- Email: [email protected]
Diurnal variations in the inorganic solute content of water draining from an alpine snowpatch
CATENA
vol. 16, p. 153-162
Cremlingen 1989
DIURNAL VARIATIONS IN THE INORGANIC S O L U T E C O N T E N T OF WATER D R A I N I N G FROM AN ALPINE SNOWPATCH N. Caine, Heidelberg Summary
ergy input to the snow cover (COLLINS & Y O U N G 1981, C L A R K 1988). Since The results of short-interval sampling for the routing and rates of meltwater transwater quality over seven diurnal flow cy- mission change during the diurnal cycle, cles from the Martinelli basin, site of a equivalent fluctuations in solute and sedlong-lasting snowpatch in the Colorado iment concentrations should occur with Front Range, are reported here. They a diurnal frequency. Like those reported show an inverse relationship between dis- from glacierized basins (e.g. COLLINS charge and the concentration of most 1983), these will probably involve the disolute species which reflects varying pro- lution of solute concentrations and inportions of melt water transferred to the creased sediment loads at higher flows. channel through different routes. This However, the response of any catchment suggests that variations in water quality will depend on its bedrock and soil, the over the diurnal cycle might be used to nature of dissolved and adsorbed matepartition the flow hydrograph. It also in- rials within the snow, and the extent of dicates a source of bias in mass yield esti- snowcover ( Z E M A N & S L A Y M A K E R mates that are based upon water samples 1975). taken with a daily (or longer) interval at Such short-term temporal fluctuations the same point in the flow cycle. On the in water quality are important for at least Martinelli site, this bias is less than 5% two reasons. First, they should provide for most solutes but is much greater than information to allow deconvolution of that in the case of chloride, silicon and the hydrologic record and the identifisuspended sediment. cation of flow sources within the catchment (BROWN 1986). Second, they may introduce bias or error to estimates of 1 Introduction material budgets which are based upon The streamflow generated from snow- samples taken consistently at one point melt in mountain catchments commonly in the daily flow cycle. To identify these, fluctuates with a 24 hour periodicity effects, a detailed temporal record of flow which reflects the cycle of radiative en- and water quality from an alpine snowpatch in the Colorado Front Range is ISSN 0341-8162 reported here. (~)1989 by CATENA VERLAG, D-3302 Cremlingen-Destedt, W. Germany 0341-8162/89/5011851/US$ 2.00 + 0.25
CATENA An Interdisciplinary Journal of SOIL SCIENCE
HYDROLOGY -GEOMORPHOLOGY
154
2
Caine
Study site
urnal periodicity (fig.l).
This paper treats discharge records from the 8 hectare basin of the Martinelli Snowpatch on the south face of Niwot Ridge at 3415 m elevation (40 ° 2' 45" N; 105 ° 35' 00" W). This basin has been the site of studies conducted through the Institute of Arctic & Alpine Research for almost 30 years ( M A R T I N E L L I 1959, O S B U R N 1963, T H O R N 1975, 1976). Since 1981, hydrologic and geochemical work in the Martinelli catchment has been conducted as part of the Long Term Ecological Research (LTER) program (SWANSON & F R A N K L I N 1988) which has provided a more complete record of the basin dynamics than was previously available (CAINE & SWANSON 1989). The catchment is underlain by granodiorite bedrock, although none is exposed within the basin itself. Instead, it is mantled by a regolith of variable thickness derived from the bedrock and from pre late-Pleistocene diamictons mantling Niwot Ridge ( M A D O L E 1982). About 45% of the catchment area is occupied by a long-lasting snowpatch in which more than 10 m of accumulated snow depth have been recorded in late winter. This part of the catchment produces almost all the streamflow from snowmelt and summer rainstorms. During mid-June 1987, the period treated here, streamflow was entirely due to melting of the residual snowpatch which was up to 4 m deep and covered about 25% of the catchment area. Only 2 mm of rainfall was recorded at the Saddle Station on Niwot Ridge, about 500 m from the Martinelli site, in the period June 10-28, 1987 and this had negligible effect on stream flows. The resulting snowmelt discharges show a regular di-
3
Procedures
Data from two periods in the early summer of 1987 are treated here (fig.la). The period June 10--12 includes the highest flows from the Martinelli basin during the 1987 runoff season when discharges varied between 600 and 700 m 3 d -1 (fig.lb). The second period, June 17-21, is on the falling limb of the seasonal hydrograph when flows amounted to 250 to 325 m 3 d -1 (fig.lc). For both periods, flow levels were recorded continuously at a calibrated control section in the channel. Water samples were taken automatically by an ISCO pump sampler programmed to collect on a 15 minute interval and combine four aliquots into a single hourly sample during the first period. The sampler was reprogrammed to combine 6 hourly aliquots during the second period. Samples were held at 0°C in an insulated ice-box prior to removal from the field. No other means of preservation was used. In the laboratory, samples were refrigerated until analysis: within 24 hours of collection for anions and within 6 days for cations. Samples were not routinely filtered prior to chemical analysis because clastic sediment concentrations at all flows are low (CAINE 1986): no more than 0.5 mg 1-1 for more than 90% of all samples taken at this site over 5 years. Chemical analysis involved Atomic Absorption Spectrophotometry (PerkinElmer model 2280) to determine concentrations of calcium, magnesium, sodium, potassium and silicon. Ion Chromatography (Dionex 2110i interfaced to a Spectra-Physics SP4270 integrator), using 3 replicates of each sample, was used
CATENA An Interdisciplinary Journal of SOIL SCIENCE
HYDROLOGY~EOMORPHOLOGY
Inorganic Solute Content, Alpine Snowpatch
800
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F i g . l : Discharge records at Martinelli, 1987.
