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Chemical studies of chloride and stable oxygen isotopes in two conifer afforested and moorland sites in the British uplands
Journal of Hydrology, 115 (1990) 269-283
269
Elsevier Science Publishers B.V., Amsterdam - - Printed in The Netherlands
[21 CHEMICAL S T U D I E S OF CHLORIDE A N D S T A B L E O X Y G E N I S O T O P E S IN TWO C O N I F E R A F F O R E S T E D A N D M O O R L A N D S I T E S IN THE BRITISH U P L A N D S
COLIN NEAL and PAUL T.W. ROSIER
Institute of Hydrology, Wallingford, OXIO 8BB (U.K.) (Received December 16, 1988; accepted after revision July 28, 1989)
ABSTRACT Neal, C. and Rosier, P.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. Chloride concentration variations for three streams in the Crinan Canal region of southwest Scotland, draining h e a t h e r (Calluna vulgaris) and immature and mature sitka spruce conifer forest (Picea sitchensis (Bong.) Cart) are compared with incident rainfall values. Results show the importance, and correspondence, of in-catchment processes for a t t e n u a t i n g atmospheric inputs of chloride before its entry to the stream. All three streams exhibit a damped chloride concentration variation compared with rainfall. The chloride chemistries for the three streams are intercorrelated. The weekly data show a net accumulation of chloride into the catchment over the 1 year sampling period. This is associated with a n inappropriate stream sampling frequency and]or large yearly fluctuations, in the rainfall input; 80% of the rainfall chloride contribution was introduced during four of the 52 weeks of sampling. The yearly flux of chloride leaving the catchment as stream flow is remarkably similar for all three catchments. The study highlights the need for long and detailed data records from which information can be obtained on chemical budgets and the deficiencies in the measurement of atmospheric inputs of chloride. Although mature conifers are known to capture significant inputs of chemicals to a catchment, not measured by a standard rainfall collector in uplands areas, analogous results are not inferred in the lowland Crinal Canal case. Far more detailed studies are required if a definitive statement is to be made. The chloride results from the Crinan Canal Study are compared with analogous data for streams draining the Hafren Forest at Plynlimon in mid-Wales. Similar degrees of damping are observed and seasonal oscillations occur which do not have the same phase or amplitude. In addition, oxygen isotopic data are presented for the Plynlimon case to highlight a higher degree of damping t h a n for chloride.
INTRODUCTION
Studies of the relation between solute concentrations in rainfall and runoff, for constituents such as chloride and the isotopes of hydrogen and oxygen which are chemically unreactive in the hydrological cycle for temperate regions, can provide important information on the hydrological pathways operating in catchments (Sklash and Farvolden, 1979; Rodhe, 1981; Bottomley
0022-1694/90/$03.50
© 1990 Elsevier Science Publishers B.V.
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C. NEAL AND P.T.W. ROSIER
et al., 1984/1985; Sklash et al., 1986; Christophersen and Neal, 1987; Neal et al., 1988). For many catchments, the variations in chloride concentration and oxygen isotope ratio are high in rainfall but low in the stream (Kennedy et al., 1986; Christophersen and Neal, 1987; Neal et al., 1988; Reynolds and Pomeroy, 1988). Results for catchments studied in the above references indicate t h a t the major portion of water entering the stream during high-flow periods is not derived directly from the rain promoting the hydrological response (Sklash and Farvolden, 1979). However, different catchments show different chemical responses to hydrological change, and a 'damped' response is not always found (Skartveit, 1981; Langan, 1986); in such circumstances, rainfall rapidly passes through the catchment to provide the major component of the stream response. It is unclear at present why geomorphically similar catchments should behave in such different ways, but the water residence time within the catchment must be a relevant factor. As part of a study of the effects of afforestation on the runoff from the catchments supplying the Crinan Canal reservoirs (Calder et al., 1982), the opportunity was taken to collect rainfall and runoff chloride data. This was initiated (1) to see if a chloride balance could provide an estimate of evaporation from different vegetation types, and (2) to provide additional information on the nature of the hydrological response for the catchments with a thin soil cover and an impermeable bedrock. As part of the detailed hydrochemical studies for the spruce forested catchment at Plynlimon, oxygen isotopic data were collected. This was undertaken (1) to see if the degree of damping is similar to t h a t previously shown for chloride (Neal et al., 1988), and (2) to provide further data for detailed modelling studies (cf. Christophersen and Neal, 1987). The results of these two experiments are presented and intercomparisons are made. CATCHMENT DETAILS The Crinan Canal area
The Crinan Canal study area at the northern end of the Kintyre peninsula in southwest Scotland (Fig. 1) has an altitude range of 150-350 m and an annual average rainfall of about 2000mm. The catchment has approximately 50% afforestation (mainly Sitka spruce). The underlying geology of the catchment is composed of metamorphic rocks of the Dalradian Assemblage together with metamorphic igneous rocks, and the soils are mainly peaty podzols, peaty gleys and brown earths (Soil Survey of Scotland, 1982). Three streams (catchment area in brackets) were monitored: Glac Connaidh - - heather, (33.8 ha), High Daill Burn - - immature forest (62.5 ha) and Achantheanbhaile - - mature forest (ll.3ha). Rainfall was sampled from a standard weekly integrated storage gauge at Dunardry, and streamwaters were collected as weekly 'instantaneous' samples. Snow deposition was not encountered in the sampling period. Stream flow was measured at the nearest gauging point, Carndubh Burn, as no suitable
CHLORIDE
AND OXYGEN
ISOTOPES
IN UPLAND
271
STREAMS
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measurements were available for the three streams. The streams showed marked hydrograph storm responses. Samples were taken on a weekly basis for 1 year (June 1985-June 1986) and analysed for chloride using an automated colorimetric method (Zall et al., 1956) within 6 weeks of sampling.
