Tracing runoff sources with deuterium and oxygen-88 during spring melt in a headwater catchment, southern Laurentians, Quebec

Journal of Hydrology, 112 (1989) 135-148

135

Elsevier Science Publishers B.V., Amsterdam - - Printed in The Netherlan& [3]

TRACING RUNOFF SOURCES WITH DEUTERIUM AND OXYGEN-18 DURING SPRING MELT IN A HEADWATER CATCHMENT, SOUTHERN LAURENTIANS, QUEBEC

R.D. MOORE

Geography Department, Simon Fraser University, Burnaby, B.C. VSA lS6 (Canada) (Received September 2, 1988; accepted after revision February 14, 1989)

ABSTRACT

Moore, R.D., 1989. Tracing runoff sources with deuterium and oxygen-18 during spring melt in a headwater catchment, southern Laurentians, Quebec. J. Hydrol., 112: 135-148. Deuterium and oxygen-18 were used as passive tracers to separate streamflow into "old" or "pre.event" and "new" or "event" components during spring snowmelt in 1987 in a small catchment in the southern Laurentians of Q,Jebec. The old water component of flow appeared to respond to the diurnal snowmelt cycle, accounting for up to 85% of the hydrograph rise from morning to late afternoon. Assumptions concerning constancy of new water concentrations and homogeneity of old water were not satisfied; however, sensitivity analyses indicated that it can be concluded that old water was a major component of streamflow. Mobilization of old vadose storage may have been an important process in runoff generation. Sampling variability of new water concentrations is documented and consequent uncertainty in separations is discussed.

INTRODUCTION

Studies of watershed hydrology have used the stable environmental isotopes, deuterium (D) and oxygen-18 (1sO), to separate streamflow in'co its "old" and "new" components for both rainfall and snowmelt events to aid in identifying runoff" processes (e.g., Sklash ar, ~ Farvolden, 1979; Bottomley et al., 1986; Kennedy et al., 1986; Hooper and Shoemaker, 1086; Sklash et al., 1986). Separations are based upon the following mixing equation for conservative tracers:

cQ

-

coQ o + CnQ

(1)

where c is the concentration of some tracer in streamwater, Q is streamflow (l s-l), Qo and Qn are the contributions to streamflow respectively from the old and new waters, and Co and cn are the concentrations of tracer in the old and new waters. Equation (1) can be combined with the mass balance constraint: Q

=

Qo +

(2)

• ,. a~.~.,~ ,ho ¢.nn.,;,~o

0022-1694/89]$03.50

-,-,,,~,~a~;a,~

for f.ho. ~nnt~h, lLinn_ of old water:

© 1989 Elsevier Science Publishers B.V.

136

(3)

Qo = Q(c - Cn)/(Co -- C.)

This paper describes the use of eqn. (3) to separate runoff into old and new components during snowmelt in a headwater catchment in the southern Laurentians, Quebec. Particular attention is paid to the variability of Cn and the resulting uncertainty in application of eqn. (3) during snowmelt. DESCRIPTION OF THE HELD

SITE

The study during spring 1987 was located in the Hermine basin, approximately 85 k m north of Montreal, Quebec, in the southern Laurentians at an elevation of approximately 400m. The catchment has an area of 5.3ha and approximately 30 m of relief(see Fig. 1).The regolithis composed of glacialtill overlying Canadian Shield bedrock. The tillhas an average depth of about lm, but tends to be thinner with rock outcrops in upslope areas and thicker in the depressions and valley bottoms. Soil textures range from loam tn loamy sand with variable contents of gravel, stones and boulders. Deciduous forestcovers the catchment completely; it is dominantly sugar maple and beech (Acer saccharum and Fagus grandifolia), with some birch (Betula alleghanensis). The Hermine basin is drained by a first-order stream, although channelized flow can occur over some hillslopes during high-flow episodes. Overland flow appears to occur only where the water table rises tc the surface. Frozen soil, occasionally observed during excavation of snow pits and taking of snow cores,

J

- ),,

,"

-sY /

/

ms

0

I

5ore

Fig. 1. M a p of Hermine basin and sampling sites.Squares indicatelysimeters,circlesindicatewells and the triangle indicates the weir. Elevations are in",- above ,k^~..~ ...~... l

