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Variations in stable oxygen isotope and solute concentrations in small submediterranean montane streams
Journal of Hydrology, 144 (1993) 283-290
283
Elsevier Science Publishers B.V., A m s t e r d a m
[31
Variations in stable oxygen isotope and solute concentrations in small submediterranean montane streams P. Durand, M. Neal and C. Neal Institute of Hydrology, Walling.ford OXIO 8BB, UK (Received 24 April 1992; revision accepted 15 August 1992)
ABSTRACT Durand, P., Neal, M. and Neal, C., 1993. Variations in stable oxygen isotope and solute concentrations in small submediterranean montane streams. J. Hydrol., 144: 283-290. Stable oxygen isotope concentrations in rainwater and streamwater were determined, combined with routine hydrochemical monitoring of three granitic catchments in the South of France. 6~80 in streamwater showed much less variation than in rainwater. A seasonal pattern in baseflow conditions (higher 6~80 in summer) suggests that the groundwater store is small and subject to evaporation. Variations in high flow conditions show that pre-event water can contribute significantly to streamflow, especially in non-forested areas. Alkalinity and silica concentration are the only other parameters that show consistent behaviour during storm events. The latter could be used as an alternative to 6~80 for hydrograph separation.
INTRODUCTION
Environmental oxygen and hydrogen isotopes are widely used in catchment studies as tracers for hydrograph separation (Martinec, 1975; Sklash and Farvolden, 1979; Rhode, 1981; M6rot et al., 1981; Loye-Pilot and Jusserand, 1990). This usually involves detailed sampling of some flood events, which can be expensive and time-consuming as the calculations can be performed only if the rainfall isotopic composition is sufficiently different from the pre-event water composition (Sklash and Farvolden, 1979). Before such detailed studies, a simple monitoring of the isotopic ratios in rainwater and streamwater,, combined with the routine chemical monitoring, provides valuable information on the hydrology of the study area (Neal and Rosier, 1990). This can help focus investigation on the most relevant parameters and periods. The results of such an approach are presented here for three small granitic catchments in the southern Massif Central (France). C o r r e s p o n d e n c e to: P. D u r a n d , Institute of Hydrology, Wallingford O X I 0 8BB, U K .
0022-1694/93/$06.00
© 1993 - - Elsevier Science Publishers B.V. All rights reserved
284
P. DURAND ET AL.
TABLE 1 Statistics o f ~ 8 0
d a t a (in %o with respect to S t a n d a r d M e a n O c e a n W a t e r ) for the s t r e a m w a t e r s
a n d r a i n w a t e r for the p e r i o d S e p t e m b e r 1 9 9 0 - S e p t e m b e r 1991 (N, n u m b e r o f d e t e r m i n a t i o n s ) N
Mean
SD
Minimum
Maximum
Rain Grassland stream
30 20
- 7.47 - 7.27
3.22 0.95
- 13.50 - 8.54
- 1.09 - 4.70
Beech s t r e a m Spruce stream
23 27
- 7.31 - 7.38
0.66 0.56
- 8.33 - 8.66
- 6.00 - 6.62
STUDY AREA AND METHODS
Three small catchments, located on the south-facing slope of a granitic mountain in southeastern France, have been extensively studied in a wideranging environmental research programmes (Lelong et al., 1990; Durand et al., 1991). The altitude range is 1200-1500m and the average annual temperature is 6°C. The mean annual rainfall is 1950 mm and precipitation is unevenly distributed over the year, which results in very dry summers and strong flood events in spring and autumn. In one catchment, La Sapine, which is covered with beech coppice, the evaporation loss is about 30%, whereas in the two others (a grassland area, Les Cloutasses, and a recently clear-felled spruce forest, La Latte) it is only 20%. A detailed description of the research programme can be found elsewhere (Durand et al., 1991). The chemical composition of rainwater and streamwater has been monitored, on an event basis, since 1981. Subsamples for oxygen isotope analysis were taken from September 1990 to September 1991, and stored in hermetically sealed glass bottles. 6180 determinations were performed at the British Geological Survey-Institute of Hydrology joint isotope facility at Wallingford, using a VG-602E (VG Analytical, Wythenshawe, Manchester, UK) mass spectrometer and a CO2 equilibration technique (Darling et al., 1992). RESULTS
Stable oxygen isotope variations
The three catchments exhibit a 'damped' response to the rainfall signal: the standard deviation of 61SO in rainwater is 3-6 times higher than in streamwater (Table 1, Fig. 1). This feature has also been noted for the major element concentrations (Durand et al., 1991). The 6~so variations are similar in the three streams. Two types of variation can be observed, one in baseflow and the other in high flow conditions.