The Hafren Forest area, Plynlimon Two streams were surveyed, the Afon Hafren and the Afon Hore, both draining sub-catchments of conifer plantation (mainly Sitka spruce) and seminatural moorland. The catchments are of similar size (about 340ha), with typically 0.5-1 m-thick peat/podzol/gley/brown earth soils with a Silurian to Ordovician bedrock comprised of a mixture of mudstones, sandstones and grits. Annual rainfall is typically 2400 mm with annual evapotranspirational losses of about 430 and 700 mm for the Hafren and Hore catchments respectively. They have different areal forest cover: 50% for the Afon Hafren and 77% for the Afon Hore (Newson, 1976a; Newson, 1976b; Neal et al., 1986, 1988). Samples for isotopic analysis was collected as part of the weekly water sampling programme; the waters were stored in sealed glass bottles before analysis to ensure insignificant evaporative effects; analysis was performed, for a 1 year sub-set of samples (January-December 1984) using a VG Micromass spectrometer and a CO2 extraction technique (Darling, 1982). THEORETICALAND PRACTICALCONSIDERATIONS Strictly, a chemical budget for a catchment requires an accurate and precise water balance and mean solute compositions, from which estimates of the flux of elements entering and leaving can be made. This can only be achieved with a detailed hydrological and chemical monitoring programme of (1) rainfall input, consisting of a network of rainfall gauges, and (2) stream flow from a single flow-measuring device or a network of such devices (I. H. Rep. Ser., 1976; Newson 1976a). In practice as in the Crinan Canal case, this detailed information is not available due to the large scale of the undertaking and the costs involved. Instead, rainfall and flow are monitored at single locations within or near to the catchments to allow flow/volume-weighted chloride concentrations to be obtained; estimates of the water fluxes are obtained using the limited rainfall data together with either the limited flow data or some theoretical estimate of the evapotranspiration (see next section for the Crinan Canal case). However, in using the more limited data, simplifying assumptions are made. These assumptions are described here as a prelude to data interpretation and the assessment of the applicability of these limited data. It is assumed t h a t all the measurements are precise and t h a t weathering and ion exchange inputs are negligible for chloride and the oxygen isotopes. ESTIMATING MEAN CHLORIDECONCENTRATIONS Comment is made here on the estimation of mean chloride concentrations on the basis of a limited hydrological and chemical data set, to provide a rigorous
CHLORIDE AND OXYGEN ISOTOPES IN UPLAND STREAMS
273
well-defined framework on which to base conclusions. In particular, the important condition of 'linear scaling' for which mean weighted concentrations can be obtained is shown; this has not been adequately stressed previously. Let R(t) and Q(t) be instantaneous (time t) or averaged (time interval t - d to t + d) 'point in time' measurements (in dimensions of flux per unit time or flux per unit time per unit area), where R and Q denote rainfall and stream flow. R' (t) and Q' (t) are the ~true' values for the catchment, which are not measured because of the spatial variability of rainfall across the catchments and the lack of flow gauging equipment on the three sub-catchments. Thus in calculating flow- and volume-weighted means for stream chloride and rainfall chloride concentrations, respectively, it is necessary to introduce scaling factors RS and QS where
R' (t) = R(t)RS and
Q' (t) = Q(t)QS Taking RC1 and QC1 to be the corresponding measured concentrations of chloride in the rainfall and the stream for the catchment, RClm and QClm, the volume-weighted mean rainfall and flow-weighted mean streamwater chloride concentration, are given by = ~ R ' ( t ) RCI(t) = ~R(t) RSRCI(t) RClm
~R" (t)
~R(t) RS
and QClm
= ~ Q ' ( t ) QCl(t) = ~Q(t) QSQCl(t)
~,Q" (t)
~ Q (t) Qs
In virtually all hydrochemical studies, where restricted data have been collected, it is assumed implicitly, if not explicitly, that these scaling factors RS and QS are constant. In such circumstances, ~ R (t) RC1 (t) RClm =
~R(t)
QClm =
~Q(t) QC1 (t) ~Q(t)
Thus volume-weighted means and flow-weighted means can be calculated using estimated rainfall and stream flow respectively, given a linear relationship between these estimated and 'true' values (i.e. RS and QS constant). It should be noted that errors in QC1 and RC1 will always give rise to errors in the flowand volume-weighted chloride levels. In conditions where linear scaling is not applicable, reliable mean concentration values are unattainable from studies of the type presented here. FINE WATERPARTICLEAND DRY DEPOSITION Catchment vegetation is known to scavenge chloride from mist and small solid particles in the atmosphere (Fowler, 1984; Cryer, 1986; Unsworth and
274
C. NEAL AND P.T.W. ROSIER
Crossley, 1986). On the other hand, standard rain gauges are usually inefficient collectors, and underestimate effective rainfall by typically 20% (Rodda and Smith, 1986). They are particularly inefficient collectors of fine water and solid particles (Robinson and Rodda, 1969; Green and Helliwell, 1972). These fine water and solid particles can supply significant salt inputs to the catchments. Thus, standard rainfall collectors may underestimate the flux of chloride entering the catchment. The degree of this underestimation cannot easily be gauged but it will vary according to vegetation type, altitude and climate. For example, enhanced scavenging is observed for various vegetation covers in the U.K; this is usually associated with upland areas, such as Plynlimon, where a low cloud base occurs for several days during the year (Cryer, 1986; Neal et al., 1988). The term 'intercepted deposition' is used hereafter to denote the discrepancy. Thus 'intercepted deposition' is that component of the input to a catchment (rainfall, mist and dry deposition) which is not collected by a standard rainfall collector. EVAPORATION ESTIMATES To estimate annual evaporation using a limited data input the equations derived by Calder and Newson (1979) for mature and immature forest catchments, and Calder (1985) for the heather catchment have been used. The equations are (1) for mature and immature forest catchments: annual evaporation
=
E t + f(pa
-
WEt)
where E t is the Penman (1948) estimate of annual evaporation from grass (ram), f is the fraction of catchment area with complete canopy coverage, p is the annual precipitation (mm), a is the interception ratio for forest, and w is the fraction of year when canopy is wet. (2) for heather catchment: annual evaporation
=
[3Et ( 1 - w) + ap
where ~ is the ratio of heather transpiration to Penman evaporation and ~ is the interception ratio for heather. A full description of the equations and the experimental details for determining the parameters were given by Calder (1986). In making the evaporation estimates the rainfall for the study period was 2070 mm and the Penman Et was 410 ram. Also, the following assumptions have been made: (1) for the mature forest catchment the canopy coverage f is 100%; (2) for the immature forest catchment the canopy coverage f is 66% with the remainder being grass; and (3) for the heather catchment the coverage is 100%. This gives annual evaporation estimates of 1052, 838 and 567 mm respectively for the mature, immature forest and heather catchments.
