The effects of antecedent moisture conditions on the relationship of hydrology to hydrochemistry in...

The e�ects of antecedent moisture conditions on therelationship of hydrology to hydrochemistry in a

small forested watershed

Pascale M. Biron1*, AndreÂ G. Roy2, FrancË ois Courschesne2, William H. Hendershot3,BenoõÃ t CoÃ teÂ 3 and Jim Fyles3

1Department of Geography, Concordia University, 1455, de Maisonneauve Blvd. W., MontreÂal, QueÂbec, H3G1M8, Canada2DeÂpartement de GeÂographie, UniversiteÂ de MontreÂal, C.P. 6128, succ. Centre-ville, MontreÂal, QueÂbec, H3C3J7, Canada

3Department of Renewable Resources, McGill University, MacDonald Campus, Ste-Anne-de-Bellevue, QueÂbec, H9X3V9, Canada

Abstract:The relationship between stream discharge and the concentration of di�erent solutes re¯ects the hydrologicalprocesses occurring in a catchment, but is also strongly a�ected by antecedent moisture conditions. In this

study, the e�ects of antecedent conditions were examined for a small forested watershed near MontreÂ al(QueÂ bec) by comparing data from sequences of fall events for two years of contrasting antecedent moistureconditions. The relationship between discharge and stream water chemistry was also quanti®ed using cross-

correlation analysis. Results show that concentrations of dissolved organic carbon, nitrate and calciumincreased during storm events with dry antecedent conditions but that the relationships were weaker under wetconditions. For both years, concentrations of sulfate, silicon and sodium and the pH were negatively correlated

with discharge (cross-correlations from ÿ0.20 to ÿ0.53). With dry antecedent conditions, there was a generaldecrease in the concentration of all solutes with time, whereas concentrations remained at about the same levelunder wet conditions. The concentration±discharge relationships for sulfate shifted frequently from higherconcentrations during the rising limb to higher concentrations during the falling limb from one event to another

as a result of changing antecedent conditions. Although the contrast in antecedent moisture conditions betweenthe two years was moderate in comparison with that reported in other studies (25% dryer than average in 1995and 12% wetter than average in 1996), the hydrochemistry of the stream was markedly di�erent. Copyright# 1999 John Wiley & Sons, Ltd.

KEY WORDS forest ecosystem; chemograph; hydrograph; storm event; stream chemistry; moisture conditions;cross-correlation analysis

INTRODUCTION

The interrelationship between river discharge and hydrochemistry has provided a useful and widely usedmeans of examining hydrological processes within forested watersheds in temperate climates (Walling andWebb, 1986; Jenkins, 1989; Giusti and Neal, 1993; Elsenbeer et al., 1994). The relationship between streamdischarge and changes in concentrations of chemical constituents can indicate the di�erent ¯ow paths ofwater and how these change with time. During low ¯ow, the origin of stream water is generally believed to be

CCC 0885±6087/99/111541±15$17.50 Received 12 June 1998Copyright # 1999 John Wiley & Sons, Ltd. Received 30 August 1998

Accepted 14 September 1998

HYDROLOGICAL PROCESSESHydrol. Process. 13, 1541±1555 (1999)

*Correspondence to: Dr P. Biron, Department of Geography, Concordia University, 1455 de Maisonneuve Blvd. W., MontreÂ al,QueÂ bec, H3G IM8, Canada. E-mail: [email protected]

Contract Grant Sponsor: MinisteÁ re des Resources Naturelles du QueÂ bec.Contract Grant Sponsor: The Fonds FCAR.

solely groundwater, therefore the chemical signature should re¯ect the high concentrations of solutes foundin the deepest part of the groundwater system (Reynolds et al., 1987; AÂ vila et al., 1992; Soulsby, 1995).According to the variable source area concept (Hewlett, 1982), as the groundwater level rises, the areacontributing to direct and rapid runo� will increase with increasing discharge. Hence, the chemical responseshould show an increase in the concentration of solutes found at higher concentrations in the upper ratherthan the lower soil horizons. This type of ¯ood hydrograph separation can, however, be complicated byvariations in the relative importance of contributing sources which may be strongly a�ected by antecedentmoisture conditions (Muscutt et al., 1990; AÂ vila et al., 1992). Antecedent moisture conditions are increas-ingly recognized as an important factor in the chemical di�erences from storm to storm even when rainfallcharacteristics are similar (Jenkins et al., 1994). Only a few studies have explicitly examined this question andthey have shown that periods of relatively dry weather have a marked e�ect on the hydrochemicalcharacteristics during storm events (Muscutt et al., 1990; AÂ vila et al., 1992). However, more detailed ®eldstudies in a variety of environments are required to generalize these results.