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Fig. l : Discharge records at Martinelli, 1987. b: Discharge, Conductance and Sediment Concentrations, June 10-12, 1987. The conductance and sediment concentration data are based on samples taken every 15 minutes with composites of four representing a one hour interval. Two missing values in sediment concentrations are shown by breaks in the line joining data points.
CATENA An Interdisciplinary Journal of SOIL S C I E N C E - - H Y D R O L O G Y ~ E O M O R P H O L O G Y
156
Caine 6,0-
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Fig. 1: Discharge records at Martinelli, 1987. c: Discharge, Conductance and Sediment Concentrations, June 17-21, 1987. The conductance and sediment concentration data are based on ssmples taken each hour with composites of six representing a six hour interval.
to determine sulfate, nitrate, chloride and fluoride concentrations. Bicarbonate concentrations were determined by titration to a pH 4.5 end point. In waters with low solute concentrations, like those treated here which have less than 100 /~eq 1-1 of cations, this is likely to overestimate alkalinity and may explain ion balances with a 10% excess of anions at times. Such an interpretation is empirically supported by the inverse correlation of bicarbonate alkalinity and the sample ion balance: r = -0.877 with N = 47 and r = -0.962 with N = 18 for the samples of this study. Suspended sediment concentrations and size distributions have been determined by laser particle sizer and counter
(Spectrex) with the output calibrated against results from filtration at 0.45 #m.
4
Water quality
Discharges from the Martinelli basin during both study periods show a strong diurnal cycle (fig.l). The 24 hr frequency explains 82% of the variance in hourly flows during the June 10-12 period, and 69% in the June 17-21 interval. These proportions are similar to those defined by Fourier tansformation of residual hourly discharges (after removal of the trend due to annual hydrograph recession) during six-day periods in the preceding five seasons at this site
CATENA An Interdisciplinary Journal of SOIL SCIENCE HYDROLOGY~EOMORPHOLOGY
Inorganic Solute Content, Alpine Snowpatch
Ca (mg/l) M g (mg/l) Na (mg/1) K (mg/l) HCO3 (rag/l) SO4 (mg/1) NO3 (mg/1) C1 (rag/l)
F (mg/l) Si (rag/l) Sediment (mg/l) Cond. (#S/cm) pH Water (I/s)
157
Reproducibility
June 10-12 ~ S
0.01 0.002 0.005 0.01 0.10 0.03 0.008 0.005 0.005 0.20 0.04 2% 0.10
0.89 0.10 0.35 0.14 4.15 0.67 0.73 0.05 0.01 0.86 0.13 7.61 6.45 8.01
June 17-21 ~ S
0.09 0.01 0.05 0.03 0.75 0.06 0.08 0.02 0.00 0.53 0.10 0.72 0.10 2.45
1.06 0.12 0.42 0.07 5.67 0.76 0.58 0.07 0.01 1.62 0.33 8.38 6.35 3.27
0.03 0.01 0.04 0.01 0.67 0.04 0.05 0.01 0.00 1.12 0.36 0.43 0.11 0.73
Notes: is the discharge-weighted mean. S is the discharge-weighted standard deviation. Reproducibility is the standard deviation of repeated analyses of the same sample. Specific conductance (Cond.) has been corrected to 25°C.
T a b . 1: Stream water quality at Martinelli Snowpatch, June 1987.
r Ca Mg Na K HCO3 SO4 NO3 C1 F Si Sed. Cond. pH
-0.497 -0.644 -0.715 (0.119) -0.412 -0.952 -0.947 (0.051) -0.548 (0.130) 0.387 -0.979 -0.664
June 10-12 (N = 47) a
b
r
1.286 0.152 0.700
-0.175 -0.227 -0.333
7.078 1.253 1.446
-0.261 -0.297 -0.325
-0.800 -0.808 -0.745 (-0.138) (-0.325) -0.842 (-0.403)
0.014
-0.180
0.009 14.537 6.676
1.126 -0.308 -0.026
June 17 21 (N = 18) a
b
1.232 0.163 0.614
-0.123 -0.255 -0.307
0.987
-0.208
0.009 11.087
2.465 -0.222
(-0.001)
(-0.381) (0.295) 0.549 -0.913 (-0.243)
Notes: In all cases except for that of pH, the model y = a - x b has been fitted with x = discharge (l/s), and y = concentration (mg/1), except for Conductance (/zS/cm). For pH, the linear model y = a + b - x has been fitted. Correlation coefficients (r) that are not significant (0.05 probability) are in parentheses. T a b . 2 : Discharge - water quality relationships.
CATENA An Interdisciplinary Journal of SOIL SCIENCE HYDROLOGY
GEOMORPHOLOGY
Caine
158 10,0-
MARTINELLI SNOWPATCH JUNE 1987
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Fig. 2: Discharge and Water Quality, June 1987. The regressions fitted to the Conductance, Calcium and Sulfate data for each period are summarized in tab.2.