CHLORIDE AND OXYGEN ISOTOPES IN UPLAND STREAMS
275
CHEMICALAND WATERMASS BALANCERELATIONSHIPS The relationship between the estimates of the chemical and water mass balances are explored here to identify some of the salient hydrochemical features. As a starting point, it is assumed initially that (1) chloride is chemically unreactive, (2) the store of water and chloride remains constant within the catchment, (3) 'interception deposition' of chloride is insignificant and (4) other factors are not important (see section on estimating mean chloride concentrations). The limitations of the approach taken and 'data gaps' will then be highlighted. To aid this assessment a new term, Clf, is introduced; this is the ratio of the volume-weighted chloride concentration in rainfall to the flow-weighted chloride concentration in the stream: elf
-
RClm QClm
Because of evapotranspiration losses in the catchment, chloride concentrations in the outflow should be higher t h a n the incoming rainfall to maintain the mass balance. Thus Clf represents the ratio of water leaving the catchment as stream flow to the water entering the catchment as rainfall, because for a long-term mass balance RClm ~ R ' (t) = QCl~ ~ Q ' (t) From a knowledge of evapotranspiration and rainfall this ratio can be represented as Ef = ~ R ' ( t ) - ~ E ' ( t ) ~ R ' (t)
_ ~V'(t) ~ R ' (t)
In theory Clf and Ef should be the same, given the above assumptions of conservative chloride and constant scaling factors as discussed previously. This has been tested using the Crinan Canal catchments. RESULTS AND DISCUSSION
Crinan Canal The input of chloride to the three Crinan Canal catchments is very variable through the year (Figs. 2 and 3). Over 70% of the chloride input by wet deposition occurs during 4 weeks of the sampling period when chloride concentrations are at their highest. Chloride outputs are also very variable (Fig. 2), but in this case the degree of variation is related to the large fluctuations in stream flow and concentration changes are relatively low (Fig. 3). Given the large variations in the rainfall chloride input and the use of weekly 'instantaneous' sampling in the stream it is possible t h a t stream outputs are underestimated. Consequently, a much longer, and more intensive sampling
276
C. NEAL AND P.T.W. ROSIER
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Jul. '85
Sept. '85
Nov. '85
Jan. '86
Mar. '86
May '86
Fig. 2. Chloride flux variations in rainfall and stream waters for the Crinan Canal catchments. period is required to obtain reliable long-term chloride input fluxes and mean concentrations• The need for a long-term study to obtain chloride budgets is well illustrated in the heather and immature forest cases where flow/volume-weighted chloride concentrations are higher in the rain inputs than in the stream outflow; evapotranspiration should lead to evaporative concentration of chloride and Clf values of less than unity (the annual evaporation estimates derived using the Calder and Newson equation give Er values of 0,73 and 0.60 for the heather and immature forest cases, respectively; Ef should equal Clf, given the above assumptions). The calculated values of Clf, using instantaneous flow weightings, are 1.3 and 1.25 (integrated flow weightings give essentially the same result, Table 1). The high Clf values reflect long-term ( > yearly) changes in the chloride storage of the catchment• (Analytical errors associated with chloride, rainfall and flow measurement would not give rise to these large discrepancies, whereas analytical procedures for chloride determination are both accurate and precise• Errors associated with incomplete assessment of the atmospheric input (i.e. 'interception deposition') would lead to even greater disparity between Ef and Clf. Within the sampling period, chloride shows a net accumulation in the heather and immature forest catchments relative to water
277
CHLORIDE AND OXYGEN ISOTOPES IN UPLAND STREAMS 120
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Nov. '85
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May '86
Fig. 3. Chloride variations in rainfall and stream waters for the Afon Hafren and the Afon Hore catchments.
storage. However, the weekly stream-sampling programme may have resulted in important chloride episodes being missed and chloride stream fluxes being underestimated. It should be noted t h a t it may also be argued that a single rain gauge is not representative enough for comparisons to be made between catchments; with no information on rainfall solute composition across the sites it may not be realistic to draw specific conclusions. Set against this, however, the stream Cl% flux outputs are remarkably similar for the three sub-catchments, which suggests constant precipitation chemistry and a uniform catchment delay mechanism.) Similar results are observed in the mature forest case, as the estimate of Ef based on the Calder and Newson equation (0.49) is lower than the observed Clr value (0.78), although in this case Cl~ is less t h a n unity. The mature forested catchment is also probably showing a net accumulation of chloride in the catchment relative to water storage. However, the use of weekly data may result in important episodes of chloride release to the stream being missed, as storm response occurs over a matter of hours rather t h a n weeks.
278
c. NEALAND P.T.W. ROSIER
TABLE 1 Summary statistics of the chloride concentrations (mg 1-1) in the rainfall and stream water Stream water
Average (volume-weighted 1) Average (volume-weighted 2) Average (arithmetic) Min. value Max. value S.D. Sample size Clfa (weighted 1) Clfa (weighted 2)
Rainfall
Heather
Immature forest
Mature forest
10.95
11.34
18.31
14.20
11.49
11.75
18.07
14.20
13.96
15.08
22.40
13.28
6 31.0 5.75 53
6 32.4 7.46 53
6 46.0 9.39 53
0 112.0 22.81 53
1.29 1.24
1.25 1.21
0.78 0.79
-
Stream-flow weightings undertaken in two ways. (1) Instantaneous flow values used. (2) Integrated weekly flow values used. a Clf is the ratio of the volume-weighted mean concentration of chloride in rainfall to the flowweighted mean concentration of chloride in the stream, as calculated using weightings 1 and 2.
The temporal trends in stream chloride concentration are the same (Table 2, Fig. 3); chloride concentrations are highest from J a n u a r y to March and lowest at the end of September. The reason for this strong seasonality remains unclear, as it does not correspond directly with hydrological factors such as rainfall distribution and soil moisture deficits; a much lengthier sampling period is required to assess the regularity of the trend. Whatever the detailed hydrological processes operating, this regularity in trend between catchments implies that they are remarkably similar for all three catchments. The importance of 'interception deposition' in supplying chloride to the Crinan Canal catchments cannot quantitatively be established owing to the large uncertainties. However, there are three aspects which, combined, imply that this phenomenon may be less significant in lowland than in upland areas. First, low CIF values would be expected in areas where 'intercepted deposition' is high and Clf should be lower than El, if this term were significant; this is not the case for the Crinan Canal catchments, but it is the case for upland areas such as Plynlimon, where Clf is up to 40% lower than Ef (Neal et al., 1988; Reynolds and Pomeroy, 1988). Second, the flux of chloride in the Crinan Canal streams are approximately equal (15.8, 14.5 and 16.4gm -2 year -1 for the heather, immature and mature forest cases respectively). The anticipated degree of capture would be in the order, mature forest > immature forest >~ heather, based on vegetation surface area available to scavenging, and conse-
279
CHLORIDE AND OXYGEN ISOTOPES IN UPLAND STREAMS
•"
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Fig. 4. lsO/~60 variations in rainfall and stream waters for the Afon Hafren and the Afon Hore catchments. I
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'
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Fig. 5. Chloride concentration variations in rainfall and stream waters for the Afon Hafren and the Afon Hore catchments.
280
C. NEAL AND P.T.W. ROSIER
TABLE 2 Rainfall and stream-water chloride concentration, and flow inter-correlations (significance level in brackets, N = 53 in all cases)
CLHE CLIF
CLHE
CLIF
CLMF
1.0000 (0.0000)
0.9033 (0.0000) 1.0000 (0.0000)
0.8696 (0.0000) 0.9184 (0.0000) 1.0000 (0.0000)
CLMF CLRAIN~ RAINa FLOW
CLRAINa 0.0478 (0.7341) 0.0345 (0.8065) - 0.0427 (0.7615) 1.0000 (0.0000)
RAINa 0.0748 (0.5944) - 0.1127 (0.4217) - 0.1322 (0.3454) 0.0652 (0.6568) 1.0000 (0.0000)
FLOW - 0.3780 (0.0053) - 0.3627 (0.0076) - 0.3317 (0.0153) 0.0706 (0.6154) 0.4923 (0.0002) 1.0000 (0.0000)
Where CLHE is the chloride concentration in the Glac Connaidh stream draining heather. CLIF is the chloride concentration in the High Daill Stream draining immature forest. CLMF is the chloride concentration in the Achantheanbhaile stream draining mature forest. CLRAIN is the chloride concentration in the rainfalP RAIN is the rainfall amount a FLOW is the flow in Carndubh Burn. These are weekly bulked data; the others are 'instantaneous grab' samples. Caution must be taken when comparing the two types of data: low correlations are to be expected as the stream response times to rainfall will be in the order of hours.