Several approaches have been used to analyse the relationship between solute concentration and streamdischarge. Data have been bulked to derive regression relationships (power functions) between soluteconcentration and discharge (Foster, 1978; Walling and Webb, 1986; Webb and Walling, 1992), sometimesdistinguishing seasonal trends (Foster, 1978; Walling and Webb, 1986). There is much scatter in theserelationships, however, and power functions are perhaps not the most appropriate technique to describesolute response to changing stream ¯ow (O'Connor, 1976). Furthermore, it is generally recognized that verydi�erent responses are to be expected from event to event owing to both storm characteristics (rainfallamount and intensity) and antecedent moisture conditions (Muscutt et al., 1990; AÂ vila et al., 1992; Jenkinset al., 1994; Soulsby, 1995). Therefore, examining solute concentrations as a function of discharge usingregression analysis may obscure some important phenomenon at the scale of the storm (AÂ vila et al., 1992).However, quanti®cation of these storm responses remains di�cult, and most existing analyses consist ofvisual comparisons between peaks in discharge and peaks in dilution of solute concentrations (e.g. Muscuttet al., 1990; Elsenbeer et al., 1994).

The objectives of the study described in this paper were: (1) to examine the e�ects of antecedent moistureconditions on the relationships between solute concentrations and discharge during autumn in a smallforested watershed and (2) to characterize the relationship between discharge and stream water chemistryduring a sequence of autumn storm events using a novel statistical approach (cross-correlation).

METHODS

Study site

The Hermine catchment is located at the Station de Biologie des Laurentides de l'UniversiteÂ de MontreÂ alnear St-Hippolyte, about 80 km north of MontreÂ al, QueÂ bec, Canada (458590N, 748010W, altitude c. 400 m)(Figure 1a). An intermittent ®rst-order stream drains the catchment area of 5.1 ha (Figure 1b). The bedrockis Precambrian anorthosite of the Morin series (McGerrigle, 1976) which is covered by till (1±2 m) except onbare outcrops in the downstream area and on some of the catchment divides. The till has a mineral com-position similar to anorthosite. Soils are orthic or gleyed humo-ferric and ferro-humic podzols (AgricultureCanada Expert Committee on Soil Survey, 1987). Details of the soil chemistry can be found elsewhere(Courchesne et al., 1995). Mean annual precipitation is 1100 mm of which 30% is snow (Figure 1c). Thecanopy vegetation (mean age: 85 years, density: 28 m2/ha) is 78% sugar maple (Acer saccharum) with someAmerican beech (Fagus grandifolia, 9%) and yellow birch (Betula alleghaniensis, 6%).

Field study

This study is part of an ongoing long-term monitoring programme initiated during 1993 in the Herminecatchment aimed at examining the e�ects of intra- and inter-year climatic variations on nutrient availabilityand cycling and forest productivity. To examine the e�ects of antecedent moisture conditions, we present

Copyright # 1999 John Wiley & Sons, Ltd. HYDROLOGICAL PROCESSES, VOL. 13, 1541±1555 (1999)

1542 P. M. BIRON ET AL.

data collected during the autumn of two contrasting years: a dry (1995) and a relatively wet (1996) year. Theamount of rain over the Hermine area from 1 June to September 15 was 279 mm in 1995 and 414 mm in1996, whereas the mean for the past 25 years is 371 mm for this period. Thus, the summer of 1995 was 25%dryer than the average, whereas the summer of 1996 was 12% wetter than usual. The stream had been drybefore the period of investigation for 21 days in 1995 and 5 days in 1996.

Stream discharge was calculated from the water level above a 908V-notch weir measured with a GlobalLevel sensor bubbler (model BT101). The water table height was also recorded by a bubbler in two wellsconstructed of 5 cm diameter ABS plastic pipe, open at the bottom and pierced every 7.5 cm with two 10 mmholes on opposite sides. The wells were located close to the stream (well #1) and near the top of the hillslope(well #2) (Figure 1b). None of the wells reached the bedrock because of the presence of a compacted/hardened BC horizon. The measurement frequency at all three bubblers was four per hour. The moisturecontent of the soil was measured at a depth of 25 cm using time domain re¯ectometry (TDR) near the twowells and at seven other locations within the watershed, although at a much lower sampling frequency (everytwo weeks from mid-May until the end of October and every four weeks thereafter). These data were notcalibrated to give absolute percentages, and they are reported here as relative values. Daily rainfall totalswere obtained from a weather station at the Station de Biologie de l'UniversiteÂ de MontreÂ al located about1 km from the Hermine catchment. Comparisons between these data and the amount of rain collected everytwo weeks in open samplers located within the watershed showed close agreement (regression slope: 1.08,r � 0.98). Except at the very beginning of the sampling period in 1996, no leaves were present on trees, so theprecipitation data from the weather station have been used.

The stream water samples were collected at the weir using an automatic water sampler. During the autumnof 1995, samples were collected every six hours. In contrast, the stream was sampled once a day in 1996. Thissampling frequency may not be optimal for a detailed study of the link between stream concentration anddischarge in a small watershed. However, we believe our analysis based on these relatively low samplingfrequencies is valid, especially since we examined these relationships for a series of events spread over 33 daysin 1995 and 73 days in 1996. Soil water samples were collected in the zone close to the stream in accordancewith the hypothesis of `horizons of last contact' (Figure 1b), meaning that the chemistry of the water reachingthe stream depends on the chemical characteristics of the latest horizon encountered (O'Brien andHendershot, 1993). The samples were collected every two weeks until late October and every four weeksafterwards using the three zero-tension lysimeters ( funnel with silica sand connected to a plastic bottle) (seeFigure 1b) located at the interface between the organic and mineral horizons (0 cm) and in the lower Bhorizon (ÿ50 cm). During periods of low moisture, however, no solution could be obtained from the lower Bhorizon lysimeters.