STABLE
S+O
OXYGEN
ISOTOPE
VARIATIONS
IN
MONTANE
285
STREAMS
(=/~)
0
rain -
beech
.•
grass spruce
+ 4+ ,~-
-5 -
:r :
-
,
-+
+'
'
~
-10
-15 60
o<,,s,i,
20 21109190
21/12•90
21/03/91
21/06/91
19/09/91
Date Fig. 1. Oxygen isotope concentrations in rainwater and streamwater in the three Mont-Loz6re catchments and instantaneous discharge during sampling in the Sapine stream, from 7 September 1991 to 19 September 1992.
There is a seasonal pattern in baseflow conditions, with the 'heaviest' water (i.e. that most enriched in 5180) occurring in late summer and the lightest water in spring. As the input of rainwater to the groundwater store is insignificant in summer, this means that some evaporation takes place from the groundwater, probably from the peat zones and free drainage channel (the summer increase of 5t80 is more marked at Les Cloutasses, which has the largest peat area and the greatest channel length). However, this possibility needs to be confirmed by hydrogen isotope measurements. In high flow conditions, the signal of the event water sometimes reaches the outlet. This was observed during two very different episodes. In October 1991, the flood was due to 300 m m of rainfall in a few days. This is an event of normal magnitude for the period, which did not result in a major flood because of the high saturation deficit of the catchments before the onset of the rainfall. 6t80 of pre-event streamwater was around - 6.5 to - 7%o compared with -9%o for the event rainwater. During the flood, streamwater 5~80
286
P. D U R A N D ET AL.
decreased by 0.5-1%o in the three rivers. The second episode was a summer thunderstorm, with an intense rainfall of 90 m m in a few hours. The rainwater 6~80 was high (-2.8%o), and the streamwater exhibited a strong increase in 3180, especially in the Cloutasses catchment, 6~sO increased from -7%o before the event to - 4.7%o in the rising part of the hydrograph. The major flood of the study period occurred in March 1991, as a result of the coincidence of snowmelt and rainfall. No significant pattern was observed in the oxygen isotope variations. However, it cannot be concluded that no rainwater reached the stream, primarily because only one sample per river was collected during the event, and all the samples were taken at times other than that of the peak flood. Moreover, the snow and rainwater producing the flood had very variable 6J80 values ( - 12 to - 6.6%o), whereas the streamwater 6~80 value was around - 7.5%o. This difficulty has also been pointed out by Hooper and Shoemaker (1986) as a serious limitation on the use of stable isotopes for the study of snowmelt events. However, 6~sO in streamwater remained very negative during the weeks after this event; even if the ~SO-depleted winter rainwater and snow did not contribute to the discharge as surface flow, they made up most of the spring groundwater without significant fractionation.
Other hydrochemical variations There is no significant relationship between chemical composition and isotope concentrations in rainwater. However, the highest 6180 was measured for a small event with very high alkalinity and Ca concentrations; these features characterize a particular type of event known as 'red rain', i.e. rainfall with a large amount of aeolian dust from the Sahara (Avila and Roda, 199 l; Durand et al., 1992a). Further measurements are required to determine whether high 6t80 values characterize this type of rainwater. Few parameters showed systematic patterns in high flow conditions. In most cases, there are overlaps between concentration ranges in rainwater and streamwater, which result in either dilution or concentration during floods (Durand et al., 1991). The exceptions are for alkalinity and silica concentrations, which decreased during each episode. The variations of these two parameters confirm the conclusions of the isotope study, at least in the Sapine and Cloutasses catchments (Fig. 2). The two floods showing the rainwater signal in streamwater also showed a strong decrease in alkalinity and silica concentration. Conversely, during the rainfall-snowmelt episode of March, only a slight decrease was observed. In the clear-felled spruce catchment, La Latte, the hydrochemical variations are determined by the strongly seasonal release of N O 3 a s a result of the mineralization of the organic pools after the felling.