(40% to 80% of the residual variance in the 24-hour frequency). The double peak in the daily hydrographs of the June 1721 period (fig.lc) is a response to separation of the main snowpatch into two segments, one upslope of the other, on June 14. This pattern, too, is repeated in other years at this site.
and is followed by sodium, magnesium and potassium in molar proportions of 10:7:2:1 (tab.l). Bicarbonate is the main anion (even if overestimation is conceded) followed by nitrate, sulfate and chloride in molar proportions of 44:4:3:1 (tab.l). Suspended sediment concentrations are low, averaging only 0.1 mg 1-l in the first period and 0.28 mg 1-1 in the Solute concentrations are low and sim- second. The general increase in sediment ilar to those found in weekly sampling concentration with the lower discharges during the 1982-1987 flow seasons. Cal 7 of the second period has been noted at cium is the dominant cation in solution CATENA--An
Interdisciplinary Journalof SOIL S C I E N C E - - H Y D R O L O G Y ~ E O M O R P H O L O G Y
Inorganic Solute Content, Alpine Snowpatch
159
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Fig. 3: Discharge and Sediment Concentration, June 1987. The arrows show the temporal sequence of samples in the two periods adn define a clockwise hysteresis, except in the June 18-19 cycle (solid arrows) which is counterclockwise. other times on this site and seems to reflect exposure of the unvegetated riparian zone and channel as the snowcover retreats. During both sampling periods, solute concentrations vary inversely with the water discharge (fig.lb,c). The shortterm variability in solute concentrations (as measured by the standard deviation in tab.l) is relatively high: up to 70% of the standard deviation estimated from weekly samples over entire flow seasons. The inverse relationship between concentration and discharge shows a diluting effect due to increased meltwater flows during the afternoon and evening hours (tab.2). This is best defined for specific conductance but is also evident in the response of individual ions (fig.2). Values of the exponent (b) in the solute con-
CATENA
An Interdisciplinary Journal of SOIL SCIENCE
centration : discharge relationship are variable but are all appreciable less than 1.0 (tab.2). This shows that, despite dilution, water discharge will dominate in estimates of solute yield, the highest yields being associated with the highest flows ( G U N N E R S O N 1967). Within each study period, sediment concentrations increase with the flow volume (tab.2). Peak concentrations tend to lead the highest flows by about one hour (fig.l) which gives a clockwise hysteresis to the concentration - discharge relation (fig.3) and reduces the simple correlation between flow and sediment concentration (tab.2). An exception to this occurs in the June 18-19 part of the record which loops in a counter-clockwise direction (fig.3). This follows a time of relatively high sediment concentration and
HYDROLOGY~;EOMORPHOLOGY
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Caine
may indicate depletion of stored sediment in the channel by that event.
5
Discussion
This empirical record shows that flow volumes and solute and sediment concentrations are closely related in a nival hydrologic regime. It suggests that simple mixing models, such as those of COLLINS (1983) and S O U C H E Z & L E M M E N S (1987) might be used to identify discharge sources in the Matrinelli catchment. For that purpose, specific conductance seems to be the best water quality charactgeristic for it represents an integration of all ionic solutes and is relatively simple to measure on a continuous basis ( F E N N 1987). Empirically, conductance is the water quality characteristic most closely correlated with stream discharge (tab.2). However, changes in the discharge - concentration relationships between the two periods in June 1987 introduce a further complication to their use in hydrograph separation. In 10 days, ionic concentrations measured at equivalent discharges are reduced by between 10% and 20% (fig.2, tab.2), part of a trend to reduced concentrations as the melt-season progresses. This is a late stage in snowpack "flushing" reported by other workers (e.g. J O H A N N E S S E N & H E N R I K S E N 1978, B R I M B L E C O M B E et al. 1985, S C H O N D O R F & H E R R M A N N 1987). These data do not permit clear identification of the fractionation effects found in earlier studies because more than 40% of the annual flow had been released from the catchment by June 10, 1987. Thus, the high, and highly differentiated, concentrations of the early flows were not defined. Nevertheless, the seasonal trend would require continuous adjustment of CATENA
a meltwater conductance used as input to a mixing model. The magnitude of bias in mass estimates of material removed from the basin can also be evaluated by these data. In this case, at and soon after the highest flows of the season, the bias in yield due to a midday sampling is relatively slight: an overestimate of less than 5% for most solutes (tab.3). It is greatest (up to 20%) for those solute species most poorly correlated to discharge and so is unlikely to be removed by matching sampling times to those of mean daily discharge. Later in the season, the diurnal peak discharge occurs earlier and a midday sampling time approaches that of peak flow. This will tend to correct the bias shown in tab.3 or convert it into a slight underestimate. When integrated across the entire flow season, errors due to sampling between 12:00 hr and 16:00 hr are probably negligible in this basin. The relatively constant concentration of chloride in these records conforms to the pattern found in longer-term records. If the first 10-15 days of streamflow are ignored (i.e. a time when snowpack flushing gives chloride concentrations as high as 0.5 mg/1-1 on ocasions), they average only 0.08 mg/1-1 (with S.D = 0.03) over 4 seasons (52 samples). The poor correlation with discharge could introduce error to estimates of chloride yield. Tab.3 suggests that one-point sampling may overestimate it by more than 10% during high flows. If these estimates are then used to "correct" the estimated yields of other solutes (e.g. CLAASSEN et al. 1986), the error will be transferred. This suggests that corrections based on the conservative behavior of the chloride ion not be applied automatically to nival streams.
An Interdisciplinary Journal of SOIL SCIENCE
HYDROLOGY
GEOMORPHOLOGY
Inorganic Solute Content, Alpine Snowpatch
Actual* Ca Mg Na K HCO3 SO4 NO3 C1 F Si Sed. Cond. pH
June 10-12 Est.'*
600 64 235 91 2788 452 492 33 7 575 90 5113 4336
595 64 259 97 3024 460 479 37 7 662 118 5154 4368
161
Diff.
Actual*
-0.9% 0.0% 10.0% 6.6% 8.5% 1.8% -2.6% 12.2% 5.0% 15.1% 30.6% 0.8% 0.7%
299 33 118 21 1602 215 163 21 3 458 92 2369 1794
June 17-21 Est.*" 291 32 119 23 1617 210 151 21 3 554 162 2269 1777
Diff. -2.5% -3.4% 0.5% 9.6% 1.0% -2.1% -7.3% 4,1% 10.9% 20.9% 75,4% -4,2% -1,0%
Notes: * Actual yields (g/day) evaluated from the continuous flow record and concentrations in water samples taken at 1 or 6 hour intervals. ** Estimate yields (d/day) derived from the continuous flow record and concentrations in the 12:00 hr water sample for each day. Solute and sediment yields are in g/day. The values for specific conductance are m 3./zS/cm.day and for pH are units.m3/day.