Chemical analyses

Un®ltered water samples were analysed for pH and electrical conductivity (EC) within 24 h of collectionusing appropriate meters. A separate aliquot of each sample was then passed through a 0.4 mm poly-carbonate membrane ®lter prior to analysis of calcium (Ca) concentrations by atomic absorptionspectrometry (Perkin±Elmer) and nitrate (NO3), and sulfate (SO4) and sodium (Na) by ion chromatography(Waters). Dissolved organic carbon (DOC) was estimated from the absorbance at 254 nm using a linearrelationship between absorbance and DOC measured with a Shimadzu TOC analyser (Shimadzu, Kyoto,Japan). The precision of this method is 0.01 mg/l and the bias in the comparison with true values is between1 and 3%. Dissolved silicon (Si) was analysed by an automated colorimitic molybdate method (EnvironmentCanada, 1993).

Cross-correlation analysis

Cross-correlation analysis was used to quantify the relationship between solute concentrations and streamdischarge. Cross-correlation analysis allows one to determine if the ¯uctuations in two time-series areconcomitant (Box et al., 1994). It also permits one to quantify a temporal lag, for example between a peak in


ANTECEDENT MOISTURE AND HYDROCHEMISTRY 1543

discharge and a peak in a given solute concentration. This type of time-series analysis can only be performedon two series with identical sampling frequencies. Therefore, the frequency of the time-series of the chemistryof stream water was increased up to a frequency of four sample/hour, i.e. the bubbler sampling frequency,using linear interpolation. Although linear interpolation will not recreate the variability of small-scale¯uctuations in the chemical time-series, it preserves initial data. This is important for cross-correlationanalysis, where delays in the position of the peaks in discharge and in solute concentrations are examined.Furthermore, both the stream discharge and the chemical time series have trends over time: the dischargegenerally increases over time whereas, in most cases, solute concentrations decrease with time. Cross-correlation analysis, as any time-series analysis, requires that the time-series are stationary so these trendshad to be removed. This was done using least-squares regression. The regression slope of the relationshipbetween time and either discharge or solute concentrations was calculated ®rst. The expected regressionvalues were subtracted from the observed values and the residuals were added to the mean ordinates (meandischarge or mean concentration solutes) so that the slope became zero.

Figure 1. (a) Location of the study site (Hermine catchment). (b) Map of the Hermine catchment with the position of the weir, the twowells and the location of the soil solution samplers near the stream (elevations in m). (c) Mean monthly precipitation and temperature

for the catchment (based on data from the last 25 years)



RESULTS AND DISCUSSION

Hydrology

The contrast between moisture conditions in 1995 and 1996 is clearly shown by the water levels in the wells(Figure 2). In 1995, the water level near the stream was below the base of the well (5ÿ 93 cm) until21 October (Figure 2a), indicating very dry conditions despite 77 mm of rain in the ®rst week (Table I). Onthe upper part of the hillslope, the water level increased only slightly on the same date and continued toincrease during subsequent events (Figure 2b). In 1996, both wells were dry at the beginning of the period ofinvestigation (Figure 2). After the ®rst event, the water table increased approximately to the same height bothnear the stream and in the upslope wells (c. ÿ60 cm), albeit with a slight delay (13 hours) at the upslope well.The sharp increase in the water level in the wells in 1996 re¯ected the wetter antecedent moisture conditionsduring the year. TDR data collected 11 days before the period of investigation (17 September, 1996) gave soilvolumetric moisture percentages of 21 and 26% near wells #1 and #2, respectively. In 1995, these values were13 and 9% near wells #1 and #2 four days prior to the ®rst event (2 October). The simultaneous increase ofthe water level in both wells also indicates that the water table on the hillslope does not respond as a wedgeextending from the base of the slope to the top of the hillslope, a phenomenon also observed by Wilson et al.(1990).

The antecedent moisture conditions are contrasted in Table I which provides information on the rainfallinput, runo� output and the ratio between output and input in the catchment for di�erent events, for bothyears. The end of the events was taken when the di�erence between three consecutive discharge values wasless than 0.1 l/s. Note that although the ®rst events of each year were similar in terms of duration andquantity of precipitation (Table I), the hydrological response of the wells and at the weir were markedlydi�erent. In 1995, the tendency is for the ratio of output on input (Q/R) to increase progressively with time,as would be expected according to the variable area concept, where the contributing area increases with anincrease in the volume of water stored in the system (Hewlett, 1982). In 1996, the ratio Q/R increasedmarkedly from the ®rst to the second event, with over 70% of the input water exiting the catchment during

Figure 2. Water table height below ground in wells: (a) near stream, 1995; (b) hillslope top, 1995; (c) near stream, 1996; (d) hillslope top,1996. The bases of the wells are at ÿ93 cm near the stream and ÿ91 cm on the upslope. Event numbers are also indicated (cf. Table I)



this event owing to the wet antecedent conditions. The stream yield signi®cantly decreased thereafter,indicating that a greater proportion of the water in®ltrated and was stored in the soil during the later events.