287
STABLE OXYGEN ISOTOPE VARIATIONS IN MONTANE STREAMS
/~eq/I
mg/I 2
Si
1.5
• ii
~ ......
h
•
A
0,5
[] --
A
Beech
'
'
o
Spruce
'
' - -
Alkalinity
oo
©
Grassland
,,
.-
".
~
"',
," " ' - - .
' o
P&
A'"
''(3
0
,
,,-'"
80
°
9
/
", ,"
40
0 ,
~
,
,
L
-40
Cl o o
oP'
o
',
i
i
21/09/90
21/12/90
o
i
21/03/91
o
c~ 0
:
L
21/06/91
/ 19/09/91
Date Fig. 2. D i s s o l v e d silica concentrations, alkalinity and chloride c o n c e n t r a t i o n s in s t r e a m w a t e r (and rainwater for chloride) during the study period.
To verify the agreement of the results of the chemical and isotopic approaches, the proportion of pre-event water at two points of the summer flood was calculated, in the Cloutasses and the Sapine catchment, using the 6~80 and the Si concentration data. The two-component mixing model formula was used: Qp
cv - CE
QT
Cp-
(1)
C E
where Q is the discharge, C is the tracer concentration, and subscripts P, E and
288
P. t)tJRAr~DETAL
TABLE 2 Hydrograph separation for a summer storm event using ~80 and Si as tracers % of pre-event water (180)
% of pre-event water (Si)
46 73
43 78
85 68
82 64
Grassland
30/7/91, 18:00h 31/7/91, 12:00h Beech .forest
30/7/91, 14:00h 31/7/91, 14:00h
T refer to pre-event component, event component and total discharge, respectively. The silica concentration of rainwater was assumed to be negligible. This is a safe assumption, as the values are generally low (the 10 year volume weighted mean is 0.01 mgl-~), except in 'red rains', coinciding with Saharan dust fallout, which did not occur here. This assumption is invalid for alkalinity, which shows large variations in rainwater, even during the same event (Durand et al., 1992a). Insufficient data make it impossible to use alkalinity for hydrograph separation. The results given by the two tracers are consistent (Table 2), as was found elsewhere (Hooper and Shoemaker, 1986; Loye-Pilot and Jusserand, 1990; Wels et al., 1991). Although the sampling was not sufficiently detailed to obtain a precise estimate of the proportion of the two components for the whole event, the results show a higher proportion of event water in the grassland catchment, Les Cloutasses. This corroborates other observations in the area (e.g. by Cosandey, 1993). The variations in conductivity and chloride concentration are often used to provide information on the hydrological behaviour of catchments (e.g. Nakamura, 1971; Neal and Rosier, 1990). This approach was not possible in the present case, as the variations were different from one stream to another, and the values in streamwater overlapped without a clear link with the rainfall values (Fig. 2). The conductivity variations integrate both dilution and concentration patterns. In the case of chloride, the complexity may well be due to the temporal and spatial variations of dry deposition, depending on the vegetation type and the meteorological conditions. CONCLUSION The seasonal pattern in the stable oxygen isotope composition of the three streams suggests that the catchment water stores are small, shallow and subject to evaporation in summer. Summer and early autumn are the periods
STABLE O X Y G E N ISOTOPE V A R I A T I O N S IN M O N T A N E STREAMS
289