quently the stream chloride flux should vary in the same manner. The constancy of this flux may also suggest that the storage effects are similar for all three catchments. Third, ~intercepted deposition' would be expected to be m o s t s i g n i f i c a n t i n t h e u p l a n d a r e a s w h e r e t h e r e is a f a r g r e a t e r c h a n c e o f t h e v e g e t a t i o n b e i n g i n c o n t a c t w i t h m i s t (i.e. a l o w c l o u d b a s e ) . M u c h m o r e d e t a i l e d s t u d y is r e q u i r e d , h o w e v e r , b e f o r e a d e f i n i t i v e s t a t e m e n t c a n b e m a d e o n t h i s p o i n t . T h e o b s e r v a t i o n is p r e s e n t e d h e r e s i m p l y a s a p o i n t e r t o encourage further work in this field for lowland areas.
Hafren Forest, Plynlimon F o r t h e H a f r e n F o r e s t c a t c h m e n t s t h e r e is a s i g n i f i c a n t v a r i a t i o n i n t h e is O/160 r a t i o i n r a i n f a l l , a l t h o u g h s t r e a m w a t e r s s h o w a v e r y d a m p e d b e h a v i o u r ( F i g . 4). T h i s d e g r e e o f d a m p i n g is e v e n g r e a t e r t h a n t h a t f o r c h l o r i d e ( F i g s 4 a n d 5) a n d n o c o r r e s p o n d i n g s e a s o n a l e f f e c t is o b s e r v e d . T h e l o s s o f w a t e r f r o m the catchment to the atmosphere by transpiration and complete evaporation from the wetted vegetation surfaces causes insignificant isotopic fractionation a n d h e n c e a m i n i m a l v a r i a t i o n i n 18O/160 r a t i o . ' E v a p o r a t i v e c o n c e n t r a t i o n ' o f c h l o r i d e w i l l o c c u r , h o w e v e r , a s w a t e r is l o s t f r o m t h e c a t c h m e n t t o t h e
CHLORIDE AND OXYGEN ISOTOPES IN UPLAND STREAMS
281
atmosphere. The even greater damping of 180/160 ratio, compared with chloride, is thus to be expected; the evaporative effect leads to a much more heterogeneous distribution of chloride, compared with the lsO/160 ratio, in the catchment. The isotopic study provides further evidence suggesting t h a t only a minor portion of the water which enters the streams during a storm event at the Hafren forest catchment is derived from the rain generating the hydrological response. CONCLUSIONS (1) For estimates of either evapotranspiration from chloride concentration measurements or chloride budgets much longer sampling periods are required. Furthermore, the atmospheric input has to be much more accurately assessed, with the inclusion of a quantified value for the 'intercepted deposition' contribution. High-quality rainfall and stream-flow measurements are also required as volume/flow-weighted mean chloride concentrations are used in the calculations. (2) Seasonal variations in the chloride concentrations in stream waters in the Crinan Canal and the Plynlimon regions are observed. The patterns differ from area to area but are similar for a given area. For example, (a) in the Plynlimon case the highest stream-water concentrations are to be found from October to April (cf. Neal et al., 1988; Reynolds and Pomeroy, 1988) compared with J a n u a r y to March in the Crinan Canal case and, (b) the amplitude of the variation is much greater for the Crinan Canal case. The regularity of these patterns for a given area and the differences in pattern observed from area to area has not yet been explained but in some way this phenomenon must be related to longer-term changes in climate pattern (cf. Reynolds and Pomeroy, 1988). F u r t h e r study is warranted as the trends provide important clues to the nature of water storage and the flow pathways through catchments. ACKNOWLEDGEMENTS Thanks must go to the staff of the British Waterways Board at Ardrishaig for the work carried out in the Crinan Canal study, to George Darling of the British Geological Survey for the isotopic analysis and to the staff of the Institute of Hydrology for sample collection at Plynlimon and analysis at Wallingford. Valuable criticism of the draft was given by Brian Smith and Paul Whitehead. Financial support was received from a number of Agencies: British Waterways Board, North of Scotland Hydro-Electric Board, Scottish Development Department, Department of Energy, Department of the Environment, Water Research Centre and the Natural Environment Research Council. REFERENCES Bottomley,D.J., Craig, J. and Johnson, L.M., 1984/1985. Neutralisation of acid runoff by groundwater discharge to streams in Canadian Precambrian shield watersheds. J. Hydrol.,75: 1--26. Calder, I.R., 1985.Influenceof woodlandson water quantity. Inst. Biol. Proc. Environ. Div. Symp., Weather, Woodlands and Water, Edinburgh, March, 23 1984.
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c. NEALANDP.T.W.ROSIER
Calder, I.R., 1986. The influence of land use on water yield in upland areas of the U.K.J. Hydrol., 88: 201-211. Calder, I.R. and Newson, M.D., 1979. Land use and upland water resources in Britain--a strategic look. Water Resour. Bull., 16: 1628-1639. Calder, I.R., Hall, R.L., Harding, R.J., Rosier, P.T.W. and Wright, I.R., 1982. Report on the collaborative project with the British Waterways Board on the effects of afforestation on the runoff from the catchments supplying the Crinan Canal reservoirs. Institute of Hydrology Report, Wallingford. Christophersen, N. and Neal C., 1987. Some results important for further development of hydrochemical models describing freshwater acidification. In: B. Moldan and T. Paces (Editors), Proc. GEOMON International Workshop on Geochemistry and Monitoring in Representative Basins. Geol. Survey, Prague, pp. 39-141. Cryer, R., 1986. Atmospheric solute inputs. In: S.T. Trudgill (Editor), Solute Processes. Wiley, New York, N.Y., pp. 15-84. Darling, W.G., 1982. Revised procedures for the measurement of 2H/1H and lsO/~60 in water samples. Br. Geol. Surv., Hydrogeol. Res. Group, Stable Isotope Tech. Rep., 16: 1-29. Fowler, D., 1984. Transfer to terrestrial surfaces. Phil. Trans. R. Soc. Lond., Ser. B, 305: 281-297. Green, M.J. and Helliwell, P.R., 1972. The effect of wind on the rainfall catch. Proc. Symp. Distribution of Precipitation in Mountainous Areas, Geilo, Norway, World Meteorol. Organisation, OMH NO. 326, Vol. 2, pp. 27-46. I.H. Rep. Ser., 1976. Water balance of the headwater catchments of the Wye and Severn. Inst. Hydrol. Rep. Ser., 33: 142. 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 environmental isotopes, Mattole River Basin, California, J. Hydrol., 84: 107-140. Langen, S., 1986. Atmospheric deposition, afforestation and water quality at Loch Dee, SW Scotland. Ph.D. Thesis, University of St. Andrews. Neal, C., Smith, C.J., Walls, J. and Dunn, C.S., 1986. Major, minor and trace element mobility in the acidic upland forested catchment of the upper River Severn, Mid Wales. J. Geol. Soc. Lond. 143: 635-648. Neal, C., Christophersen, N., Neale, R., Smith, C.J., Whitehead, P.G. and Reynolds, B., 1988. Chloride in precipitation and streamwater for the upland catchment of River Severn, midWales; some consequences for hydrochemical models. Hydrol. Proc., 2: 155-165. Newson, A.J., 1976a. Some aspects of the rainfall of Plynlimon, mid-Wales. Inst. Hydrol. Rep. Ser. 34: 1-36. Newson, M.D., 1976b. The physiography, deposits and vegetation of the Plynlimon catchments. Inst. Hydrol. Rep. Ser., 30: 1-59. Penman, H,L., 1948. Natural evaporation from open water, bare soil and grass. Proc. R. Soc., Lond., Ser. A, 193: 120-146. Reynolds, B. and Pomeroy, A.B., 1988. Hydrogeochemistry of chloride in an upland catchment in mid-Wales. J. Hydrol., 99: 19-32. Robinson, A.C. and Rodda, J.C., 1969. Rain, wind and the aerodynamic characteristics of raingauges. Meteorol. Mag., 98: 113-120. Rodda, J.C. and Smith, S.W., 1986. The significance of the systematic error in rainfall measurement for assessing wet deposition. Atmos. Environ., 20: 1059-1064. Rodhe, A., 1981. Spring flood. Meltwater or groundwater? Nord. Hydrol., 12: 21-30. Skarveit, A., 1981. Relationships between precipitation, chemistry, hydrology and runoff acidity. Nord. Hydrol., 12: 65~0. Sklash, M.G. 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. Soil Survey of Scotland, 1982. Sheet 6 S.W. Scotland.