Tab. 3' Bias estimates.
6
Conclusion
The empirical data summarized here clearly verify the suggestion that water qality characteristics in snowmelt streams respond through a dilution mechanism to changes in flow volume. However, the resulting relationship between solute concentration and discharge is not constant through time but changes along the falling limb of the seasonal hydrograph. On such a longer temporal scale, the relationship is reversed, lower concentrations associated with the lower, later discharges. This will complicate, though not prevent, modeling of the flows as a mixture of water from two or more sources because it suggests that the geochemical characteristics of at least one of them are not constant in the long term. The periodic variation in water quality also introduces a possible CATENA An Interdisciplinary Journal of SOIL SCIENCE
HYDROLOGY
bias to many estimates of material yield thoug, in the case evaluated here, the effect is not great. It is most serious with respect to the chloride ion and its use in calibrating basin budgets.
Acknowledgements Work in the Martinelli Catchment has been supported by the National Science Foundation as part of the LongTerm Ecological Research program under Grants BSR 8514329 and BSR 8745637. I thank J.M. Caine for chemical analyses and M.I. Litaor and two anonymous reviewers whose critical comments have much improved this paper.
References BRIMBLECOMBE, P., TRANTER, M., ABRAHAMS, P.W., BLACKWOOD, L, DAVIES, T.D. & VINCENT, C.E. (1985): Relocation and
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preferential elution of acidic solutes through the snowpack of a small, high-altitude Scottish catchment. Annals of Glaciology, v. 7, 141-147. BROWN, M.J. (1986): Use of stream chemistry to estimate hydrologic parameters. Water Resources Research, v. 22, 805-811. CAINE, N. (1986): Sediment movement and storage on alpine slopes in the Colorado Rocky Mountains. In: Hillslope Processes. (Ed. A.D. Abrahams), London, Allen and Unwin, 115137. CAINE, N. & SWANSON, F.J. (1988): Geomorphic coupling of hillslope and channel systems in two small mountain basins. Zeitschrift fiir Geomorphologie, v. 33 (in press). CLAASSEN, H.C., REDDY, M.M. & HALM, D.R. (1986): Use of the chloride ion in determining hydrologic-basin water budgets - - a 3-year case study in the San Juan Mountains, Colorado, U.S.A. Journal of Hydrology, v. 85, 49-71. CLARK, M.J. (1988): Periglacial hydrology. In: Advances in Periglaical Geomorphology. (Ed. M.J. Clark), Chichester, J. Wiley and Sons, 415462. COLLINS, D.N. (1983): Solute yields from a glacierized high mountain basin. In: Dissolved Load of Rivers and Surface Water: Quantity / Quality Relationships. International Association Scientific Hydrology Publication No. 141, 41-49.
MARTINELLI, M.B. Jr. (1959): Some hydrologic aspects of alpine snowfields under summer conditions. Journal of Geophysical Research, v. 64, 451-455. OSBURN, W.S., Jr. (1963): The dynamics of fallout distribution in a Colorado alpine tundra snow accumulation ecosystem. In: Radiobiology: Proceedings of the First National Symposium on Radioecology. (Eds. V. Sehulz & A.W. Klement, Jr.), 51-71. SCHONDORF, T. & HERRMANN, R. (1987): Transport and chemodynamics of organic micropollutants and ions during snowmelt. Nordic Hydrology, v. 18, 259-278. SOUCHEZ, R.A. & LEMMENS, M.M. (1987): Solutes. In: Glacio-Fluvial Sediment Transfer. (Eds. A.M. Gurnell & M.J. Clark), Chichester, John Wiley & Sons, 285-303. SWANSON, F.J. & FRANKLIN, J.F. (1988): The long-term ecological research program. Eos, v. 69, p. 34, 36, 46. THORN, C.E. (1975): Influence of late-lying snow on rock-weathering rinds. Arctic and Alpine Research, v. 7, 373-378. THORN, C.E. (1976): Quantitative evaluation of nivation in the Colorado Front Range. Geological Society of America Bulletin, v. 87, 11691178. ZEMAN, L.J. & SLAYMAKER, H.O. (1975): Hydrochemical analysis to discriminate variable runoff source areas in an alpine basin. Arctic and Alpine Research, v. 7, 341-351.
COLLINS, D.N. & YOUNG, G.J. (1981): Meltwater hydrology and hydrochemistry in snow and ice-covered mountain catchments. Nordic Hydrology, v. 12, 319-334. FENN, C.R. (1987): Electrical conductivity. In: Glacio-fluvial sediment transfer: an alpine persoective. (Ed. A.M. Gurnell & M.J. Clark), Chichester, J. Wiley and Sons, 377-420. GUNNERSON, C.G. (1967): Streamflow and quality in the Columbia River basin. Proceedings ASCE, Journal of Sanitary Engineering Division, v. 93, No. SA6, 1-16. JOHANNESSEN, M. & HENRIKSEN, P. (1978): Chemistry of snow meltwater - - changes during melt. Water Resources Research, v. 14, 615-619. MADOLE, R.F. (1982): Possible origins of tilllike deposits near the summit of the Front Range in north-central Colorado. U.S. Geological Survey Professional Paper 1243, 31 pp.