137

is areally discontinuous and restricted to a thin surface layer, and does not restrict infiltration.Concrete frost has not been observed. The streamflow regime is dominated by the spring melt period, with a secondary peak due to autumn rainfalls.The stream dries up during summer droughts, but flows throughout winter. A drilling survey with an auger during November 1986 indicated that phreatic water at that time was limited to the lowest portions of the slopes. METHODS

Streamflow was determined from observations of stage at a V-notch weir at the basin outlet starting February 22, 1987. The rating curve for the weir was obtained from potassimn-dichromate dilution gauging at high flows and by volumetric gauging at low flows. Continuous recordings of stage were available only after day 80 (March 21) because of problems with ice on the weir and in the stilling well of the water-level recorder during the earlier part of the study period. Daily precipitation and m a x i m u m and minimum temperatures were obtained from a Quebec Ministry of the Environment climate station (St-Hippolyte) located approximately 1.5k m from the basin. Snow-pack water equivalent was determined from measurements of depth and density at thirty fixed points equally spaced along a traverse through the basin. Trenches were dug through the snow across hillslopes near wells 1, 3 and 4 to observe the occurrence of overland flow. Isotopic concentrations in the snow pack near the beginning and end of the melt period were determined by melting and analysing bulked snow samples. During the main melt period, outflow from the snow cover was collected from plastic trays approximately 30 cm by 60 cm in size with 20 cm-high walls. Five lysimeters were installed on day 81, and three more on day 83. Sampling was carried out in mid-afternoon, to coincide roughly with the time of m a x i m u m daily melt. Samples of shallow groundwater were collected from four wells (see Fig. 1 for locations). Samples of streamwater for isGtopic analyses were collected on several days throughout the melt period; an attempt was made to sample at the lowest and highest points of the diurnal hydrograph cycle. Rainfall samples for isotopic analyses were taken from a gauge located within the basin. Each sample was decanted into a 60 ml polyethylene bottle which was filled to its brim, and then tightly sealed and stored at 4°C until analysed. Samples were analysed for 180 at the G E O T O P E laboratory at the University of Quebec at Montreal and for D at the Ontario Hydro Research Division. Concentrations of ~sO were given in "delta" notation: that is: ~X

=

(I000~)

(Rxsmple -

RXmow)/RXsmow

where x represents the isotopic abundance (D:H or IsO:160) and the subscript "smow" refers to "Standard M e a n Ocean Water." Concentrations of D were

138

supplied in parts per million but were converted for ease of comparison with other studies to per rail using RD~mow= 155.76 parts per million (Gat, 1980). Repeated analyses of samples for 6180 indicate an analytical precision of + 0.2 %0, while the precision for 6D is approximately _+ 1%0. DESCRIPTION OF THE STUDY PERIOD

Figure 2 summarizes several hydroclimatic variables during the study period. Observations began on day 53 (February 22),when the discharge was 0.0961 s- i,the lowest observed winter flow for the period of record (1985-1987). Prior to day 80 (March 21), only small amounts of melt occurred during occasional sunny, warm days. More rapid melt and runoff occurred between days 80 and 91. The firstrainfallof the study period occurred on day 85, and heavy rainfall

_

3o1 o

,

,,il,

,ll

llll,

-3O

,, |

I, Ii 11

, ,,,,,llii,,li,, ,I1",,,,,llll lil lllll, llll "" 11

g

|

•

•

o.eao] O, 20,m,

.J 10' O T

,EE'-t'u300 E 1 50

60

7

B0 90 JULIAN DAY 1987

100

I

1

Fig. 2. Time series plot of several hydroclimatic variables during the study period, including from the top: (a) daily temperature range (°C); Co) daily precipitation in mm (s indicates sncv,~alls); (c) streamfiow in I s-1; and (d) mean water equivalent of the snow cover in nun.