w h e n the event a n d pre-event water isotope ratios are m o s t likely to be very different, thus allowing a precise h y d r o g r a p h separation. This is f o r t u n a t e , as early a u t u m n is a critical period for the hydrological a n d h y d r o c h e m i c a l functioning o f these catchments, as was s h o w n by previous field and modelling studies ( D u r a n d et al., 1991, 1992a, b). The stream response to the fi~80 variations in rainwater is usually d a m p e d , as m o s t o f the rainwater infiltrates a n d mixes with soilwater a n d g r o u n d w a t e r . However, the r a i n w a t e r signal is sometimes observed in streamwater, especially in the grassland c a t c h m e n t . This is in a g r e e m e n t with the variations o f silica c o n c e n t r a t i o n a n d alkalinity; the other chemical p a r a m e t e r s exhibit m u c h m o r e complex variations. The isotopic separation o f the h y d r o g r a p h a n d the chemical separation using silica give similar results. Silica could therefore be used as a alternative tracer w h e n isotopic separation is n o t possible, e.g. w h e n the 6~80 o f rainwater is too variable or too close to the 6180 o f the streamwater. REFERENCES Avila, A. and Rod~t, F., 1991. Red rains as major contributors of nutrients and alkalinity to terrestrial ecosystems at Montseny (NE Spain). Orsis, 6: 215-229. Cosandey, C., 1993. For~ts et 6coulements: r61e de la for~t sur la formation des crues et le bilan d'6coulement annuel; impact d'une coupe foresti6re. European Communities Commission sectorial report, 52 pp,, in press. Darling, W.G., Talbot, J.C. and Warrington, A.G., 1992. Updated procedures for the measurement of 2H/~H and ~80/160 in water samples. Rep. Br. Geol. Surv. WD/92/11, 22 pp. Durand, P., Neal, C., Lelong, F. and Didon-Lescot, J.F., 1991. Hydrochemical variations in spruce, beech and grassland areas, Mont-Loz6re, Southern France. J. Hydrol., 129: 57-70. Durand, P., Neal, C. and Lelong, F., 1992a. Anthropogenic and natural contributions to the rainfall chemistry of a mountainous area in the Cevennes National Park (Mont-Loz6re, Southern France). J. Hydrol., 130: 71-85. Durand, P., Robson, A.J. and Neal, C., 1992b. Modelling the hydrology of submediterranean montane catchments (Mont-Loz+re, France) using TOPMODEL: initial results. J. Hydrol., 139: 1-14. Hooper, R.P. and Schoemaker, C.A., 1986. A comparison of chemical and isotopic hydrograph separation. Water Resour. Res., 22: 1444-1454. Lelong, F., Dupraz, C., Durand, P. and Didon-Lescot, J.-F., 1990. Effects of vegetation type on the biogeochemistry of small catchments (Mont-Loz6re, France). J. Hydrol., 116: 125145. Loye-Pilot, M.-D. and Jusserand, C., 1990. D6composition chimique et isotopique d'un hydrogramme de crue d'un torrent m6diterran6en--r6flexions m6thodologiques. Rev. Sci. Eau, 3: 211-231. Martinec, J., 1975. Subsurface flow from snow-melt traced by tritium. Water Resour. Res., 11: 496-497. M6rot, P., Bourguet, M. and le Leuch, M., 1981. Analyse d'une crue fi l'aide du traqage naturel par l'oxyg6ne 18 mesur6 dans les pluies, le sol, le ruisseau. Catena, 8: 69-81.
290
1'. D U R A N D ET AL.
Nakamura, R., 1971. Runoff analysis by electrical conductance of water. J. Hydrol., 14: 197-212. Neal, C. and Rosier, T.W., 1990. Chemical studies of chloride and stable oxygen isotopes in two conifer afforested and moorland sites in the British Uplands. J. Hydrol., 115: 269-283. Rhode, A., 1981. Spring flood: Meltwater or groundwater? Nord. Hydrol., 12: 21-30. Sklash, M.G. and Farvolden, R.N., 1979. The role of groundwater in storm runoff. J. Hydrol., 43: 45-65. Wels, C., Cornett, R.J. and Lazerte, B.D., 1991. Hydrograph separation: a comparison of geochemical and isotopic tracers. J. Hydrol., 122: 253-274.
