Seasonal variations of deuterium concentration in runoff from a giacierized basin

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1 Hydrological Sciences -Journal - des Sciences Hydrologiques, 27,1, 3/1982 Seasonal variations of deuterium concentration in runoff from a giacierized basin W, AMBACH,..P, KIRCHLECHNER Institut fur Medizinische Physik, Universitat Innsbruck, Mullerstrasse 44, A-6020 Innsbruck, Austria H, WSER & W. STICHLER Institut fur Radiohydrometrie, Gesellschaft fur Strahlen- und Umweltforschung mbh, Munich, FR Germany ABSTRACT Seasonal variations of the deuterium content in Rofenache, Vent, Oetztal Alps, are discussed and compared for the years The basin covers an area of 98 km, 44% being glacierized. The onset and course of the seasonal variations are closely connected with the snowmelt in the lower region of the basin and with the ice melt in the ablation area which sets in later. Due to the meltwater from the winter snow pack in the lower part of the basin, minimum deuterium contents are observed in the runoff, whereas progressing ice melt in the ablation area causes the deuterium concentration in the runoff to rise. If the ice melt sets in late, due to the weather, this is well reflected in a delayed increase in the deuterium concentration in the runoff. Variations saisonnières de la concentration en deuterium dans 1'écoulement provenant d'un bassin couvert de glaciers RESUME Les variations saisonnières de la concentration en deuterium à Rofenache, Vent, Alpes d'oetztal sont discutées et comparées pour les années de la période La surface du bassin versant est de 98 km dont 44% sont constitués par des glaciers. Le début et le cours des variations saisonnières sont en étroite relation avec la fonte des neiges dans les parties basses du bassin et avec la fonte de la glace dans la zone d'ablation, phénomène qui commence plus tard. Par suite de la présence d'eau provenant de la fonte du stock neigeux dans les parties basses du bassin versant, on observe un minimum de concentration en deuterium dans les eaux d'écoulement, alors que la fonte progressive de la glace dans la zone d'ablation donne lieu à une augmentation de cette concentration. Si la fonte de la glace commence tard par suite des conditions météorologiques, ceci est bien reflété dans les variations de concentration en deuterium par une montée tardive de cette concentration dans l'écoulement. 29

2 30 W. Ambach ef al. INTRODUCTION This is a discussion on seasonal fluctuations of the deuterium concentration in water samples taken from Rofenache (Oetztal Alps) with a catchment area of 98 km 2, 44% being glacierized. The measurements cover the period from 1972 to The following components of the runoff influence the deuterium concentration in the runoff stream of a glacier (Ambach et al., 1976; Behrens et al., 1975): (a) Snow meltwater from the winter snow pack showing comparatively low deuterium concentrations. (b) Ice meltwater from the ablation area showing comparatively high deuterium concentrations. Snow meltwater from the accumulation area in the summer shows deuterium concentrations similar to those of ice meltwater. (c) Spring water with deuterium concentrations lower than those of ice meltwater. Because of the changing proportions of meltwater and spring water in the total runoff, the deuterium concentration in the runoff in midsummer shows daily fluctuations, caused by the weather (Ambach et al., 1978). RESULTS The deuterium concentrations of water samples from Rofenache for the years are given in Figs 1-3, part (c). As far as possible the data plotted in the graphs were measurements made on individual samples, otherwise monthly mean values were used. The seasonal variations of the deuterium concentration are characterized by a minimum during the snowmelt in spring. During the midsummer ablation period i.e., when ice meltwater comes from the ablation area, the deuterium concentration increases until a flat maximum is reached, and drops again gradually when the ablation period is over, until a value is reached which remains constant during the winter. In connection with the snowmelt, Figs 1-3 give the snow depths at Vent (part (a)) and the course of the ice ablation on Hintereisferner measured on representative ablation stakes (part (b) ). For a better understanding of the relation between ablation and deuterium concentrations in the runoff, Figs 1-3 each cover periods of two years. Figures 1 and 2 compare two years of highly different ice ablations and deuterium concentrations. Figure 3, however, shows two years largely displaying a similar course of ice ablation. Ice ablation during the summer ablation period is described by values obtained on three ablation stakes whose positions are given in Fig.4. Stake 75 was chosen in the lower part of the ablation area, thus characterizing well the onset of ice ablation in the glacierized part of the basin. Stake 21 lies in the middle, and stake 32 in the upper region of the ablation area (Figs 1-3, part (b)). Figure 1 shows that the snowmelt at Vent in the years 1973 and 1975 set in during the second half of April. The snow depths in winter, however, vary considerably. As snowmelt in both years set in nearly at the same time, the deuterium concentration in the

3 31 JAN FEB MAR APR MAY JUN JUL AUG SEP OCT NOV DEC Fig. 1-2r 6 'H (%o) h ROFENACHE VENT, JAN, FEB «MAR, APR, MAY, JUN, JUL, AUG, SEP, OCT. NOV, Fig. DEC 2 SNOW DEPTH 1m APR, MAY, JUN, JUL, AUG, SEP, OCT, NOV, DEC Fig. 3 Figs 1-3 (a) Snow depth at Vent (1890 m a.s.i.) and snow depth at ablation stakes 21, 32 and 75 on Hintereisferner. Map see Fig. 4. (b) Ice ablation on the same stakes. The plotted values are stake readings related to the surface at the onset of ice ablation. The ablation has not been calculated in water equivalents. (c) Deuterium concentrations in Rofenache, samples taken at Vent (1890 m a.s.u.