Hydrochemistry

The hydrological di�erences observed between the two years were re¯ected in di�erences between thechemographs, as is shown in Figures 3 and 4. Initial concentration values for DOC, No3 , Ca and SO4 in 1995(Figure 3) were much higher than in 1996 (Figure 4), and were followed by a marked decrease through time,to concentrations comparable with those in 1996. This decrease may be related to the presence of organicsediments and litterfall that have accumulated and were decomposed on the dry stream bed for a longerperiod in 1995 than in 1996. Although detailed analysis is not presented here, we have observed that theconcentrations of potassium were much higher at the beginning of the 1995 period than in 1996 (about8 mmol/l in 1995 and close to 0 mmol/l in 1996), and decreased rapidly afterwards (down to 4 mmol/l by9 October, 1995). The marked decrease of concentrations over time was not evident in 1996, when there was aslight increase in SO4 concentrations. An increase of SO4 concentrations with discharge was also observed byJenkins et al. (1994). Silicon was the only solute where concentrations and decreases in concentration were ofsimilar magnitudes in 1996 and in 1995. Sodium concentrations were only slightly higher in 1995 butexhibited a decrease during the high ¯ow period, whereas the concentration of Na was invariant in 1996.Values of pH were systematically lower in 1995 than in 1996, except for the 8±11 November 1996 event,consistent with a previous study that indicated that dry conditions cause pH to decrease (Courchesne et al.,1995).

The mean concentration values for the soil solution collected near the stream during the autumn of 1995and 1996 are indicated on Figures 3 and 4 (at 0 and ÿ50 cm). These values show that concentrations ofDOC, NO3 and Ca were higher near the surface than at ÿ50 cm, whereas the opposite was observed for SO4

and Na. Si concentrations were similar at 0 andÿ50 cm in 1996. A value at the surface could not be obtainedin 1995.

Stream water chemistry can be viewed as a variable mixture of soil water and groundwater end-members(Webb and Walling, 1992; Jenkins et al., 1994; Neal et al., 1997a,b). During events, the chemical composi-tion of soil solution at di�erent depths can be used to infer the ¯ow paths of water before entering the streamchannel at di�erent times. The soil water samples were collected near the stream because the chemistry of thesolum-water contribution to stream ¯ow was largely controlled by the soils along the stream banks in theHermine catchment (Hendershot et al., 1992; O'Brien and Hendershot, 1993). In cases where concentrationswere greater at 0 cm than atÿ50 cm, peaks in discharge were clearly related to peaks in solute concentrations(e.g. DOC, NO3 and Ca; Figures 3 and 4). These concentration peaks were mostly due to the contribution ofrapid runo� through the organic matter-rich surface horizons where the products of organic matter decom-position are leached during events. For NO3, there is a marked peak for the ®rst event of 1995 followed by

Table I. Characteristics of fall events in 1995 and 1996

Year Event # Dates Rain(mm)

Volume ofrain, R (m3)

Output(mm)

Volume ofoutput, Q (m3)

Output/inuput(Q/R)

1995 1 7±8 October 58 2956 0.90 46 0.021995 2 14±15 October 19 996 0.51 26 0.031995 3 21±22 October 54 2792 5.46 280 0.101995 4 28±29 October 16 842 1.48 76 0.091995 5 2±3 November 28 1457 8.42 432 0.301996 1 28±29 September 48 2453 2.87 147 0.061996 2 21±26 October 65 3336 48.00 2463 0.741996 3 8±11 November 68 3484 36.42 1869 0.541996 4 1±4 December 36 1852 9.61 493 0.27



only slight increases for the subsequent events (Figure 4). This pattern was also observed by AÂ vila et al.(1992) in a Mediterranean forested catchment as the conditions were changing from dry to wet.

Minima in solute concentrations coincided with positive discharge peaks in cases where soil waterconcentrations were greater in the deeper horizons (Na and SO4) due to the weathering of plagioclase fromanorthosite and cation exchange for Na, and anion adsorption/desorption reactions for SO4 (Figures 3

Figure 3. (a) Discharge hydrograph at the weir and chemographs in 1995 for (b) pH, (c) DOC, (d) SO4 , (e) NO3, ( f) Na, (g) Ca and (h)Si. Filled triangles (m) represent the mean concentration in soil solution close to the stream at 0 cm whereas inverted ®lled triangles (.)are concentrations at ÿ50 cm, when soil solution could be sampled. Note that no data were available for soil solution concentrations of

Si at 0 cm



and 4). Marked dilution of stream concentrations of dissolved Si were evident during storm events despitenearly identical concentrations in the soil water at 0 and ÿ50 cm depth. Although groundwater was notsampled, Si concentrations in the stream during low ¯ow periods (51 l/s) may be used to estimate the Siconcentration of the groundwater component (Hendershot et al., 1992; Jenkins et al., 1994). This approachindicates a relatively high average value of 166 mmol/l for both years (standard deviation: 17 mmol/l) thatmay be related to the weathering of Al-silicate minerals in the parent material. Dilution of this Si-rich

Figure 4. (a) Discharge hydrograph at the weir and chemographs in 1996 for (b) pH, (c) DOC, (d) SO4 , (e) NO3, ( f) Na, (g) Ca and(h) Si. Triangular symbols are as in Figure 3



groundwater component during storm events by the input of water travelling in surface horizons and havingmuch lower Si concentrations would therefore explain the marked decrease in Si concentrations as stream¯ow increases.