139

occurred between days 89 and 91: streamflow data between days 89 and 9! are considered unreliable because the stage record appeared somewhat erratic. A snowfall on day 91 followed by cooler weather produced a recession which lasted until day 95. Warmer weather, from day 95 onwards, produced rapid ablation of the remaining snowpack, which became increasingly patchy and disappeared altogether by day 105. Overland flow was first observed on day 81. The extent of surface-saturation and overland flow varied roughly in concert with streamflow variations. However, rigorous monitoring of the extent of surface-saturation and overland flow was not feasible because of its dynamic nature and detailed spatial structure: seepage areas did not extend uniformly upslope, as in the maps of Taylor (1982), but formed in the "valleys" between hmnmocks. ISOTOPIC C O N T E N T S OF OLD A N D N E W W A T E R

Definitions of old and new water Old water refers here to water which was stored in t}~e vadose, tensionsaturated and phreatic zones in the catchment on day 53. N e w water refers to the water released into the catchment by melt and rainfall following day 53. Old water contents were assumed to equal the streamwater values for day 53 ( - 11.1%0 for 180 and -67.2 %0 for D). If the isotopic content~ of meltwater and rainwater vary through time, the isotopic contents of new water in the stream would also vary through time, depending on both the time-varying contents of the water released to the catchment and the routing of this water to the stream channel. Because this routing cannot be accouvted for,constant values for c, were used, equal to the grand means of the values of ~D and ~IsO shown in Table 1 (- 18.4 %0 for IsO and - 123.4 %0 for D).

Variability of new water isotopic concentrations Isotopic contents of lysimeter outflow, bulk snow samples and rainwater are respectively given in Tables 1, 2 and 3. With the exception of the meltwater samples from day 83, the sT:owcover waters became heavier through the melt period. Hermann et al. (1981) observed similar variations during laboratory experiments. Two-way analyses of variance indicate that, for both isotopes,the difference among days is significant (p < 0.001).However, the null hypothesis that no difference existsamong lysimeters cannot be rejected at a 95% confidence level (p = 0.057 for D and p = 0.595 for 180). Estimates of the standard deviations of the spatial variability were derived by pooling estimates of the standard deviations of ~mO and ~D within days, yielding respective values of 0.7 and 6.0 %0. These standard deviations are substantially greater than the analytical errors.

140 TABLE

1

Isotopic

concentrations

Lysimeter

(%0) o f l y s i m e t e r

outflow

Day 81

83

85

88

~D 1

- 124.9

- 135.9

- 120.4

- 115.9

2

- 119.8

- 130.7

- 118.5

- 115.9

3

- 119.2

- 148.7

- 124.9

- 122.4

4

- 128.1

-

131.4

-

110.8

-

106.3

5

- 118.5

-

141.6

-

115.3

-

110.2

6

- 124.3

- 113.4

- 115.3

7

- 133.9

- 123.0

- 119.8

8

- 141.0

- 124.3

- 129.4

~Is 0 1

- 18.1

-

i9.5

-

18.5

-

17.2

2

- 17.7

-

19.0

-

18.1

-

17.9

3

- 17.0

- 20.6

- 18.7

- 16.4

4

- 18.7

-

-

-

5

- 18.4

- 20.8

- 17.7

- 16.7

6

- 20.4

- 17.2

- 17.2

7

-

-

8

- 29.7

TABLE

2

Isotopic

contents

(%0) o f b u l k

snow

19.8

19.6

16.5

18.1

- 18.6

-

17.3

17.9

- 18.7

samples

Day

~lsO

6D

67

- 18.6

- 124.9

99

- 15.2

- 103.1

TABLE

3

Isotopic

contents

(%0) o f r a i n w a t e r

Day

~180

86

- 13.3

- 87.0

87

- 14.5

-

91

- 10.4

-68.4

94

- 14.7

99

- 12.1

6D

92.8

141

Isotopic concentrations of the rain varied, but were substantially greater than those for the snowcover waters. Insufficientsample volumes precluded analyses for D for the last two rain events.