283
Elsevier Science Publishers B.V., A m s t e r d a m
[31
Variations in stable oxygen isotope and solute concentrations in small submediterranean montane streams P. Durand, M. Neal and C. Neal Institute of Hydrology, Walling.ford OXIO 8BB, UK (Received 24 April 1992; revision accepted 15 August 1992)
ABSTRACT Durand, P., Neal, M. and Neal, C., 1993. Variations in stable oxygen isotope and solute concentrations in small submediterranean montane streams. J. Hydrol., 144: 283-290. Stable oxygen isotope concentrations in rainwater and streamwater were determined, combined with routine hydrochemical monitoring of three granitic catchments in the South of France. 6~80 in streamwater showed much less variation than in rainwater. A seasonal pattern in baseflow conditions (higher 6~80 in summer) suggests that the groundwater store is small and subject to evaporation. Variations in high flow conditions show that pre-event water can contribute significantly to streamflow, especially in non-forested areas. Alkalinity and silica concentration are the only other parameters that show consistent behaviour during storm events. The latter could be used as an alternative to 6~80 for hydrograph separation.
INTRODUCTION
Environmental oxygen and hydrogen isotopes are widely used in catchment studies as tracers for hydrograph separation (Martinec, 1975; Sklash and Farvolden, 1979; Rhode, 1981; M6rot et al., 1981; Loye-Pilot and Jusserand, 1990). This usually involves detailed sampling of some flood events, which can be expensive and time-consuming as the calculations can be performed only if the rainfall isotopic composition is sufficiently different from the pre-event water composition (Sklash and Farvolden, 1979). Before such detailed studies, a simple monitoring of the isotopic ratios in rainwater and streamwater,, combined with the routine chemical monitoring, provides valuable information on the hydrology of the study area (Neal and Rosier, 1990). This can help focus investigation on the most relevant parameters and periods. The results of such an approach are presented here for three small granitic catchments in the southern Massif Central (France). C o r r e s p o n d e n c e to: P. D u r a n d , Institute of Hydrology, Wallingford O X I 0 8BB, U K .
0022-1694/93/$06.00
© 1993 - - Elsevier Science Publishers B.V. All rights reserved
284
P. DURAND ET AL.
TABLE 1 Statistics o f ~ 8 0
d a t a (in %o with respect to S t a n d a r d M e a n O c e a n W a t e r ) for the s t r e a m w a t e r s
a n d r a i n w a t e r for the p e r i o d S e p t e m b e r 1 9 9 0 - S e p t e m b e r 1991 (N, n u m b e r o f d e t e r m i n a t i o n s ) N
Mean
SD
Minimum
Maximum
Rain Grassland stream
30 20
- 7.47 - 7.27
3.22 0.95
- 13.50 - 8.54
- 1.09 - 4.70
Beech s t r e a m Spruce stream
23 27
- 7.31 - 7.38
0.66 0.56
- 8.33 - 8.66
- 6.00 - 6.62
STUDY AREA AND METHODS
Three small catchments, located on the south-facing slope of a granitic mountain in southeastern France, have been extensively studied in a wideranging environmental research programmes (Lelong et al., 1990; Durand et al., 1991). The altitude range is 1200-1500m and the average annual temperature is 6°C. The mean annual rainfall is 1950 mm and precipitation is unevenly distributed over the year, which results in very dry summers and strong flood events in spring and autumn. In one catchment, La Sapine, which is covered with beech coppice, the evaporation loss is about 30%, whereas in the two others (a grassland area, Les Cloutasses, and a recently clear-felled spruce forest, La Latte) it is only 20%. A detailed description of the research programme can be found elsewhere (Durand et al., 1991). The chemical composition of rainwater and streamwater has been monitored, on an event basis, since 1981. Subsamples for oxygen isotope analysis were taken from September 1990 to September 1991, and stored in hermetically sealed glass bottles. 6180 determinations were performed at the British Geological Survey-Institute of Hydrology joint isotope facility at Wallingford, using a VG-602E (VG Analytical, Wythenshawe, Manchester, UK) mass spectrometer and a CO2 equilibration technique (Darling et al., 1992). RESULTS
Stable oxygen isotope variations
The three catchments exhibit a 'damped' response to the rainfall signal: the standard deviation of 61SO in rainwater is 3-6 times higher than in streamwater (Table 1, Fig. 1). This feature has also been noted for the major element concentrations (Durand et al., 1991). The 6~so variations are similar in the three streams. Two types of variation can be observed, one in baseflow and the other in high flow conditions.