Individuals storm events

Close observation of individual storm events reveals the complexity of the relationship between dischargeand solute concentrations and how this relationship may vary with antecedent moisture conditions. Thisimportant contrast in solute and discharge responses during at storm event may be promoted by the ¯ushinge�ect, where biogeochemical solutes in soils are ¯ushed to the stream by the formation of a rising water tableduring storm ¯ow (Creed et al., 1996), especially after periods of prolonged drought (Walling and Foster,1975; Slack, 1977). Patterns of SO4 concentrations are presented here to illustrate this point (Figures 5 and6). Note that concentration values of SO4 for the ®rst two events of 1995 (Figure 5a,b) were much higherthan mean values at 0 and ÿ50 cm measured in the zero-tension lysimeters (Figure 3). These high SO4

concentrations may have been related to the accumulation of a pool of soluble SO4 in surface soil horizonsduring the prolonged dry period that preceded the ®rst two events. Evaporation, the decomposition oforganic matter and the dry deposition of S compounds could contribute to such a pool (Robarge andJohnson, 1992). This pool of readily leachable SO4 was evident in stream water following a rainfall event.The ®rst event in 1995 was characterized by well-developed hysteresis for SO4 : concentrations were higher onthe falling limb (Figure 5a). In comparison, the ®rst event in 1996, when the catchment was wetter, showedonly limited hysteresis of 1±2 mmol/l (Figure 6a). In 1996, concentration values were in the 48±52 mmol/lrange, typical of a strong contribution to stream ¯ow originating from the B horizon, where retention±release reactions maintain dissolved SO4 at this level (Hendershot and Courchesne, 1994). The second eventsof both years were again markedly di�erent, with the falling limb of the hydrograph in 1995 showing anincrease in SO4 concentrations relative to the rising limb and the falling limb of the hydrograph in 1996showing lower concentrations of SO4 compared with the rising limb (Figure 6b). As the conditions becamemore humid in 1995, i.e. events 3±5 (Figure 5c±e), the pattern of hysteresis in SO4 concentrations was similarto that of the earlier event of 1996 (#2, Figure 6b) where there was a greater soil water content. During thelast event of 1996 (Figure 6d), in which antecedent conditions in the watershed were dry (c.f. Figure 2c,d), theconcentration±discharge relationship was similar to the ®rst event of 1995, i.e. an increase in SO4 con-centration in the receding limb (Figure 5a). The frequent shifts in the pattern of hysteresis contrast with theclear increase in the concentration of SO4 during the falling limb observed in a series of summer/autumnepisodes in Adirondack and Pennsylvania streams (Evans and Davies, 1998). These variations in thedirection of the hysteresis of SO4 from storm to storm, however, corresponded to the results presented byAÂ vila et al. (1992) for a change in antecedent conditions from dry to wet in regions with intense summerdryness.

Quanti®cation of the relationships between hydrochemistry and discharge

Although the slope of the power functions of solute-rating relationships can vary considerably betweencatchments and between individual constituents, most rivers are characterized by a negative value for the bexponent of the rating relationship (Webb and Walling, 1992). The slopes of the relationships between thelog of solute concentration and the log of discharge for the data collected in this study are shown in Table II.With the exception of DOC, all slopes were negative, indicating a dilution of solutes as discharge increased.

Cross-correlation analysis, however, shows that simply examining the sign of the slope exponents of powerfunctions may be misleading (Figure 7). Indeed, like DOC, both NO3 and Ca have positive cross-correlationcoe�cients at lag 0, indicating that concentrations increased during storm events, despite their negativeslopes of power functions. This pattern is due to the long-term (season-scale) temporal trends in soluteconcentrations (cf. Figures 3 and 4) which may hide the short-term (event-scale) correspondence betweenchemograph and hydrograph peaks. For example, the concentrations of Ca after the peak of 22 October1995 were lower than those prior to that event, illustrating the marked decrease of Ca concentrations



with increasing discharge (Figure 3g). The discharge, however, reached a higher stage during this eventthan it had before the event (Figure 3a). This pattern gave rise to a negative cross-correlation that partlycounter-balanced the positive correlation between the peaks in discharge and Ca concentration. By removingtrends in both time-series, the counter-balancing e�ect no longer plays a role and the match between theconcentration and discharge peaks is enhanced.

Figure 5. Hysteresis of SO4 as a function of stream discharge in 1995 for events (a) #1, (b) #2, (c), #3, (d) #4 and (e) #5. Filled triangles(m) represent the rising limb and open circles (s) the falling limb

Table II. Slope of the ordinary least-squares power function between the concentration of di�erent solutes anddischarge for the surveyed period in 1995 and 1996

Year pH Na Si Ca No3 SO4 DOC

1995 ÿ0.0011* ÿ0.078 ÿ0.048 ÿ0.056 ÿ0.17 ÿ0.08 0.12(n � 128) (0.005) (0.50) (0.54) (0.25) (0.19) (0.53) (0.25)1996 ÿ0.01 ÿ0.08 ÿ0.04 ÿ0.01* ÿ0.09* ÿ0.04 0.24(n � 74) (0.29) (0.36) (0.35) (0.02) (0.03) (0.53) (0.41)

*Not signi®cant at a � 0.01. All the other slopes are signi®cant at a � 0.001.r2 is indicated in parentheses. Slopes are in mmol s lÿ2.