Homogeneity of old water Table 4 shows isotopic contents of water samples taken from the wells, which are a mixture of new and old waters. The lack of significant variation amongst the wells (not counting the anomalous values for well 4 for day 81) does not contradict the assumption of spatial homogeneity of old water concentrations, considering that the meltwater concentrations did not appear to exhibit significant spatial variations. A necessary, but not sufficient, condition for homogeneity is that the points on a j~80-JD mixing diagram representing the streamwater, shallow groundwater and the overland flow samples should lie on a line between the points representing the old and new waters. However, in the case of uncertainty or variability in the isotopic contents of old and new waters, water samples which are mixtures of old and new water may be scattered somewhat about the mixing line, producing a mixing zone. Figure 3 is a mixing diagram showing the streamwater, shallow groundwater and overland flow samples. A mixing zone, accounting for uncertainty in the values of Co and cn, is shown. Several points lie outside the mixing zone, indicating that the assumption of homogeneity may be invalid;that is, not all of the old water had the same isotopic concentrations as the baseflow sample. SEPARATION

OF RUNOFF

INTO

OLD

AND

NEW

COMPONENTS

Separations were only considered reliable up to and including day 88. TABLE

4

Isotopic

concentrations

Well

(%0) o f s h a l l o w

groundwater

D~y 81

87

94

106

~D 1

- 66.5

- 89.6

- 89.6

- 86.4

2

- 69.1

- 92.8

- 89.0

- 88.3

3

- 69.1

- 92.8

- 88.3

- 88.3

4

- 103.7

- 89.6

- 87.1

- 90.3

~Is 0 1

- 11.8

- 14.3

- 14.1

- 13.5

2

- 12.1

- 13.7

- 14.0

- 13.7

3

- 12.1

- 13.7

- 13.7

- 13.7

4

- 17.0

- 13.3

- 13.9

- 14.0

142

-so-~

-60 []

-100 -

-120-

-1~0 -21

l

-19

-17

-15

-13

-11

8t80 (%,,)

Fig. 3. Mixing diagram for water samples. Crosses represent meltwater samples, squares represent streamwater samples, diamonds represent well samples and the triangle represents an overland flow sample. Error bars for meltwater are 95% confidence intervalsfor the daily means. Error bars for the baseflow sample represent the analytical errors.

Following day 88, rainfallshaving isotopic concentrations different from the meltwater would have substantially changed the values for c, from those used for the calculations. The separations up to those for day 85 preceded any rainfall, and by day 88, most of the rainwater from the storms of days 85 and 86, which totalled only 12mm, should have left the catchment or have been largely diluted by meltwater. In Table 5, the calculated values for Qo using IsO and D show reasonable agreement, and illustrate the apparent responsiveness of old water to the diurnal melt cycle. The ratio of the change in Q0, from morning to afternoon, to the corresponding change in Q, is shown in Table 6; the increase in the old water component accounts for up to 85% of the total increase in streamflow. The responsiveness tends to decrease through the melt period. A n estimate of the integrated value of Qo, for the period from day 53 to day 88 inclusive, has been calculated by assuming a conservative mean value for Qo/Q of 0.5,which is lower than all calculated values. The cumulative outflow up to the end of day 88 is approximately 5100m 3 (flow prior to day 80 has been estimated on the basis of the manual observations), of which old water therefore constitutes at least 2500 m 3. The isotopic contents of an overland flow sample collected near lysimeter 1 at 15:55 on day 83 (the firstday overland flow was observed, and on which melt rates were relatively high) are - 13.5 %0 for 180 and - 8 6 . 4 %0 for D. A p p l y i n g

143 TABLE

5

Isotopic concentrations

of streamwater

and calculated

old water contributions

to stre-mflow

Time

Q

6'sO

5D

Qo(180)

QofD)

(day)

(1 s - ' )

(%0)

(%0)

(1 s - ' )

(1 s - ' )