CHLORIDE AND OXYGENISOTOPES IN UPLAND STREAMS
283
Unsworth, M.H. and Crossley, A., 1986. Consequences of cloud water deposition on vegetation at high elevation. In: T.C. Hutchinson and K.M. Meemer (Editors), NATO Advanced Research Workshop on Effects of Acid Deposition of Forests, Wetland and Agricultural Ecosystems, Springer, Berlin, Vol. G16, pp. 171-188. Zall, D.M., Fisher, D. and Garner, M.Q., 1956. Photometric determination of chlorides in waters. Anal. Chem., 28: 1665-1668.
269
Elsevier Science Publishers B.V., Amsterdam - - Printed in The Netherlands
[21 CHEMICAL S T U D I E S OF CHLORIDE A N D S T A B L E O X Y G E N I S O T O P E S IN TWO C O N I F E R A F F O R E S T E D A N D M O O R L A N D S I T E S IN THE BRITISH U P L A N D S
COLIN NEAL and PAUL T.W. ROSIER
Institute of Hydrology, Wallingford, OXIO 8BB (U.K.) (Received December 16, 1988; accepted after revision July 28, 1989)
ABSTRACT Neal, C. and Rosier, P.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. Chloride concentration variations for three streams in the Crinan Canal region of southwest Scotland, draining h e a t h e r (Calluna vulgaris) and immature and mature sitka spruce conifer forest (Picea sitchensis (Bong.) Cart) are compared with incident rainfall values. Results show the importance, and correspondence, of in-catchment processes for a t t e n u a t i n g atmospheric inputs of chloride before its entry to the stream. All three streams exhibit a damped chloride concentration variation compared with rainfall. The chloride chemistries for the three streams are intercorrelated. The weekly data show a net accumulation of chloride into the catchment over the 1 year sampling period. This is associated with a n inappropriate stream sampling frequency and]or large yearly fluctuations, in the rainfall input; 80% of the rainfall chloride contribution was introduced during four of the 52 weeks of sampling. The yearly flux of chloride leaving the catchment as stream flow is remarkably similar for all three catchments. The study highlights the need for long and detailed data records from which information can be obtained on chemical budgets and the deficiencies in the measurement of atmospheric inputs of chloride. Although mature conifers are known to capture significant inputs of chemicals to a catchment, not measured by a standard rainfall collector in uplands areas, analogous results are not inferred in the lowland Crinal Canal case. Far more detailed studies are required if a definitive statement is to be made. The chloride results from the Crinan Canal Study are compared with analogous data for streams draining the Hafren Forest at Plynlimon in mid-Wales. Similar degrees of damping are observed and seasonal oscillations occur which do not have the same phase or amplitude. In addition, oxygen isotopic data are presented for the Plynlimon case to highlight a higher degree of damping t h a n for chloride.
INTRODUCTION
Studies of the relation between solute concentrations in rainfall and runoff, for constituents such as chloride and the isotopes of hydrogen and oxygen which are chemically unreactive in the hydrological cycle for temperate regions, can provide important information on the hydrological pathways operating in catchments (Sklash and Farvolden, 1979; Rodhe, 1981; Bottomley
0022-1694/90/$03.50
© 1990 Elsevier Science Publishers B.V.
270
C. NEAL AND P.T.W. ROSIER
et al., 1984/1985; Sklash et al., 1986; Christophersen and Neal, 1987; Neal et al., 1988). For many catchments, the variations in chloride concentration and oxygen isotope ratio are high in rainfall but low in the stream (Kennedy et al., 1986; Christophersen and Neal, 1987; Neal et al., 1988; Reynolds and Pomeroy, 1988). Results for catchments studied in the above references indicate t h a t the major portion of water entering the stream during high-flow periods is not derived directly from the rain promoting the hydrological response (Sklash and Farvolden, 1979). However, different catchments show different chemical responses to hydrological change, and a 'damped' response is not always found (Skartveit, 1981; Langan, 1986); in such circumstances, rainfall rapidly passes through the catchment to provide the major component of the stream response. It is unclear at present why geomorphically similar catchments should behave in such different ways, but the water residence time within the catchment must be a relevant factor. As part of a study of the effects of afforestation on the runoff from the catchments supplying the Crinan Canal reservoirs (Calder et al., 1982), the opportunity was taken to collect rainfall and runoff chloride data. This was initiated (1) to see if a chloride balance could provide an estimate of evaporation from different vegetation types, and (2) to provide additional information on the nature of the hydrological response for the catchments with a thin soil cover and an impermeable bedrock. As part of the detailed hydrochemical studies for the spruce forested catchment at Plynlimon, oxygen isotopic data were collected. This was undertaken (1) to see if the degree of damping is similar to t h a t previously shown for chloride (Neal et al., 1988), and (2) to provide further data for detailed modelling studies (cf. Christophersen and Neal, 1987). The results of these two experiments are presented and intercomparisons are made. CATCHMENT DETAILS The Crinan Canal area
The Crinan Canal study area at the northern end of the Kintyre peninsula in southwest Scotland (Fig. 1) has an altitude range of 150-350 m and an annual average rainfall of about 2000mm. The catchment has approximately 50% afforestation (mainly Sitka spruce). The underlying geology of the catchment is composed of metamorphic rocks of the Dalradian Assemblage together with metamorphic igneous rocks, and the soils are mainly peaty podzols, peaty gleys and brown earths (Soil Survey of Scotland, 1982). Three streams (catchment area in brackets) were monitored: Glac Connaidh - - heather, (33.8 ha), High Daill Burn - - immature forest (62.5 ha) and Achantheanbhaile - - mature forest (ll.3ha). Rainfall was sampled from a standard weekly integrated storage gauge at Dunardry, and streamwaters were collected as weekly 'instantaneous' samples. Snow deposition was not encountered in the sampling period. Stream flow was measured at the nearest gauging point, Carndubh Burn, as no suitable
CHLORIDE
AND OXYGEN
ISOTOPES
IN UPLAND
271
STREAMS
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measurements were available for the three streams. The streams showed marked hydrograph storm responses. Samples were taken on a weekly basis for 1 year (June 1985-June 1986) and analysed for chloride using an automated colorimetric method (Zall et al., 1956) within 6 weeks of sampling.