STABLE
S+O
OXYGEN
ISOTOPE
VARIATIONS
IN
MONTANE
285
STREAMS
(=/~)
0
rain -
beech
.•
grass spruce
+ 4+ ,~-
-5 -
:r :
-
,
-+
+'
'
~
-10
-15 60
o<,,s,i,
20 21109190
21/12•90
21/03/91
21/06/91
19/09/91
Date Fig. 1. Oxygen isotope concentrations in rainwater and streamwater in the three Mont-Loz6re catchments and instantaneous discharge during sampling in the Sapine stream, from 7 September 1991 to 19 September 1992.
There is a seasonal pattern in baseflow conditions, with the 'heaviest' water (i.e. that most enriched in 5180) occurring in late summer and the lightest water in spring. As the input of rainwater to the groundwater store is insignificant in summer, this means that some evaporation takes place from the groundwater, probably from the peat zones and free drainage channel (the summer increase of 5t80 is more marked at Les Cloutasses, which has the largest peat area and the greatest channel length). However, this possibility needs to be confirmed by hydrogen isotope measurements. In high flow conditions, the signal of the event water sometimes reaches the outlet. This was observed during two very different episodes. In October 1991, the flood was due to 300 m m of rainfall in a few days. This is an event of normal magnitude for the period, which did not result in a major flood because of the high saturation deficit of the catchments before the onset of the rainfall. 6t80 of pre-event streamwater was around - 6.5 to - 7%o compared with -9%o for the event rainwater. During the flood, streamwater 5~80
286
P. D U R A N D ET AL.
decreased by 0.5-1%o in the three rivers. The second episode was a summer thunderstorm, with an intense rainfall of 90 m m in a few hours. The rainwater 6~80 was high (-2.8%o), and the streamwater exhibited a strong increase in 3180, especially in the Cloutasses catchment, 6~sO increased from -7%o before the event to - 4.7%o in the rising part of the hydrograph. The major flood of the study period occurred in March 1991, as a result of the coincidence of snowmelt and rainfall. No significant pattern was observed in the oxygen isotope variations. However, it cannot be concluded that no rainwater reached the stream, primarily because only one sample per river was collected during the event, and all the samples were taken at times other than that of the peak flood. Moreover, the snow and rainwater producing the flood had very variable 6J80 values ( - 12 to - 6.6%o), whereas the streamwater 6~80 value was around - 7.5%o. This difficulty has also been pointed out by Hooper and Shoemaker (1986) as a serious limitation on the use of stable isotopes for the study of snowmelt events. However, 6~sO in streamwater remained very negative during the weeks after this event; even if the ~SO-depleted winter rainwater and snow did not contribute to the discharge as surface flow, they made up most of the spring groundwater without significant fractionation.
Other hydrochemical variations There is no significant relationship between chemical composition and isotope concentrations in rainwater. However, the highest 6180 was measured for a small event with very high alkalinity and Ca concentrations; these features characterize a particular type of event known as 'red rain', i.e. rainfall with a large amount of aeolian dust from the Sahara (Avila and Roda, 199 l; Durand et al., 1992a). Further measurements are required to determine whether high 6t80 values characterize this type of rainwater. Few parameters showed systematic patterns in high flow conditions. In most cases, there are overlaps between concentration ranges in rainwater and streamwater, which result in either dilution or concentration during floods (Durand et al., 1991). The exceptions are for alkalinity and silica concentrations, which decreased during each episode. The variations of these two parameters confirm the conclusions of the isotope study, at least in the Sapine and Cloutasses catchments (Fig. 2). The two floods showing the rainwater signal in streamwater also showed a strong decrease in alkalinity and silica concentration. Conversely, during the rainfall-snowmelt episode of March, only a slight decrease was observed. In the clear-felled spruce catchment, La Latte, the hydrochemical variations are determined by the strongly seasonal release of N O 3 a s a result of the mineralization of the organic pools after the felling.