Address of author: Nel Caine
Geographisches Institut Universit/it Heidelberg Im Neuenheimer Feld 348 D-6900 Heidelberg 1 West Germany
C A T E N ~ A n Interdisciplinary Journal of SOIL S C I E N C E - - H Y D R O L O G Y ~ E O M O R P H O L O G Y
vol. 16, p. 153-162
Cremlingen 1989
DIURNAL VARIATIONS IN THE INORGANIC S O L U T E C O N T E N T OF WATER D R A I N I N G FROM AN ALPINE SNOWPATCH N. Caine, Heidelberg Summary
ergy input to the snow cover (COLLINS & Y O U N G 1981, C L A R K 1988). Since The results of short-interval sampling for the routing and rates of meltwater transwater quality over seven diurnal flow cy- mission change during the diurnal cycle, cles from the Martinelli basin, site of a equivalent fluctuations in solute and sedlong-lasting snowpatch in the Colorado iment concentrations should occur with Front Range, are reported here. They a diurnal frequency. Like those reported show an inverse relationship between dis- from glacierized basins (e.g. COLLINS charge and the concentration of most 1983), these will probably involve the disolute species which reflects varying pro- lution of solute concentrations and inportions of melt water transferred to the creased sediment loads at higher flows. channel through different routes. This However, the response of any catchment suggests that variations in water quality will depend on its bedrock and soil, the over the diurnal cycle might be used to nature of dissolved and adsorbed matepartition the flow hydrograph. It also in- rials within the snow, and the extent of dicates a source of bias in mass yield esti- snowcover ( Z E M A N & S L A Y M A K E R mates that are based upon water samples 1975). taken with a daily (or longer) interval at Such short-term temporal fluctuations the same point in the flow cycle. On the in water quality are important for at least Martinelli site, this bias is less than 5% two reasons. First, they should provide for most solutes but is much greater than information to allow deconvolution of that in the case of chloride, silicon and the hydrologic record and the identifisuspended sediment. cation of flow sources within the catchment (BROWN 1986). Second, they may introduce bias or error to estimates of 1 Introduction material budgets which are based upon The streamflow generated from snow- samples taken consistently at one point melt in mountain catchments commonly in the daily flow cycle. To identify these, fluctuates with a 24 hour periodicity effects, a detailed temporal record of flow which reflects the cycle of radiative en- and water quality from an alpine snowpatch in the Colorado Front Range is ISSN 0341-8162 reported here. (~)1989 by CATENA VERLAG, D-3302 Cremlingen-Destedt, W. Germany 0341-8162/89/5011851/US$ 2.00 + 0.25
CATENA An Interdisciplinary Journal of SOIL SCIENCE
HYDROLOGY -GEOMORPHOLOGY
154
2
Caine
Study site
urnal periodicity (fig.l).
This paper treats discharge records from the 8 hectare basin of the Martinelli Snowpatch on the south face of Niwot Ridge at 3415 m elevation (40 ° 2' 45" N; 105 ° 35' 00" W). This basin has been the site of studies conducted through the Institute of Arctic & Alpine Research for almost 30 years ( M A R T I N E L L I 1959, O S B U R N 1963, T H O R N 1975, 1976). Since 1981, hydrologic and geochemical work in the Martinelli catchment has been conducted as part of the Long Term Ecological Research (LTER) program (SWANSON & F R A N K L I N 1988) which has provided a more complete record of the basin dynamics than was previously available (CAINE & SWANSON 1989). The catchment is underlain by granodiorite bedrock, although none is exposed within the basin itself. Instead, it is mantled by a regolith of variable thickness derived from the bedrock and from pre late-Pleistocene diamictons mantling Niwot Ridge ( M A D O L E 1982). About 45% of the catchment area is occupied by a long-lasting snowpatch in which more than 10 m of accumulated snow depth have been recorded in late winter. This part of the catchment produces almost all the streamflow from snowmelt and summer rainstorms. During mid-June 1987, the period treated here, streamflow was entirely due to melting of the residual snowpatch which was up to 4 m deep and covered about 25% of the catchment area. Only 2 mm of rainfall was recorded at the Saddle Station on Niwot Ridge, about 500 m from the Martinelli site, in the period June 10-28, 1987 and this had negligible effect on stream flows. The resulting snowmelt discharges show a regular di-
3
Procedures
Data from two periods in the early summer of 1987 are treated here (fig.la). The period June 10--12 includes the highest flows from the Martinelli basin during the 1987 runoff season when discharges varied between 600 and 700 m 3 d -1 (fig.lb). The second period, June 17-21, is on the falling limb of the seasonal hydrograph when flows amounted to 250 to 325 m 3 d -1 (fig.lc). For both periods, flow levels were recorded continuously at a calibrated control section in the channel. Water samples were taken automatically by an ISCO pump sampler programmed to collect on a 15 minute interval and combine four aliquots into a single hourly sample during the first period. The sampler was reprogrammed to combine 6 hourly aliquots during the second period. Samples were held at 0°C in an insulated ice-box prior to removal from the field. No other means of preservation was used. In the laboratory, samples were refrigerated until analysis: within 24 hours of collection for anions and within 6 days for cations. Samples were not routinely filtered prior to chemical analysis because clastic sediment concentrations at all flows are low (CAINE 1986): no more than 0.5 mg 1-1 for more than 90% of all samples taken at this site over 5 years. Chemical analysis involved Atomic Absorption Spectrophotometry (PerkinElmer model 2280) to determine concentrations of calcium, magnesium, sodium, potassium and silicon. Ion Chromatography (Dionex 2110i interfaced to a Spectra-Physics SP4270 integrator), using 3 replicates of each sample, was used
CATENA An Interdisciplinary Journal of SOIL SCIENCE
HYDROLOGY~EOMORPHOLOGY
Inorganic Solute Content, Alpine Snowpatch
800
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155
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F i g . l : Discharge records at Martinelli, 1987.