Cross-correlation analysis also allows us to quantify delays between discharge and solute concentrationpeaks as well as the strength of the relationship. DOC had by far the highest cross-correlation coe�cient(0.86 and 0.79 in 1995 and 1996, respectively, Figure 7b,i). This largest correlation, however, did not occur atlag 0, but at ÿ1.5 hours in 1995 and ÿ7.25 hours in 1996, thus indicating that peaks in DOC occurredslightly before peaks in discharge. Note that cross-correlation curves in 1996 were always more spread outthan in 1995. This may be an artefact of the sampling frequency in 1996, which was less than in 1995.Maximum peaks in cross-correlation at negative lags were also present for NO3 and Ca (Figure 7d,f,k,m).The occurrence of a concentration maxima of nitrate in stream water well in advance of peak ¯ow has beenattributed to the initial ¯ushing of a limited supply of readily soluble materials (Walling and Foster, 1975;AÂ vila et al., 1992; Creed et al., 1996) or to changes in the proportional contributions of di�erent ¯ow pathsduring the event (AÂ vila et al., 1992). However, the correlations for NO3 were weak, because peaks in NO3

concentration decreased sharply with each succeeding event (Figure 3) (AÂ vila et al., 1992). Note that thecurves for CAwere relatively di�erent in 1995 and 1996, with a positive cross-correlation in 1995 and a cross-correlation close to zero in 1996 at lag 0 (Figure 7f,m). AÂ vila et al. (1992) also observed that the behaviour ofCa in stream water was linked to peak ¯ow in dry antecedent conditions. Under wet antecedent conditions,they noted that chemical changes in the stream were complex and variable, and that a dilution e�ect of Caconcentrations occurred at peak ¯ow. This may explain the absence of cross-correlation at lag 0 in 1996.However, part of this di�erence between 1995 and 1996 may be due to the di�erent sampling frequenciesbetween those two years. Further research is needed on the precise e�ects of the post-processing of thechemical and discharge time-series (e.g. oversampling by linear interpolation, trend removal) on the cross-correlation analysis, particularly when sampling frequencies vary.

Strong negative cross-correlations at lag 0 were evident for SO4, particularly in 1995 (r � ÿ0.57,Figure 7c). The two other solutes which show a decrease in concentration during storm events, Si and Na,both exhibited absolute maximum cross-correlation values at positive lags, thus indicating that the decreasein concentration occurs after the peak in discharge (Figure 7e,g,l,n). This pattern also applied to pH(Figure 7a,h).

Figure 6. Hysteresis of SO4 as a function of stream discharge in 1996 for events (a) #1, (b) #2, (c), #3 and (d) #4. Filled triangles (m)represent the rising limb and open circles (s) the falling limb



Figure 7. Cross-correlation functions in 1995 for (a) pH, (b) DOC, (c) SO4 , (d) NO3, (e) Na, ( f) Ca, (g) Si and in 1996 for (h) pH, (i)DOC, ( j) SO4 , (k) NO3, (l) Na, (m) Ca and (n) Si. The y-axis is the cross-correlation coe�cient. Dashed lines represent the signi®cancelevel. Negative lags indicate that positive or negative peaks in solute concentration occur before the peak in discharge whereas positive

lags indicate that solute concentration peaks after discharge



The cross-correlation plots revealed a statistically signi®cant reversal in the sign of coe�cients as the(positive or negative) lag increased, as can be seen by absolute values of peaks larger than the signi®cancelevels. This was most clearly indicated for Ca during both years (Figure 7f,m), and Na and Si in 1995(Figure 7e). For Ca, a peak in concentration occurred shortly prior to the peak in discharge and was followedby a decrease in concentration some time after the storm event. For Na and Si, an increase in concentrationpreceded the peak in storm event but peak ¯ow was related to a marked decrease in dissolved Naconcentration.

Causes of rapid runo�

The correspondence between discharge peaks, peaks in the concentration of solutes that have a higherconcentration near the soil surface (DOC, NO3, Ca) and minima in the concentration of those that haveelevated concentrations deeper in lower soil solutions (SO4 , Na, Si), indicates that runo� is occurring inupper soil horizons during events. This phenomenon is not surprising during 1996, when the water table wasalready high in the early autumn (Figure 2). However, the same phenomenon occurred during the ®rst eventsof 1995 when the water table close to the stream was low, i.e. prior to 21 October (Figure 2). Rapid surfaceruno� occurred even though the soils close to the stream were not saturated and, hence, the contributingarea, if any, was very limited. Thus, the rapidity of the response and the dry antecedent moisture conditionspresent con¯icting evidence.