287
STABLE OXYGEN ISOTOPE VARIATIONS IN MONTANE STREAMS
/~eq/I
mg/I 2
Si
1.5
• ii
~ ......
h
•
A
0,5
[] --
A
Beech
'
'
o
Spruce
'
' - -
Alkalinity
oo
©
Grassland
,,
.-
".
~
"',
," " ' - - .
' o
P&
A'"
''(3
0
,
,,-'"
80
°
9
/
", ,"
40
0 ,
~
,
,
L
-40
Cl o o
oP'
o
',
i
i
21/09/90
21/12/90
o
i
21/03/91
o
c~ 0
:
L
21/06/91
/ 19/09/91
Date Fig. 2. D i s s o l v e d silica concentrations, alkalinity and chloride c o n c e n t r a t i o n s in s t r e a m w a t e r (and rainwater for chloride) during the study period.
To verify the agreement of the results of the chemical and isotopic approaches, the proportion of pre-event water at two points of the summer flood was calculated, in the Cloutasses and the Sapine catchment, using the 6~80 and the Si concentration data. The two-component mixing model formula was used: Qp
cv - CE
QT
Cp-
(1)
C E
where Q is the discharge, C is the tracer concentration, and subscripts P, E and
288
P. t)tJRAr~DETAL
TABLE 2 Hydrograph separation for a summer storm event using ~80 and Si as tracers % of pre-event water (180)
% of pre-event water (Si)
46 73
43 78
85 68
82 64
Grassland
30/7/91, 18:00h 31/7/91, 12:00h Beech .forest
30/7/91, 14:00h 31/7/91, 14:00h
T refer to pre-event component, event component and total discharge, respectively. The silica concentration of rainwater was assumed to be negligible. This is a safe assumption, as the values are generally low (the 10 year volume weighted mean is 0.01 mgl-~), except in 'red rains', coinciding with Saharan dust fallout, which did not occur here. This assumption is invalid for alkalinity, which shows large variations in rainwater, even during the same event (Durand et al., 1992a). Insufficient data make it impossible to use alkalinity for hydrograph separation. The results given by the two tracers are consistent (Table 2), as was found elsewhere (Hooper and Shoemaker, 1986; Loye-Pilot and Jusserand, 1990; Wels et al., 1991). Although the sampling was not sufficiently detailed to obtain a precise estimate of the proportion of the two components for the whole event, the results show a higher proportion of event water in the grassland catchment, Les Cloutasses. This corroborates other observations in the area (e.g. by Cosandey, 1993). The variations in conductivity and chloride concentration are often used to provide information on the hydrological behaviour of catchments (e.g. Nakamura, 1971; Neal and Rosier, 1990). This approach was not possible in the present case, as the variations were different from one stream to another, and the values in streamwater overlapped without a clear link with the rainfall values (Fig. 2). The conductivity variations integrate both dilution and concentration patterns. In the case of chloride, the complexity may well be due to the temporal and spatial variations of dry deposition, depending on the vegetation type and the meteorological conditions. CONCLUSION The seasonal pattern in the stable oxygen isotope composition of the three streams suggests that the catchment water stores are small, shallow and subject to evaporation in summer. Summer and early autumn are the periods
STABLE O X Y G E N ISOTOPE V A R I A T I O N S IN M O N T A N E STREAMS
289