5S.50

0.096

- 11.1

- 67.2

0.096

0.096

66.61

0.592

- 12.7

- 80.0

0.462

0.457

67.40

0.183

- 12.8

- 76.1

0.140

0.154

67.62

0.592

- 12.3

- 75.5

0.495

0.505

70.62

0.166

- 11.6

- 72.3

0.155

0.151

73.51

0.108

- 11.4

- 72.3

0.104

0.098

75.43

0.446

- 13.3

- 87.1

0.312

0.~ 0.093

76.44

0.108

- 11.3

- 74.9

0.105

80.57

0.218

- 11.9

- 78.7

0.194

0.173

80.72

0.321

- 12.9

- 78.1

0.242

0.259

81.39

0.393

- 12.7

- 81.9

0.307

0.290

81.56

0.952

- 13.9

- 85.1

0~587

0.649

81.70

1.947

- 14.1

- 90.9

1.147

1.126

83.42

2.451

- 12.7

- 81.3

1.914

1.836

83.54

3.322

- 13.1

- 83.8

2.412

2.341

83.67

9.027

- 13.7

- 87.7

5.812

5.734

84.51

4.611

- 12.2

- 87.1

3.916

2.978

84.66

12.214

- 14.0

- 87.1

7.362

7.889

85.39

13.335

- 14.2

- 89.0

7.672

8.162

87.44

4.611

- 14.1

- 87.1

2,716

2.978

87.58

6.456

- 14.2

- 92.2

3.714

3.584

88.44

4.426

- 14.1

- 88.3

2.607

2.764

88.58

6.911

- 14.3

- 90.9

3.882

3.997

88.65

8.762

- 14.7

- 92.2

4.441

4.864

88.69

9.707

- 14.5

- 91.6

5.186

5.493

91.61

5.595

- 14.3

- 90.3

91.64

4.993

- 14.3

- 90.9

92.40

2.451

- 14.0

- 88.3

94.39

0.952

- 13.8

- 88.3

95.50

4.518

- 14.8

- 92.2

95.60

6.018

- 14.0

- 88.3

99.43

2.129

- 14.4

- 84.5

102.49

1.166

- 14.0

- 85.8

eqn. (3), with previously-defined values for Coand cn, indicates that the overland flow contained over 60% old water and this is an underestimate because the lysimeter outflow was depleted that day compared to the mean values used to define c,. This result indicates that seepage rates were greater than the rate of production of overland flow through deflection of meltwater. SENSITIVITY

OF RESULTS

TO VIOLATIONS

OF ASSUMPTIONS

Precise application of eqn. (3) requires five conditions (after Sklash and Farvolden, 1979): (1) the old and new waters have different isotopic contents;

144 TABLE 6 Ratio of maximum observed diurnal iacrease in Qo to corresponding increase in Q (based on separations using D) Day

Change in Qo (1 s -1)

Change in Q (1 s -1)

Ratio (%)

67 80 81 83 84 87 88

0.35 0.086 0.84 3.90 4.91 0.61 2.73

0.41 0.103 1.56 6.58 7.60 1.85 5.28

85 83 54 59 65 33 52

(2) the isotopic contents of the new water remain constant throughout the event; (3) the isotopic contents of waters in the vadose and phreatic zones present in the watershed prior to the event are either identical, or else only one of these components makes significant contributions to streamflow; (4) isotopic contents of old water are spatially uniform; and (5) contributions to flow from surface storage are negligible. Assumptions (1) and (5) were satisfied during the 1987 spring melt, but assumption (2) was not and assumptions (3) and (4) may not have been.

Sensitivity to uncertainty in c, To assess the sensitivity of the results to variations in cn, calculations were made with c, set equal to the minimum and maximum observed mean daily concentrations calculated from the data in Table 1; these were respectively -20.1 and -17.4%o for ~180 and -135.9 and -116.1%o for ~D. Although the meltwater contents may have had a greater range than that used in this analysis, cn at a given time is a weighted mean of all previous values of the isotopic concentrations in meltwater; the range of c~ will therefore be less than the range of values for meltwater. The potential error in Qo/Q due to errors in c, increases with decreasing c (that is, decreasing contributions from old water). In the case of separations using 180, the greatest uncertainty occurs for c = -14.7%o, which was observed on day 88: the range of values for Qo/Q is 0.43 to 0.55. For separations using D, the greatest uncertainty occurs for c = -92.2%0, which occurred on days 87 and 88: the corresponding range of values for Qo/Q is 0.50 to 0.65. This analysis indicates that the uncertainty due to using a constant value for c~ does not affect the qualitative finding that old water made a major contribution to streamflow in this study, comprising at least half the streamflow up to the end of day 88.