These con¯icting data may be partly resolved by invoking soil hydrophobicity which limits in®ltrationduring dry conditions and allows rapid surface runo�. The hydrophobic e�ect has been observed in forestedcatchments in seasonally dry environments and following long dry periods in wetter locations (Burch et al.,1989; Wilson et al., 1990; Crockford et al., 1991; Burcar et al., 1994; Doerr et al., 1996). Hydrophobicityis often associated with burning (Imeson et al., 1992; Doerr et al., 1996), but is also observed in unburnedsoils where the coating of soil mineral particles by organic substances limits in®ltration into the soilmatrix following dry spells, and forces water to ¯ow towards the stream more rapidly than under morehydrophilic conditions (Wilson et al., 1990; Burcar et al., 1994; Doerr et al., 1996). One other possibility isthat the sharp contrast in hydraulic conductivity between the organic and mineral horizons favours arelatively rapid runo� at the interface between these two horizons (Chappell et al., 1990; McDonnell et al.,1991).

CONCLUSIONS

Even when the hydrology of a catchment is well documented, ¯ow-related variation in stream waterchemistry may be complex when storm events are examined in detail (Soulsby, 1995). Our results indicate adi�erent relationship between hydrology and hydrochemistry during the same season for two years. Initialconcentrations of DOC, NO3, Ca and SO4 were much higher after a dry period in 1995 than during wetterantecedent conditions in 1996. Furthermore, the direction of the hysteresis for SO4 changed from an increasein concentration during the falling limb to an increase during the rising limb as moisture content increased.Cross-correlation analysis also revealed that the concentration values of Ca increased with an increase indischarge in 1995, whereas the cross-correlation was close to zero in 1996. Some of the hydrochemicalpatterns reported in this study are similar to observations of the e�ects of antecedent moisture conditions onstream ¯ow and chemistry reported for a Mediterranean forested catchment by AÂ vila et al. (1992). Althoughantecedent conditions in the study catchment di�ered in 1995 and 1996, they were not as sharply di�erent asthose observed by AÂ vila et al. (1992) in regions with intense summer dryness. Thus, even moderate year-to-year variations in antecedent moisture conditions may signi®cantly a�ect the hydrochemistry of a catchment.This temporal variability is in addition to the well-known spatial heterogeneity of stream chemistry inwatersheds (Neal et al., 1997a) and con®rms the di�culty of producing predictive hydrochemical models ofcatchment systems.



ACKNOWLEDGEMENTS

This research was funded by the MinisteÁ re des Ressources Naturelles du QueÂ bec and by The Fonds FCAR.We thank Alain Dufresne for his technological support with the bubblers and laboratory analyses, HeÂ leÁ neLalande and Marie-Claude Turmel for laboratory analyses and ®eld assistance. We also thank threeanonymous reviewers for their helpful and constructive comments.

REFERENCES

Agriculture Canada Expert Committee on Soil Survey 1987. The Canadian System of Soil Classi®cation (2nd edn), Agriculture CanadaPubl. 1646. Canadian Government Publishing, Ottawa, 164 pp.

AÂ vila, A., PinÄ ol, J., RodaÁ , F., and Neal, C. 1992. `Storm solute behaviour in a montane Mediterranean forested catchment', J. Hydrol.,140, 143±161.

Box, G. E. P., Jenkins, G. M., and Reinsel, G. C. 1994. Time Series Analysis: Forecasting and Control (3rd edn). Prentice Hall,Englewood Cli�s.

Burcar, S., Miller, W. W., Tyler, S. W., and Johnson, D. W. 1994. `Season preferential ¯ow in two Sierra Nevada soils under forestedand meadow cover', Soil Sci. Soc. Am. J., 58, 1555±1561.

Burch, G. J., Moore, I. D., and Burns, J. 1989. `Soil hydrophobic e�ects on in®ltration and catchment runo�', Hydrol. Process., 3,211±222.

Chappell, N. A., Ternan, J. L., Williams, A. G., and Reynolds, B. 1990. `Preliminary analysis of water and solute movement beneath aconiferous hillslope in Mid-Wales, UK', J. Hydrol., 116, 201±215.

Courchesne, F. F., Savoie, S., and Dufresne, A. 1995. È�ects of air-drying on the measurement of soil pH in acidic forest soils ofQuebec, Canada', Soil Sci., 160, 56±68.

Creed, I. F., Band, L. E., Foster, N. W., Morrison, I. K., Nicolson, J. A., Semkin, R. S., and Je�ries, D. S. 1996. `Regulation of nitrate-N release from temperate forests: a test of the N ¯ushing hypothesis', Wat. Resour. Res., 32, 3337±3354.

Crockford, S., Topalidis, S., and Richardson, D. P. 1991. `Water repellency in a dry sclerophyll forest: measurements and processes',Hydrol. Process., 5, 405±420.

Doerr, S. H., Shakesby, R. A., and Walsh, R. P. D. 1996. `Soil hydrophobicity variations with depth and particle size fraction in burnedand unburned Eucalyptus globulus and Pinus pinaster forest terrain in the AÂ gueda Basin, Portugal', Catena, 27, 25±47.

Elsenbeer, H., West, A., and Bonnell, M. 1994. `Hydrologic pathways and storm¯ow hydrochemistry at South Creek, northeastQueensland', J. Hydrol., 162, 1±21.