The Hafren Forest area, Plynlimon Two streams were surveyed, the Afon Hafren and the Afon Hore, both draining sub-catchments of conifer plantation (mainly Sitka spruce) and seminatural moorland. The catchments are of similar size (about 340ha), with typically 0.5-1 m-thick peat/podzol/gley/brown earth soils with a Silurian to Ordovician bedrock comprised of a mixture of mudstones, sandstones and grits. Annual rainfall is typically 2400 mm with annual evapotranspirational losses of about 430 and 700 mm for the Hafren and Hore catchments respectively. They have different areal forest cover: 50% for the Afon Hafren and 77% for the Afon Hore (Newson, 1976a; Newson, 1976b; Neal et al., 1986, 1988). Samples for isotopic analysis was collected as part of the weekly water sampling programme; the waters were stored in sealed glass bottles before analysis to ensure insignificant evaporative effects; analysis was performed, for a 1 year sub-set of samples (January-December 1984) using a VG Micromass spectrometer and a CO2 extraction technique (Darling, 1982). THEORETICALAND PRACTICALCONSIDERATIONS Strictly, a chemical budget for a catchment requires an accurate and precise water balance and mean solute compositions, from which estimates of the flux of elements entering and leaving can be made. This can only be achieved with a detailed hydrological and chemical monitoring programme of (1) rainfall input, consisting of a network of rainfall gauges, and (2) stream flow from a single flow-measuring device or a network of such devices (I. H. Rep. Ser., 1976; Newson 1976a). In practice as in the Crinan Canal case, this detailed information is not available due to the large scale of the undertaking and the costs involved. Instead, rainfall and flow are monitored at single locations within or near to the catchments to allow flow/volume-weighted chloride concentrations to be obtained; estimates of the water fluxes are obtained using the limited rainfall data together with either the limited flow data or some theoretical estimate of the evapotranspiration (see next section for the Crinan Canal case). However, in using the more limited data, simplifying assumptions are made. These assumptions are described here as a prelude to data interpretation and the assessment of the applicability of these limited data. It is assumed t h a t all the measurements are precise and t h a t weathering and ion exchange inputs are negligible for chloride and the oxygen isotopes. ESTIMATING MEAN CHLORIDECONCENTRATIONS Comment is made here on the estimation of mean chloride concentrations on the basis of a limited hydrological and chemical data set, to provide a rigorous
CHLORIDE AND OXYGEN ISOTOPES IN UPLAND STREAMS
273
well-defined framework on which to base conclusions. In particular, the important condition of 'linear scaling' for which mean weighted concentrations can be obtained is shown; this has not been adequately stressed previously. Let R(t) and Q(t) be instantaneous (time t) or averaged (time interval t - d to t + d) 'point in time' measurements (in dimensions of flux per unit time or flux per unit time per unit area), where R and Q denote rainfall and stream flow. R' (t) and Q' (t) are the ~true' values for the catchment, which are not measured because of the spatial variability of rainfall across the catchments and the lack of flow gauging equipment on the three sub-catchments. Thus in calculating flow- and volume-weighted means for stream chloride and rainfall chloride concentrations, respectively, it is necessary to introduce scaling factors RS and QS where
R' (t) = R(t)RS and
Q' (t) = Q(t)QS Taking RC1 and QC1 to be the corresponding measured concentrations of chloride in the rainfall and the stream for the catchment, RClm and QClm, the volume-weighted mean rainfall and flow-weighted mean streamwater chloride concentration, are given by = ~ R ' ( t ) RCI(t) = ~R(t) RSRCI(t) RClm
~R" (t)
~R(t) RS
and QClm
= ~ Q ' ( t ) QCl(t) = ~Q(t) QSQCl(t)
~,Q" (t)
~ Q (t) Qs
In virtually all hydrochemical studies, where restricted data have been collected, it is assumed implicitly, if not explicitly, that these scaling factors RS and QS are constant. In such circumstances, ~ R (t) RC1 (t) RClm =
~R(t)
QClm =
~Q(t) QC1 (t) ~Q(t)
Thus volume-weighted means and flow-weighted means can be calculated using estimated rainfall and stream flow respectively, given a linear relationship between these estimated and 'true' values (i.e. RS and QS constant). It should be noted that errors in QC1 and RC1 will always give rise to errors in the flowand volume-weighted chloride levels. In conditions where linear scaling is not applicable, reliable mean concentration values are unattainable from studies of the type presented here. FINE WATERPARTICLEAND DRY DEPOSITION Catchment vegetation is known to scavenge chloride from mist and small solid particles in the atmosphere (Fowler, 1984; Cryer, 1986; Unsworth and
274
C. NEAL AND P.T.W. ROSIER
Crossley, 1986). On the other hand, standard rain gauges are usually inefficient collectors, and underestimate effective rainfall by typically 20% (Rodda and Smith, 1986). They are particularly inefficient collectors of fine water and solid particles (Robinson and Rodda, 1969; Green and Helliwell, 1972). These fine water and solid particles can supply significant salt inputs to the catchments. Thus, standard rainfall collectors may underestimate the flux of chloride entering the catchment. The degree of this underestimation cannot easily be gauged but it will vary according to vegetation type, altitude and climate. For example, enhanced scavenging is observed for various vegetation covers in the U.K; this is usually associated with upland areas, such as Plynlimon, where a low cloud base occurs for several days during the year (Cryer, 1986; Neal et al., 1988). The term 'intercepted deposition' is used hereafter to denote the discrepancy. Thus 'intercepted deposition' is that component of the input to a catchment (rainfall, mist and dry deposition) which is not collected by a standard rainfall collector. EVAPORATION ESTIMATES To estimate annual evaporation using a limited data input the equations derived by Calder and Newson (1979) for mature and immature forest catchments, and Calder (1985) for the heather catchment have been used. The equations are (1) for mature and immature forest catchments: annual evaporation
=
E t + f(pa
-
WEt)
where E t is the Penman (1948) estimate of annual evaporation from grass (ram), f is the fraction of catchment area with complete canopy coverage, p is the annual precipitation (mm), a is the interception ratio for forest, and w is the fraction of year when canopy is wet. (2) for heather catchment: annual evaporation
=
[3Et ( 1 - w) + ap
where ~ is the ratio of heather transpiration to Penman evaporation and ~ is the interception ratio for heather. A full description of the equations and the experimental details for determining the parameters were given by Calder (1986). In making the evaporation estimates the rainfall for the study period was 2070 mm and the Penman Et was 410 ram. Also, the following assumptions have been made: (1) for the mature forest catchment the canopy coverage f is 100%; (2) for the immature forest catchment the canopy coverage f is 66% with the remainder being grass; and (3) for the heather catchment the coverage is 100%. This gives annual evaporation estimates of 1052, 838 and 567 mm respectively for the mature, immature forest and heather catchments.