w h e n the event a n d pre-event water isotope ratios are m o s t likely to be very different, thus allowing a precise h y d r o g r a p h separation. This is f o r t u n a t e , as early a u t u m n is a critical period for the hydrological a n d h y d r o c h e m i c a l functioning o f these catchments, as was s h o w n by previous field and modelling studies ( D u r a n d et al., 1991, 1992a, b). The stream response to the fi~80 variations in rainwater is usually d a m p e d , as m o s t o f the rainwater infiltrates a n d mixes with soilwater a n d g r o u n d w a t e r . However, the r a i n w a t e r signal is sometimes observed in streamwater, especially in the grassland c a t c h m e n t . This is in a g r e e m e n t with the variations o f silica c o n c e n t r a t i o n a n d alkalinity; the other chemical p a r a m e t e r s exhibit m u c h m o r e complex variations. The isotopic separation o f the h y d r o g r a p h a n d the chemical separation using silica give similar results. Silica could therefore be used as a alternative tracer w h e n isotopic separation is n o t possible, e.g. w h e n the 6~80 o f rainwater is too variable or too close to the 6180 o f the streamwater. REFERENCES Avila, A. and Rod~t, F., 1991. Red rains as major contributors of nutrients and alkalinity to terrestrial ecosystems at Montseny (NE Spain). Orsis, 6: 215-229. Cosandey, C., 1993. For~ts et 6coulements: r61e de la for~t sur la formation des crues et le bilan d'6coulement annuel; impact d'une coupe foresti6re. European Communities Commission sectorial report, 52 pp,, in press. Darling, W.G., Talbot, J.C. and Warrington, A.G., 1992. Updated procedures for the measurement of 2H/~H and ~80/160 in water samples. Rep. Br. Geol. Surv. WD/92/11, 22 pp. Durand, P., Neal, C., Lelong, F. and Didon-Lescot, J.F., 1991. Hydrochemical variations in spruce, beech and grassland areas, Mont-Loz6re, Southern France. J. Hydrol., 129: 57-70. Durand, P., Neal, C. and Lelong, F., 1992a. Anthropogenic and natural contributions to the rainfall chemistry of a mountainous area in the Cevennes National Park (Mont-Loz6re, Southern France). J. Hydrol., 130: 71-85. Durand, P., Robson, A.J. and Neal, C., 1992b. Modelling the hydrology of submediterranean montane catchments (Mont-Loz+re, France) using TOPMODEL: initial results. J. Hydrol., 139: 1-14. Hooper, R.P. and Schoemaker, C.A., 1986. A comparison of chemical and isotopic hydrograph separation. Water Resour. Res., 22: 1444-1454. Lelong, F., Dupraz, C., Durand, P. and Didon-Lescot, J.-F., 1990. Effects of vegetation type on the biogeochemistry of small catchments (Mont-Loz6re, France). J. Hydrol., 116: 125145. Loye-Pilot, M.-D. and Jusserand, C., 1990. D6composition chimique et isotopique d'un hydrogramme de crue d'un torrent m6diterran6en--r6flexions m6thodologiques. Rev. Sci. Eau, 3: 211-231. Martinec, J., 1975. Subsurface flow from snow-melt traced by tritium. Water Resour. Res., 11: 496-497. M6rot, P., Bourguet, M. and le Leuch, M., 1981. Analyse d'une crue fi l'aide du traqage naturel par l'oxyg6ne 18 mesur6 dans les pluies, le sol, le ruisseau. Catena, 8: 69-81.
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Nakamura, R., 1971. Runoff analysis by electrical conductance of water. J. Hydrol., 14: 197-212. Neal, C. and Rosier, T.W., 1990. Chemical studies of chloride and stable oxygen isotopes in two conifer afforested and moorland sites in the British Uplands. J. Hydrol., 115: 269-283. Rhode, A., 1981. Spring flood: Meltwater or groundwater? Nord. Hydrol., 12: 21-30. Sklash, M.G. and Farvolden, R.N., 1979. The role of groundwater in storm runoff. J. Hydrol., 43: 45-65. Wels, C., Cornett, R.J. and Lazerte, B.D., 1991. Hydrograph separation: a comparison of geochemical and isotopic tracers. J. Hydrol., 122: 253-274.