400-
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Fig. l : Discharge records at Martinelli, 1987. b: Discharge, Conductance and Sediment Concentrations, June 10-12, 1987. The conductance and sediment concentration data are based on samples taken every 15 minutes with composites of four representing a one hour interval. Two missing values in sediment concentrations are shown by breaks in the line joining data points.
CATENA An Interdisciplinary Journal of SOIL S C I E N C E - - H Y D R O L O G Y ~ E O M O R P H O L O G Y
156
Caine 6,0-
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Fig. 1: Discharge records at Martinelli, 1987. c: Discharge, Conductance and Sediment Concentrations, June 17-21, 1987. The conductance and sediment concentration data are based on ssmples taken each hour with composites of six representing a six hour interval.
to determine sulfate, nitrate, chloride and fluoride concentrations. Bicarbonate concentrations were determined by titration to a pH 4.5 end point. In waters with low solute concentrations, like those treated here which have less than 100 /~eq 1-1 of cations, this is likely to overestimate alkalinity and may explain ion balances with a 10% excess of anions at times. Such an interpretation is empirically supported by the inverse correlation of bicarbonate alkalinity and the sample ion balance: r = -0.877 with N = 47 and r = -0.962 with N = 18 for the samples of this study. Suspended sediment concentrations and size distributions have been determined by laser particle sizer and counter
(Spectrex) with the output calibrated against results from filtration at 0.45 #m.
4
Water quality
Discharges from the Martinelli basin during both study periods show a strong diurnal cycle (fig.l). The 24 hr frequency explains 82% of the variance in hourly flows during the June 10-12 period, and 69% in the June 17-21 interval. These proportions are similar to those defined by Fourier tansformation of residual hourly discharges (after removal of the trend due to annual hydrograph recession) during six-day periods in the preceding five seasons at this site
CATENA An Interdisciplinary Journal of SOIL SCIENCE HYDROLOGY~EOMORPHOLOGY
Inorganic Solute Content, Alpine Snowpatch
Ca (mg/l) M g (mg/l) Na (mg/1) K (mg/l) HCO3 (rag/l) SO4 (mg/1) NO3 (mg/1) C1 (rag/l)
F (mg/l) Si (rag/l) Sediment (mg/l) Cond. (#S/cm) pH Water (I/s)
157
Reproducibility
June 10-12 ~ S
0.01 0.002 0.005 0.01 0.10 0.03 0.008 0.005 0.005 0.20 0.04 2% 0.10
0.89 0.10 0.35 0.14 4.15 0.67 0.73 0.05 0.01 0.86 0.13 7.61 6.45 8.01
June 17-21 ~ S
0.09 0.01 0.05 0.03 0.75 0.06 0.08 0.02 0.00 0.53 0.10 0.72 0.10 2.45
1.06 0.12 0.42 0.07 5.67 0.76 0.58 0.07 0.01 1.62 0.33 8.38 6.35 3.27
0.03 0.01 0.04 0.01 0.67 0.04 0.05 0.01 0.00 1.12 0.36 0.43 0.11 0.73
Notes: is the discharge-weighted mean. S is the discharge-weighted standard deviation. Reproducibility is the standard deviation of repeated analyses of the same sample. Specific conductance (Cond.) has been corrected to 25°C.
T a b . 1: Stream water quality at Martinelli Snowpatch, June 1987.
r Ca Mg Na K HCO3 SO4 NO3 C1 F Si Sed. Cond. pH
-0.497 -0.644 -0.715 (0.119) -0.412 -0.952 -0.947 (0.051) -0.548 (0.130) 0.387 -0.979 -0.664
June 10-12 (N = 47) a
b
r
1.286 0.152 0.700
-0.175 -0.227 -0.333
7.078 1.253 1.446
-0.261 -0.297 -0.325
-0.800 -0.808 -0.745 (-0.138) (-0.325) -0.842 (-0.403)
0.014
-0.180
0.009 14.537 6.676
1.126 -0.308 -0.026
June 17 21 (N = 18) a
b
1.232 0.163 0.614
-0.123 -0.255 -0.307
0.987
-0.208
0.009 11.087
2.465 -0.222
(-0.001)
(-0.381) (0.295) 0.549 -0.913 (-0.243)
Notes: In all cases except for that of pH, the model y = a - x b has been fitted with x = discharge (l/s), and y = concentration (mg/1), except for Conductance (/zS/cm). For pH, the linear model y = a + b - x has been fitted. Correlation coefficients (r) that are not significant (0.05 probability) are in parentheses. T a b . 2 : Discharge - water quality relationships.
CATENA An Interdisciplinary Journal of SOIL SCIENCE HYDROLOGY
GEOMORPHOLOGY
Caine
158 10,0-
MARTINELLI SNOWPATCH JUNE 1987
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Fig. 2: Discharge and Water Quality, June 1987. The regressions fitted to the Conductance, Calcium and Sulfate data for each period are summarized in tab.2.
(40% to 80% of the residual variance in the 24-hour frequency). The double peak in the daily hydrographs of the June 1721 period (fig.lc) is a response to separation of the main snowpatch into two segments, one upslope of the other, on June 14. This pattern, too, is repeated in other years at this site.