CHLORIDE AND OXYGEN ISOTOPES IN UPLAND STREAMS
275
CHEMICALAND WATERMASS BALANCERELATIONSHIPS The relationship between the estimates of the chemical and water mass balances are explored here to identify some of the salient hydrochemical features. As a starting point, it is assumed initially that (1) chloride is chemically unreactive, (2) the store of water and chloride remains constant within the catchment, (3) 'interception deposition' of chloride is insignificant and (4) other factors are not important (see section on estimating mean chloride concentrations). The limitations of the approach taken and 'data gaps' will then be highlighted. To aid this assessment a new term, Clf, is introduced; this is the ratio of the volume-weighted chloride concentration in rainfall to the flow-weighted chloride concentration in the stream: elf
-
RClm QClm
Because of evapotranspiration losses in the catchment, chloride concentrations in the outflow should be higher t h a n the incoming rainfall to maintain the mass balance. Thus Clf represents the ratio of water leaving the catchment as stream flow to the water entering the catchment as rainfall, because for a long-term mass balance RClm ~ R ' (t) = QCl~ ~ Q ' (t) From a knowledge of evapotranspiration and rainfall this ratio can be represented as Ef = ~ R ' ( t ) - ~ E ' ( t ) ~ R ' (t)
_ ~V'(t) ~ R ' (t)
In theory Clf and Ef should be the same, given the above assumptions of conservative chloride and constant scaling factors as discussed previously. This has been tested using the Crinan Canal catchments. RESULTS AND DISCUSSION
Crinan Canal The input of chloride to the three Crinan Canal catchments is very variable through the year (Figs. 2 and 3). Over 70% of the chloride input by wet deposition occurs during 4 weeks of the sampling period when chloride concentrations are at their highest. Chloride outputs are also very variable (Fig. 2), but in this case the degree of variation is related to the large fluctuations in stream flow and concentration changes are relatively low (Fig. 3). Given the large variations in the rainfall chloride input and the use of weekly 'instantaneous' sampling in the stream it is possible t h a t stream outputs are underestimated. Consequently, a much longer, and more intensive sampling
276
C. NEAL AND P.T.W. ROSIER
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Fig. 2. Chloride flux variations in rainfall and stream waters for the Crinan Canal catchments. period is required to obtain reliable long-term chloride input fluxes and mean concentrations• The need for a long-term study to obtain chloride budgets is well illustrated in the heather and immature forest cases where flow/volume-weighted chloride concentrations are higher in the rain inputs than in the stream outflow; evapotranspiration should lead to evaporative concentration of chloride and Clf values of less than unity (the annual evaporation estimates derived using the Calder and Newson equation give Er values of 0,73 and 0.60 for the heather and immature forest cases, respectively; Ef should equal Clf, given the above assumptions). The calculated values of Clf, using instantaneous flow weightings, are 1.3 and 1.25 (integrated flow weightings give essentially the same result, Table 1). The high Clf values reflect long-term ( > yearly) changes in the chloride storage of the catchment• (Analytical errors associated with chloride, rainfall and flow measurement would not give rise to these large discrepancies, whereas analytical procedures for chloride determination are both accurate and precise• Errors associated with incomplete assessment of the atmospheric input (i.e. 'interception deposition') would lead to even greater disparity between Ef and Clf. Within the sampling period, chloride shows a net accumulation in the heather and immature forest catchments relative to water
277
CHLORIDE AND OXYGEN ISOTOPES IN UPLAND STREAMS 120
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Fig. 3. Chloride variations in rainfall and stream waters for the Afon Hafren and the Afon Hore catchments.
storage. However, the weekly stream-sampling programme may have resulted in important chloride episodes being missed and chloride stream fluxes being underestimated. It should be noted t h a t it may also be argued that a single rain gauge is not representative enough for comparisons to be made between catchments; with no information on rainfall solute composition across the sites it may not be realistic to draw specific conclusions. Set against this, however, the stream Cl% flux outputs are remarkably similar for the three sub-catchments, which suggests constant precipitation chemistry and a uniform catchment delay mechanism.) Similar results are observed in the mature forest case, as the estimate of Ef based on the Calder and Newson equation (0.49) is lower than the observed Clr value (0.78), although in this case Cl~ is less t h a n unity. The mature forested catchment is also probably showing a net accumulation of chloride in the catchment relative to water storage. However, the use of weekly data may result in important episodes of chloride release to the stream being missed, as storm response occurs over a matter of hours rather t h a n weeks.
278
c. NEALAND P.T.W. ROSIER
TABLE 1 Summary statistics of the chloride concentrations (mg 1-1) in the rainfall and stream water Stream water
Average (volume-weighted 1) Average (volume-weighted 2) Average (arithmetic) Min. value Max. value S.D. Sample size Clfa (weighted 1) Clfa (weighted 2)
Rainfall
Heather
Immature forest
Mature forest
10.95
11.34
18.31
14.20
11.49
11.75
18.07
14.20
13.96
15.08
22.40
13.28
6 31.0 5.75 53
6 32.4 7.46 53
6 46.0 9.39 53
0 112.0 22.81 53
1.29 1.24
1.25 1.21
0.78 0.79
-
Stream-flow weightings undertaken in two ways. (1) Instantaneous flow values used. (2) Integrated weekly flow values used. a Clf is the ratio of the volume-weighted mean concentration of chloride in rainfall to the flowweighted mean concentration of chloride in the stream, as calculated using weightings 1 and 2.
