokingthe combined effects of matric diffusion and fissure flowprocesses at each site.

Acknowledgements

The Environment Agency, Southern Region, are gratefullyacknowledged for the financial and technical support theyprovided for this project through a Ph.D. studentship toLeonora Lee. They also provided borehole water level, rain-fall and local abstraction data, and Anne Wilkinson, RussellLong, Alison Rennie and Emily Cranch are particularlyacknowledged for their assistance. Adrian Lawrence andDowning, R.A., Pearson, F.J., Smith, D.B., 1979. The flow mech-anisms in the Chalk based on radio-isotope analysis in the Londonbasin. Journal of Hydrology 40, 6783.

Foster, S.S.D., 1975. The Chalk groundwater tritium anomaly apossible explanation. Journal of Hydrology 25, 159165.

Gardner, C.M.K., Cooper, J.D., Wellings, S.R., Bell, J.P., Hodnett,M.G., Boyle, S.A., Howard, M.J., 1990. Hydrology of theunsaturated zone of the Chalk of SE England. In: Chalk. ThomasTelford, London, pp. 611618.

Giles, D.M., Lowings, V.A., 1990. Variation in the character of thechalk aquifer in east Hampshire. In: Chalk. Thomas Telford,London, pp. 619626.

Haria, A.H., Hodnett, M.G., Johnson, A.C., 2003. Mechanisms ofgroundwater recharge and pesticide penetration to a chalkaquifer in southern England. Journal of Hydrology 275, 122137.

Hassan, M., Gregory, P.J., 2002. Dynamics of water movement onChalkland. Journal of Hydrology 257, 2741.

Headworth, H.G., 1972. The analysis of natural groundwater levelfluctuations in the Chalk of Hampshire. Journal of the Institutionof Water Engineers 26, 107124.

Headworth, H.G., Keating, T., Packman, M.J., 1982. Evidence for ashallow highly-permeable zone in the Chalk of Hampshire, UK.Journal of Hydrology 55, 93112.

Healy, R.W., Cook, P.G., 2002. Using groundwater levels toestimate recharge. Hydrogeology Journal 10 (1), 91109.

Johnson, A.C., Besien, T.J., Lal Bhardwaj, C., Dixon, A., Gooddy,D.C., Haria, A.H., White, C., 2001. Penetration of herbicides togroundwater in an unconfined chalk aquifer following normal soilapplications. Journal of Contaminant Hydrology 53, 101117.

Jones, H.K., Cooper J.D., 1998. Water transport through theunsaturated zone of the Middle Chalk: a case study from theFleam Dyke lysimeter. In: Robins, N.S. (Ed.), GroundwaterPollution, Aquifer Recharge and Vulnerability, vol. 130. Geolog-ical Society Special Publication, pp. 117128.

Keating, T., 1982. A lumped parameter model of a chalk aquifer-stream system in Hampshire, United Kingdom. Ground Water,20, 430436.

Lewis, M.A., Jones, H.K., Macdonald, D.M., Price, M., Barker, J.A.,Shearer, T.R., Wesselink, A.J., Evans, D.J., 1993. Groundwaterstorage in British aquifers: Chalk. R& D Note 169, National RiversAuthority, Bristol.

Mathias, S.A., Butler, A.P., Mcintyre, N., Wheater, H.S., 2005. Thesignificance of flow in the matrix of the Chalk unsaturated zone.Journal of Hydrology 310, 6277.

NERC, 2000a. Hydrological summary for the United Kingdom, March2000. Centre for Ecology and Hydrology/British GeologicalSurvey, Natural Environment Research Council, Wallingford.

NERC, 2000b. Hydrological summary for the United Kingdom,October 2000. Centre for Ecology and Hydrology/British Geo-logical Survey, Natural Environment Research Council,Wallingford.

Oakes, D.B., 1981. A numerical model of a stream aquifer systemsubject to delayed rainfall recharge. Transactions of theGeological Society of South Africa 84, 135144.

Owen, M., Robinson, V.K., 1978. Characteristics and yield infissured Chalk. Institution of Civil Engineers. Symposium onThames Groundwater Scheme 2, 2944.

Price, M., 1987. Fluid flow in the Chalk of England. In: Goff, J.C.,Williams, B.P.J. (Eds.), Fluid Flow in Sedimentary Basins andAquifers, vol. 34. Geological Society Special Publication, pp.141156.

Price, M., Bird, M.J., Foster, S.S.D., 1976. Chalk pore-sizemeasurements and their significance. Water Services 80, 596600.

Price, M., Morris, B.L., Robertson, A.S., 1982. A study of intergran-ular and fissure permeability in Chalk and Permian aquifers,using double-packer injection testing. Journal of Hydrology 54,401423.

Price, M., Downing, R.A., Edmunds, W.M., 1993. The Chalk as anaquifer. In: Downing, R.A., Price, M., Jones, G.P. (Eds.), TheHydrogeology of the Chalk of North-West Europe. Clarendon,Oxford, pp. 3558.

Price, M., Low, R.G., McCann, C., 2000. Mechanisms of waterstorage and flow in the unsaturated zone of the Chalk aquifer.Journal of Hydrology 233, 5471.

Reeves, M.J., 1979. Recharge and pollution of the EnglishChalk: some possible mechanisms. Engineering Geology 14,231240.

Robins, N.S., Buckley, D.K., Darling, W.G., Dance, L., Dumpleton,S., Evans, D.J., Hargreaves, R., Jones, H.K., McCann, L.,Mathias, S.A., 2001. 3-D Conceptualisation of the Central SouthDowns Aquifer. BGS Technical Report CR/01/154.

Rushton, K.R., Ward, C., 1979. The estimation of groundwaterrecharge. Journal of Hydrology 41, 345361.

Smith, D.B., Wearn, P.L., Richards, H.J., Rowe, P.C., 1970. Watermovement in the unsaturated zone of high and low permeabilitystrata by measuring natural tritium. In: Isotope Hydrology. IAEA,Vienna, pp. 259270.

Sussex River Authority, 1974. South Downs Groundwater Project.(Brighton and Worthing Chalk Blocks); Second Progress Report.July 1972March 1974, Sussex River Authority, Sussex.

Tokunaga, T.K., Wan, J., 1997. Water film flow along fracturesurfaces of porous rock. Water Resources Research 33, 12871295.

Tokunaga, T.K., Wan, J., 2001. Surface-zone flow along unsatu-rated rock fractures. Water Resources Research 37, 287296.

Wellings, S.R., 1984a. Recharge of the Chalk aquifer at a site inHampshire, England, 1. Water balance and unsaturated flow.Journal of Hydrology 69, 259273.

Wellings, S.R., 1984b. Recharge of the Chalk aquifer at a site inHampshire, England, 2. Solute movement. Journal of Hydrology69, 275285.

Wellings, S.R., Bell, J.P., 1980. Movement of water and nitrate inthe unsaturated zone of Upper Chalk near Winchester, Hants.England. Journal of Hydrology 48, 119136.

Wellings, S.R., Bell, J.P., 1982. Physical controls of water move-ment in the unsaturated zone. Quarterly Journal of EngineeringGeology 15, 235241.

620 L.J.E. Lee et al.

Analysis of water-level response to rainfall and implications for recharge pathways in the Chalk aquifer, SE EnglandIntroductionPrevious workRecharge pathways and timingRecharge estimation based on field hydrograph data

Study sitesMethodologyResultsCross-correlation of rainfall and water-level response dataUnsaturated zone thickness and water-level response

DiscussionUnsaturated flowUnsaturated zone thicknessDelayed secondary responsesLimitations

ConclusionsAcknowledgementsReferences