and is followed by sodium, magnesium and potassium in molar proportions of 10:7:2:1 (tab.l). Bicarbonate is the main anion (even if overestimation is conceded) followed by nitrate, sulfate and chloride in molar proportions of 44:4:3:1 (tab.l). Suspended sediment concentrations are low, averaging only 0.1 mg 1-l in the first period and 0.28 mg 1-1 in the Solute concentrations are low and sim- second. The general increase in sediment ilar to those found in weekly sampling concentration with the lower discharges during the 1982-1987 flow seasons. Cal 7 of the second period has been noted at cium is the dominant cation in solution CATENA--An
Interdisciplinary Journalof SOIL S C I E N C E - - H Y D R O L O G Y ~ E O M O R P H O L O G Y
Inorganic Solute Content, Alpine Snowpatch
159
10
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Fig. 3: Discharge and Sediment Concentration, June 1987. The arrows show the temporal sequence of samples in the two periods adn define a clockwise hysteresis, except in the June 18-19 cycle (solid arrows) which is counterclockwise. other times on this site and seems to reflect exposure of the unvegetated riparian zone and channel as the snowcover retreats. During both sampling periods, solute concentrations vary inversely with the water discharge (fig.lb,c). The shortterm variability in solute concentrations (as measured by the standard deviation in tab.l) is relatively high: up to 70% of the standard deviation estimated from weekly samples over entire flow seasons. The inverse relationship between concentration and discharge shows a diluting effect due to increased meltwater flows during the afternoon and evening hours (tab.2). This is best defined for specific conductance but is also evident in the response of individual ions (fig.2). Values of the exponent (b) in the solute con-
CATENA
An Interdisciplinary Journal of SOIL SCIENCE
centration : discharge relationship are variable but are all appreciable less than 1.0 (tab.2). This shows that, despite dilution, water discharge will dominate in estimates of solute yield, the highest yields being associated with the highest flows ( G U N N E R S O N 1967). Within each study period, sediment concentrations increase with the flow volume (tab.2). Peak concentrations tend to lead the highest flows by about one hour (fig.l) which gives a clockwise hysteresis to the concentration - discharge relation (fig.3) and reduces the simple correlation between flow and sediment concentration (tab.2). An exception to this occurs in the June 18-19 part of the record which loops in a counter-clockwise direction (fig.3). This follows a time of relatively high sediment concentration and
HYDROLOGY~;EOMORPHOLOGY
160
Caine
may indicate depletion of stored sediment in the channel by that event.
5
Discussion
This empirical record shows that flow volumes and solute and sediment concentrations are closely related in a nival hydrologic regime. It suggests that simple mixing models, such as those of COLLINS (1983) and S O U C H E Z & L E M M E N S (1987) might be used to identify discharge sources in the Matrinelli catchment. For that purpose, specific conductance seems to be the best water quality charactgeristic for it represents an integration of all ionic solutes and is relatively simple to measure on a continuous basis ( F E N N 1987). Empirically, conductance is the water quality characteristic most closely correlated with stream discharge (tab.2). However, changes in the discharge - concentration relationships between the two periods in June 1987 introduce a further complication to their use in hydrograph separation. In 10 days, ionic concentrations measured at equivalent discharges are reduced by between 10% and 20% (fig.2, tab.2), part of a trend to reduced concentrations as the melt-season progresses. This is a late stage in snowpack "flushing" reported by other workers (e.g. J O H A N N E S S E N & H E N R I K S E N 1978, B R I M B L E C O M B E et al. 1985, S C H O N D O R F & H E R R M A N N 1987). These data do not permit clear identification of the fractionation effects found in earlier studies because more than 40% of the annual flow had been released from the catchment by June 10, 1987. Thus, the high, and highly differentiated, concentrations of the early flows were not defined. Nevertheless, the seasonal trend would require continuous adjustment of CATENA
a meltwater conductance used as input to a mixing model. The magnitude of bias in mass estimates of material removed from the basin can also be evaluated by these data. In this case, at and soon after the highest flows of the season, the bias in yield due to a midday sampling is relatively slight: an overestimate of less than 5% for most solutes (tab.3). It is greatest (up to 20%) for those solute species most poorly correlated to discharge and so is unlikely to be removed by matching sampling times to those of mean daily discharge. Later in the season, the diurnal peak discharge occurs earlier and a midday sampling time approaches that of peak flow. This will tend to correct the bias shown in tab.3 or convert it into a slight underestimate. When integrated across the entire flow season, errors due to sampling between 12:00 hr and 16:00 hr are probably negligible in this basin. The relatively constant concentration of chloride in these records conforms to the pattern found in longer-term records. If the first 10-15 days of streamflow are ignored (i.e. a time when snowpack flushing gives chloride concentrations as high as 0.5 mg/1-1 on ocasions), they average only 0.08 mg/1-1 (with S.D = 0.03) over 4 seasons (52 samples). The poor correlation with discharge could introduce error to estimates of chloride yield. Tab.3 suggests that one-point sampling may overestimate it by more than 10% during high flows. If these estimates are then used to "correct" the estimated yields of other solutes (e.g. CLAASSEN et al. 1986), the error will be transferred. This suggests that corrections based on the conservative behavior of the chloride ion not be applied automatically to nival streams.
An Interdisciplinary Journal of SOIL SCIENCE
HYDROLOGY
GEOMORPHOLOGY
Inorganic Solute Content, Alpine Snowpatch
Actual* Ca Mg Na K HCO3 SO4 NO3 C1 F Si Sed. Cond. pH
June 10-12 Est.'*
600 64 235 91 2788 452 492 33 7 575 90 5113 4336
595 64 259 97 3024 460 479 37 7 662 118 5154 4368
161
Diff.
Actual*
-0.9% 0.0% 10.0% 6.6% 8.5% 1.8% -2.6% 12.2% 5.0% 15.1% 30.6% 0.8% 0.7%
299 33 118 21 1602 215 163 21 3 458 92 2369 1794
June 17-21 Est.*" 291 32 119 23 1617 210 151 21 3 554 162 2269 1777
Diff. -2.5% -3.4% 0.5% 9.6% 1.0% -2.1% -7.3% 4,1% 10.9% 20.9% 75,4% -4,2% -1,0%
Notes: * Actual yields (g/day) evaluated from the continuous flow record and concentrations in water samples taken at 1 or 6 hour intervals. ** Estimate yields (d/day) derived from the continuous flow record and concentrations in the 12:00 hr water sample for each day. Solute and sediment yields are in g/day. The values for specific conductance are m 3./zS/cm.day and for pH are units.m3/day.