The temporal trends in stream chloride concentration are the same (Table 2, Fig. 3); chloride concentrations are highest from J a n u a r y to March and lowest at the end of September. The reason for this strong seasonality remains unclear, as it does not correspond directly with hydrological factors such as rainfall distribution and soil moisture deficits; a much lengthier sampling period is required to assess the regularity of the trend. Whatever the detailed hydrological processes operating, this regularity in trend between catchments implies that they are remarkably similar for all three catchments. The importance of 'interception deposition' in supplying chloride to the Crinan Canal catchments cannot quantitatively be established owing to the large uncertainties. However, there are three aspects which, combined, imply that this phenomenon may be less significant in lowland than in upland areas. First, low CIF values would be expected in areas where 'intercepted deposition' is high and Clf should be lower than El, if this term were significant; this is not the case for the Crinan Canal catchments, but it is the case for upland areas such as Plynlimon, where Clf is up to 40% lower than Ef (Neal et al., 1988; Reynolds and Pomeroy, 1988). Second, the flux of chloride in the Crinan Canal streams are approximately equal (15.8, 14.5 and 16.4gm -2 year -1 for the heather, immature and mature forest cases respectively). The anticipated degree of capture would be in the order, mature forest > immature forest >~ heather, based on vegetation surface area available to scavenging, and conse-
279
CHLORIDE AND OXYGEN ISOTOPES IN UPLAND STREAMS
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Fig. 5. Chloride concentration variations in rainfall and stream waters for the Afon Hafren and the Afon Hore catchments.
280
C. NEAL AND P.T.W. ROSIER
TABLE 2 Rainfall and stream-water chloride concentration, and flow inter-correlations (significance level in brackets, N = 53 in all cases)
CLHE CLIF
CLHE
CLIF
CLMF
1.0000 (0.0000)
0.9033 (0.0000) 1.0000 (0.0000)
0.8696 (0.0000) 0.9184 (0.0000) 1.0000 (0.0000)
CLMF CLRAIN~ RAINa FLOW
CLRAINa 0.0478 (0.7341) 0.0345 (0.8065) - 0.0427 (0.7615) 1.0000 (0.0000)
RAINa 0.0748 (0.5944) - 0.1127 (0.4217) - 0.1322 (0.3454) 0.0652 (0.6568) 1.0000 (0.0000)
FLOW - 0.3780 (0.0053) - 0.3627 (0.0076) - 0.3317 (0.0153) 0.0706 (0.6154) 0.4923 (0.0002) 1.0000 (0.0000)
Where CLHE is the chloride concentration in the Glac Connaidh stream draining heather. CLIF is the chloride concentration in the High Daill Stream draining immature forest. CLMF is the chloride concentration in the Achantheanbhaile stream draining mature forest. CLRAIN is the chloride concentration in the rainfalP RAIN is the rainfall amount a FLOW is the flow in Carndubh Burn. These are weekly bulked data; the others are 'instantaneous grab' samples. Caution must be taken when comparing the two types of data: low correlations are to be expected as the stream response times to rainfall will be in the order of hours.
quently the stream chloride flux should vary in the same manner. The constancy of this flux may also suggest that the storage effects are similar for all three catchments. Third, ~intercepted deposition' would be expected to be m o s t s i g n i f i c a n t i n t h e u p l a n d a r e a s w h e r e t h e r e is a f a r g r e a t e r c h a n c e o f t h e v e g e t a t i o n b e i n g i n c o n t a c t w i t h m i s t (i.e. a l o w c l o u d b a s e ) . M u c h m o r e d e t a i l e d s t u d y is r e q u i r e d , h o w e v e r , b e f o r e a d e f i n i t i v e s t a t e m e n t c a n b e m a d e o n t h i s p o i n t . T h e o b s e r v a t i o n is p r e s e n t e d h e r e s i m p l y a s a p o i n t e r t o encourage further work in this field for lowland areas.
Hafren Forest, Plynlimon F o r t h e H a f r e n F o r e s t c a t c h m e n t s t h e r e is a s i g n i f i c a n t v a r i a t i o n i n t h e is O/160 r a t i o i n r a i n f a l l , a l t h o u g h s t r e a m w a t e r s s h o w a v e r y d a m p e d b e h a v i o u r ( F i g . 4). T h i s d e g r e e o f d a m p i n g is e v e n g r e a t e r t h a n t h a t f o r c h l o r i d e ( F i g s 4 a n d 5) a n d n o c o r r e s p o n d i n g s e a s o n a l e f f e c t is o b s e r v e d . T h e l o s s o f w a t e r f r o m the catchment to the atmosphere by transpiration and complete evaporation from the wetted vegetation surfaces causes insignificant isotopic fractionation a n d h e n c e a m i n i m a l v a r i a t i o n i n 18O/160 r a t i o . ' E v a p o r a t i v e c o n c e n t r a t i o n ' o f c h l o r i d e w i l l o c c u r , h o w e v e r , a s w a t e r is l o s t f r o m t h e c a t c h m e n t t o t h e
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atmosphere. The even greater damping of 180/160 ratio, compared with chloride, is thus to be expected; the evaporative effect leads to a much more heterogeneous distribution of chloride, compared with the lsO/160 ratio, in the catchment. The isotopic study provides further evidence suggesting t h a t only a minor portion of the water which enters the streams during a storm event at the Hafren forest catchment is derived from the rain generating the hydrological response. CONCLUSIONS (1) For estimates of either evapotranspiration from chloride concentration measurements or chloride budgets much longer sampling periods are required. Furthermore, the atmospheric input has to be much more accurately assessed, with the inclusion of a quantified value for the 'intercepted deposition' contribution. High-quality rainfall and stream-flow measurements are also required as volume/flow-weighted mean chloride concentrations are used in the calculations. (2) Seasonal variations in the chloride concentrations in stream waters in the Crinan Canal and the Plynlimon regions are observed. The patterns differ from area to area but are similar for a given area. For example, (a) in the Plynlimon case the highest stream-water concentrations are to be found from October to April (cf. Neal et al., 1988; Reynolds and Pomeroy, 1988) compared with J a n u a r y to March in the Crinan Canal case and, (b) the amplitude of the variation is much greater for the Crinan Canal case. The regularity of these patterns for a given area and the differences in pattern observed from area to area has not yet been explained but in some way this phenomenon must be related to longer-term changes in climate pattern (cf. Reynolds and Pomeroy, 1988). F u r t h e r study is warranted as the trends provide important clues to the nature of water storage and the flow pathways through catchments. ACKNOWLEDGEMENTS Thanks must go to the staff of the British Waterways Board at Ardrishaig for the work carried out in the Crinan Canal study, to George Darling of the British Geological Survey for the isotopic analysis and to the staff of the Institute of Hydrology for sample collection at Plynlimon and analysis at Wallingford. Valuable criticism of the draft was given by Brian Smith and Paul Whitehead. Financial support was received from a number of Agencies: British Waterways Board, North of Scotland Hydro-Electric Board, Scottish Development Department, Department of Energy, Department of the Environment, Water Research Centre and the Natural Environment Research Council. REFERENCES Bottomley,D.J., Craig, J. and Johnson, L.M., 1984/1985. Neutralisation of acid runoff by groundwater discharge to streams in Canadian Precambrian shield watersheds. J. Hydrol.,75: 1--26. Calder, I.R., 1985.Influenceof woodlandson water quantity. Inst. Biol. Proc. Environ. Div. Symp., Weather, Woodlands and Water, Edinburgh, March, 23 1984.
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