145

Sensitivity to uncertainty in co The mixing diagram indicates that the old water may not have been homogeneous, which would introduce uncertainty into the adopted value of Co. If some of the old water were depleted in heavy isotopes compared to the baseflow, then the calculated contributions of old water would be underestimates. On the other hand, if some of the old water were isotopically heavier t h a n the baseflow, t h e n the calculated old water contributions would be overestimates. Old water contributions were recalculated using a value of 5180 = -5%0, which is a typical value for summer rainfall in this area (Hinton, 1988) and which should represent an upper limit to the isotopic concentration of old waters. These calculations indicate that old water contr..b,.~-.,ns would be approximately half that calculated using a value of - 11.1%0 for Co, so that the minimum contribution of old water would be greater than 25% of the total streamflow to the end of day 88. However, even if some of the old water were as heavy as a rainfall sample, the effective value of Co would not be as extreme as assumed in the calculation. DISCUSSION

Problems in determining isotopic contents of new water If snow cores were used to specify c,, errors could be on the order of 2%0 for $1sO and 10%0 for 5D, based upon the difference between a linear interpolation between the isotopic concentrations of the bulk snow samples in this study and the isotopic contents of the lysimeter outflow. The resulting error in the calculation of Qo/Q dep~:lds on tile values of c and Co, and drawing of generalizations is difficult. The quantitative error in this study would have been substantial. For example, using c = - 14.75%oofor 180 (corresponding Qo/Q = 0.5), varying c, by + 2%0 results in values for Qo/Q ranging from 0.31 to 0.61. The spatial variations in 1sO and D contents of lysimeter outflow did not appear to be systematic and had standard deviations of 0.7 and 6.0%0, respectively. The sampling errors for a single lysimeter sample would therefore be approximately 1.4%0 for $lsO and 12.0%o for $D, at a 95% confidence level. These errors would have been quantitatively significaut here. Another problem is specifying c, arises when rainfall occurs, because the isotopic composition of rainfall often differ:'- substantially from that of meltwater. In principle, a three-source separation could be carried out using two tracers. In practice, however, points representing rainfall, baseflow and meltwater were roughly collinear on a mixing diagram for JIsO and t}D, precluding the calculation of unique values for Qo and Q,.

Runoff mechanisms It was observed that the water table rose through the melt period accom-

146

panied by an upslope extension of the phreatic zone, fed by infiltrating new water and/or mobilized old vadose water. The isotopic composition of the overland flow sample indicates that, on some hillslopes at least, old vadose water rather than infiltrating meltwater was important in feeding the growing phreatic zone. The responsiveness of the old water to the diurnal melt cycle and the large old water component in overland flow indicate that direct runoff of meltwater over surface-saturated areas was not the dominant runoff mechanism, although it did occur. Two alternatives are macropore flow and the groundwater ridging mechanism as described and documented by Sklash and Farvolden (1979), Abdul and Gillham (1984) and Novakowski and Gillham

(1988). Macropore flow would probably involve ponding of water above a relatively impermeable layer during meltwater infiltration and the preferential flow of this phreatic water through macropores, possible becoming return flow in convergence zones. If this mechanism were important, then old vadose water must have been mobilized during meltwater infiltration and incorporated into the ponded layer, to explain the observed responsiveness of old water. Vadose water could have been mobilized by displacement by and/or mixing with infiltrating meltwater, and/or by incorporation into the phreatic zone as the water table rose through the melt period. A check can be made on the available volume of old vadose water by assuming that the soils were at "field capacity," which would be a water content of approximately 0.15 m3m -3 for sandy loam soils. This is not unreasonable because soil moisture would be replenished by autumn rainstorms, with little evapotranspiration occurring before snow cover was established. Using a conservative mean soil depth of 0.5 m, the old vadose storage in the catchment would amount to 4000 m 3, which is indeed greater than the estimated old water contribution up to the end of day 88. If groundwater ridging were important, then downslope movement of old vadose water may have helped to maintain the responsive groundwater ridge through the melt period, as the extent and volume of old phreatic water appeared to be limited before the onset of melt. It is possible that macropore flow and groundwater ridging occurred simultaneously in different portions of the catchment, and perhaps interacted; for example, macropore flow could have fed the groundwater ridging zone(s). More detailed s~udy at a hillslope scale would be required to resolve the relative importance of the two mechansims. In either case, the apparent decrease in the relative responsiveness of old water through time could have resulted from the increasing incorpgration of new water into the phreatic zone through the melt period, rather than through the increasing importance of surface-saturated areas in generating overland flow through deflection of meltwater. Subsurface mixing of new and old waters is reflected in the change in the isotopic composition of shallow groundwater between days 81 and 87 (see Table 4).