Tab. 3' Bias estimates.
6
Conclusion
The empirical data summarized here clearly verify the suggestion that water qality characteristics in snowmelt streams respond through a dilution mechanism to changes in flow volume. However, the resulting relationship between solute concentration and discharge is not constant through time but changes along the falling limb of the seasonal hydrograph. On such a longer temporal scale, the relationship is reversed, lower concentrations associated with the lower, later discharges. This will complicate, though not prevent, modeling of the flows as a mixture of water from two or more sources because it suggests that the geochemical characteristics of at least one of them are not constant in the long term. The periodic variation in water quality also introduces a possible CATENA An Interdisciplinary Journal of SOIL SCIENCE
HYDROLOGY
bias to many estimates of material yield thoug, in the case evaluated here, the effect is not great. It is most serious with respect to the chloride ion and its use in calibrating basin budgets.
Acknowledgements Work in the Martinelli Catchment has been supported by the National Science Foundation as part of the LongTerm Ecological Research program under Grants BSR 8514329 and BSR 8745637. I thank J.M. Caine for chemical analyses and M.I. Litaor and two anonymous reviewers whose critical comments have much improved this paper.
References BRIMBLECOMBE, P., TRANTER, M., ABRAHAMS, P.W., BLACKWOOD, L, DAVIES, T.D. & VINCENT, C.E. (1985): Relocation and
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preferential elution of acidic solutes through the snowpack of a small, high-altitude Scottish catchment. Annals of Glaciology, v. 7, 141-147. BROWN, M.J. (1986): Use of stream chemistry to estimate hydrologic parameters. Water Resources Research, v. 22, 805-811. CAINE, N. (1986): Sediment movement and storage on alpine slopes in the Colorado Rocky Mountains. In: Hillslope Processes. (Ed. A.D. Abrahams), London, Allen and Unwin, 115137. CAINE, N. & SWANSON, F.J. (1988): Geomorphic coupling of hillslope and channel systems in two small mountain basins. Zeitschrift fiir Geomorphologie, v. 33 (in press). CLAASSEN, H.C., REDDY, M.M. & HALM, D.R. (1986): Use of the chloride ion in determining hydrologic-basin water budgets - - a 3-year case study in the San Juan Mountains, Colorado, U.S.A. Journal of Hydrology, v. 85, 49-71. CLARK, M.J. (1988): Periglacial hydrology. In: Advances in Periglaical Geomorphology. (Ed. M.J. Clark), Chichester, J. Wiley and Sons, 415462. COLLINS, D.N. (1983): Solute yields from a glacierized high mountain basin. In: Dissolved Load of Rivers and Surface Water: Quantity / Quality Relationships. International Association Scientific Hydrology Publication No. 141, 41-49.
MARTINELLI, M.B. Jr. (1959): Some hydrologic aspects of alpine snowfields under summer conditions. Journal of Geophysical Research, v. 64, 451-455. OSBURN, W.S., Jr. (1963): The dynamics of fallout distribution in a Colorado alpine tundra snow accumulation ecosystem. In: Radiobiology: Proceedings of the First National Symposium on Radioecology. (Eds. V. Sehulz & A.W. Klement, Jr.), 51-71. SCHONDORF, T. & HERRMANN, R. (1987): Transport and chemodynamics of organic micropollutants and ions during snowmelt. Nordic Hydrology, v. 18, 259-278. SOUCHEZ, R.A. & LEMMENS, M.M. (1987): Solutes. In: Glacio-Fluvial Sediment Transfer. (Eds. A.M. Gurnell & M.J. Clark), Chichester, John Wiley & Sons, 285-303. SWANSON, F.J. & FRANKLIN, J.F. (1988): The long-term ecological research program. Eos, v. 69, p. 34, 36, 46. THORN, C.E. (1975): Influence of late-lying snow on rock-weathering rinds. Arctic and Alpine Research, v. 7, 373-378. THORN, C.E. (1976): Quantitative evaluation of nivation in the Colorado Front Range. Geological Society of America Bulletin, v. 87, 11691178. ZEMAN, L.J. & SLAYMAKER, H.O. (1975): Hydrochemical analysis to discriminate variable runoff source areas in an alpine basin. Arctic and Alpine Research, v. 7, 341-351.
COLLINS, D.N. & YOUNG, G.J. (1981): Meltwater hydrology and hydrochemistry in snow and ice-covered mountain catchments. Nordic Hydrology, v. 12, 319-334. FENN, C.R. (1987): Electrical conductivity. In: Glacio-fluvial sediment transfer: an alpine persoective. (Ed. A.M. Gurnell & M.J. Clark), Chichester, J. Wiley and Sons, 377-420. GUNNERSON, C.G. (1967): Streamflow and quality in the Columbia River basin. Proceedings ASCE, Journal of Sanitary Engineering Division, v. 93, No. SA6, 1-16. JOHANNESSEN, M. & HENRIKSEN, P. (1978): Chemistry of snow meltwater - - changes during melt. Water Resources Research, v. 14, 615-619. MADOLE, R.F. (1982): Possible origins of tilllike deposits near the summit of the Front Range in north-central Colorado. U.S. Geological Survey Professional Paper 1243, 31 pp.
Address of author: Nel Caine
Geographisches Institut Universit/it Heidelberg Im Neuenheimer Feld 348 D-6900 Heidelberg 1 West Germany
C A T E N ~ A n Interdisciplinary Journal of SOIL S C I E N C E - - H Y D R O L O G Y ~ E O M O R P H O L O G Y