Environment Canada 1993. Analyse des silicates reÂactifs dans l'eau de surface. Laboratoire Conservation et Protection, CentreSt-Laurent.

Evans, C. and Davies, T. D. 1998. `Causes of concentration/discharge hysteresis and its potential as a tool for analysis of episodehydrochemistry', Wat. Resour. Res., 34, 129±137.

Foster, I. D. L. 1978. `Seasonal solute behaviour of storm¯ow in a small agricultural catchment', Catena, 5, 151±163.Giusti, L. and Neal, C. 1993. `Hydrological pathways and solute chemistry of storm runo� at Dargall Lane, southwest Scotland',

J. Hydrol., 142, 1±27.Hendershot, W. H. and Courchesne, F. 1994. È�ect of base cation addition on soil chemistry in a sugar maple forest of the lower

Laurentians, Quebec', Can. J. For. Res., 24, 609±617.Hendershot, W. H., Savoie, S., and Courchesne, F. 1992. `Simulation of stream-water chemistry with soil solution and groundwater

¯ow contributions', J. Hydrol., 136, 237±252.Hewlett, J. D. 1982. Principles of Forest Hydrology. University of Georgia Press, Athens, 183 pp.Imeson, A. C., Verstraten, J. M., Van Mulligen, E. J., and Sevink, J. 1992. `The e�ects of ®re and water repellency on in®ltration and

runo� under Mediterranean type forest', Catena, 19, 345±361.Jenkins, A. 1989. `Storm period hydrochemical response in an unforested Scottish catchment', Hydrol. Sci., 34, 393±404.Jenkins, A., Ferrier, R. C., Harriman, R., and Ogunkoya, Y. O. 1994. À case study in catchment hydrochemistry: con¯icting

interpretations from hydrological and chemical observations', Hydrol. Process., 8, 335±349.McDonnell, J. J., Owens, I. F., and Stewart, M. K. 1991. À case study of shallow ¯ow paths in a steep zero-order basin', Wat. Resour.

Bull., 27, 679±685.McGerrigle, J. I. 1976. `ReÂ gion de Saint-Hippolyte/Sainte-AdeÁ le (QueÂ bec)', Rapport GeÂologique No. 179. MinisteÁ re des Richesses

Naturelles, Ottawa, 41 pp.Muscutt, A. D., Wheater, H. S., and Reynolds, B. 1990. `Storm¯ow hydrochemistry of a small Welsh upland catchment', J. Hydrol.,

116, 239±249.Neal, C., Robson, A. J., and Christophersen, N. 1997a. `Towards coupling hydrological, soil and weathering processes within a

modelling perspective', in Saether, U. M. and Caritat, P. (Eds), Geochemical Processes, Weathering and Groundwater Recharge inCatchments. Brook®eld: A. A. Balkema, Rotterdam, pp. 329±380.

Neal, C., Robson, A. J., Shand, P., Edmunds, W. M., Dixon, A. J., Buckley, D. K., Hill, S., Harow, M., Neal, M., Wilkinson, J., andReynolds, B. 1997b. `The occurrence of groundwater in the lower Palaeozoic rocks of upland central Wales', Hydrol. Earth SystemSci., 1, 3±18.

O'Brien, C. and Hendershot, W. H. 1993. `Separating stream¯ow into groundwater, solum and upwelling ¯ow and its implications forhydrochemical modelling', J. Hydrol., 146, 1±12.



O'Connor, D. J. 1976. `The concentration of dissolved solids and river ¯ow', Wat. Resour. Res., 12, 279±294.Reynolds, B., Hornung, M., and Stevens, P. A. 1987. `Solute budgets and denudation rate estimates for a mid-Wales catchment',

Catena, 14, 13±23.Robarge, W. P. and Johnson, D. W. 1992. `The e�ects of acidic deposition on forested soils', Adv. Agron., 47, 1±83.Slack, J. G. 1977. `River water quality in Essex during and after the 1976 drought', E�uent Wat. Treat. J., 17, 575±578.Soulsby, C. 1995. `Contrasts in storm event hydrochemistry in an acidic a�orested catchment in upland Wales', J. Hydrol., 170,

159±179.Walling, D. E. and Foster, I. D. L. 1975. `Variations in the natural chemical concentration of river water during ¯ood ¯ows, and the lag

e�ect: some further comments', J. Hydrol., 26, 237±244.Walling, D. E. and Webb, B. W. 1986. `Solutes in river systems', in Trudgill, S. T. (Ed.), Solute Processes. John Wiley & Sons,

Chichester, pp. 251±327.Webb, B. W. and Walling, D. E. 1992. `Water Quality II. Chemical characteristics', in Calow, P. and Petts, G. E. (Eds), The Rivers

Handbook. Blackwell Scienti®c Publications, London, 73±100.Wilson, G. V., Jardine, P. M., Luxmoore, R. J., and Jones, J. R. 1990. `Hydrology of a forested hillslope during storm events',

Geoderma, 46, pp. 119±138.



The effects of antecedent moisture conditions on the relationship of hydrology to hydrochemistry in...

Documents

Transcript of The effects of antecedent moisture conditions on the relationship of hydrology to hydrochemistry in...