147 CONCLUSIONS

(1) Separation of streamflow into new and old components during snowmelt periods is imprecise because the isotopic concentrations of the new water are not constant through time. Further uncertainty in specifying the appropriate values for cn could arise from the spatial variability of meltwater concentrations and the fact that meltwater concentrations can differsubstantially from those of snow cores. Future studies should sample lysimeter outflow at a number of sites to reduce sampling error. (2) Addition of rainfall, which often has isotopic concentrations differing from those of meltwater, complicates the separation. Although in principle a three-component separation could be calculated using two tracers, the nearcollinearity of" points representing rainfall, meltwater and old water on a 5~sO-~D mixing diagram precludes such a separation in practice. (3) Direct runoff of meltwater over surface-saturated areas did not dominate runoff generation. Macropore flow and groundwater ridging are two possible alternative mechanisms. In either case, mobilization of old vadose water may have played an important role in streamflow generation. Further detailed study at plot and hillslope scales, utilizingboth tracer and hydrometric techniques as recommended by Sklash et al. (1986),would need to be ca~ied out to testthis proposition. ACKNOWLEDGEMENTS

Tbe research was funded by an operating srant from the Natural Sciences and Engineering Research Council of Canada. The assistance of the following individuals and organizations is gratefully acknowledged: W. Hendershot and F. Courchesne of Macdonald College; the staffof the Biological Station of the University of Montreal; C. Hillaire-Marcel and C. Guillemette •f the University of Quebec at Montreal; H. Mehdi of Ontario Hydro; J. Rowland and M. Hinton of McGill University; and the Geography Departments, McGill University and the University of British Columbia. I appreciate comments by G. Barrett, C. Burn and P. Jordan on drafts of this paper. REFERENCES Abdul, A.S. and Gillham, R.W., 1984. Laboratory studies of the effects of the capillary fringe on streamflow generation. J. Hydrol., 20: 691-698. Bottomley, D.J., Craig, D. and Johnston, L.M., 1986. Oxygen-18 studies of snowmelt runoff in a small precambrian shield watershed: implications for streamwater acidification in acid.sensitive terrain. J. Hydrol., 88: 213-234. Gat, J.R., 1980. The isotopes of hydrogen and oxygen in precipitation. In: P. Fritz and J. Ch. Fontes (Editors), Handbook of Environmental Isotope Geochemistry. Elsevier, New York, N.Y., pp. 21-47. Henmann, A., Lehrer~ M. and Stichler, W., 1981. Isotope input into runoff systems from melting snow covers. Nord. Hydrol., 12: 309-318. Hinton, M.J., 1988. Interception loss from a deciduous canopy and its effect on the isotopic concentration of summer precipitation. B. Sc. (Hons.) Thesis, Geography Department, McGill

148 University, Montreal, Que. (unpubl.) Hooper, R.P. and Shoemaker, C.H., 1986. A comparison of chemical and isotopic hydrograph sevaration. Water Resour. Res., 22: 1444-1454. Kennedy, V.C., Kendall, C., Zellweger, G.W., Wyerman, T.A. and Avanzino, R.J., 1986. Determination of the components of stormflow using water chemistry and evironmental isotopes, Mattole River Basin, Cahfornia. J. Hydrol., 84: 107-140. Novakowski, K.S. and Gillham, R.W., 1988. Field investigations of the nature of water-table response to precipitation in shallow water-table environments. J. Hydrol., 97: 23-32. Sklash, M. and Farvolden, R.N., 1979. The role of groundwater in storm runoff. J. Hydrol., 43: 45--65. Sklash, M.G., Stewart, M.K. and Pearce, A.J., 1986. Storm runoff generation in humid headwater catchments 2. A case study of hillslope and low-order stream response. Water Resour. Res., 22: 1273-1282. Taylor, C.H., 1982.The effect on storm runoff response of seasonal variations in contributing zones in small watersheds. Nord. Hydrol., 13: 165-182.