4 32 W. Ambachef al. HINTEREISFERNER Fig. 4 Map of the ablation stakes. The non-glacierized part is stippled. The glacier margin is shown by the thick line. glacier runoff in spring shows the same decrease. Differences are explained by weather influences. The deuterium concentration shows a minimum during the month of May, because it is then that the portion of meltwater from the snow accumulated during the low temperatures of midwinter is high. The course of snowmelt with time in the ablation area of Hintereisferner varies considerably in the two years. Ice ablation at the snout (stake 75) in the year 1975 set in a month later than in The time difference is even larger in the upper region of the ablation area at stake 32, where a time difference of nearly two months was observed. The late onset of ice ablation in 1975 causes an adequate time shift in the trend of deuterium concentrations in the glacier runoff. The increase in the deuterium concentrations for June, July and August is less distinct in 1975 than it is in 1973, reflecting the delayed ice ablation. At the end of the ablation season, i.e. at the end of September, the deuterium content shows a gradual decrease in 197 3, which is due to the decreasing portion of ice meltwater in the total runoff. The decrease in deuterium concentrations in fall and in early winter, after the ablation period, was evaluated in a former paper for calculating the mean residence time of meltwater by means of box models (Loschhorn et al., 1977). For the deuterium concentrations described in Fig.2, only monthly means are available. In the years 1976 and 1977, again a difference in deuterium concentrations can be observed under different conditions for ice ablation. Here too, late ablation of 1977 as compared to 1976 is reflected by lower deuterium concentrations in the glacier runoff during midsummer. The early melt of the winter snow cover at Vent in spring 1976, however, does not cause a significant change in the deuterium concentrations. As only monthly means are available, details caused by weather are not reflected. Figure 3 compares two years which display a similar course of ice ablation on Hintereisferner (1972 and 1974). For these two years the onset of ice ablation at each ablation stake occurred within a few days of each other. This analogous course of ice ablation is expressed in a comparable increase in deuterium concentrations in the runoff during the ablation period. The seasonal variations of deuterium concentrations in Rofenache under discussion confirm that deuterium concentrations show minimum values during melt of the winter snow cover in the lower region of the basin. This is plausible, since these snow layers had accumulated during midwinter at low temperatures, thus showing low

5 Seasonal variations of deuterium in runoff 33 deuterium concentrations. In all cases, the deuterium concentration in the runoff was found to increase after the onset of ice melt in the ablation area. This is due to higher deuterium concentrations in the ice meltwater. Simultaneously with ice ablation, snow ablation takes place in the higher parts of glacierized and non-glacierized regions of the basin. Comprehensive samples collected earlier on Hintereisferner have shown that the deuterium content of ice meltwater on average is -101 /oo, the deuterium content of snow meltwater of old snow in summer being -99 /»» (Ambach et al., 1976). In the case of snow meltwater, variation of the deuterium concentration is much wider than in ice meltwater. In midsummer, the difference between the deuterium concentrations of ice meltwater and snow meltwater from a glacierized catchment area is not significant. This is plausible, because the ice, melting in the ablation area, is actually firn displaced by glacier movement and densified by metamorphosis. The relative high deuterium concentration of these samples may be explained by isotopic fractionation during the transformation from snow to firn. Spring water on average shows a deuterium concentration of -116 / / displaying a distinct difference from the deuterium concentrations of the other runoff components (Ambach et al., 1976). CONCLUSIONS Measurements of deuterium concentration in the runoff from a glacierized basin together with measurements of the tritium concentration and electric conductivity may be used for determining the portions of snow meltwater, ice meltwater and spring water. In the Rofenache basin, a number of glacio-hydrological isotope studies have been made of this subject. An informative study by Behrens et al. (1979) presents an analysis of environmental isotopes and measurements of the electric conductivity. Combined analyses of tritium, deuterium and 0 concentrations in runoff samples for describing the percentage of runoff components have been made e.g. by Moser & Ambach (1977), Martinec (1972) and Dincer et al. (1974). For the Rofenache a numerical example was given by Behrens et al. (1975, Fig. 10) showing in midsummer a relative portion of groundwater of 27% at noon and of 64% at midnight in relation to the total discharge (100%). If only tritium measurements of runoff samples are available, the percentage of ice meltwater in the total runoff can be determined (Ambach et al., 1973). Measuring the electric conductivity makes it possible to distinguish the meltwater flowing on the surface of the glacier from the meltwater that flows on the glacier bed, being thus enriched with ions (Collins, 1979; Behrens et al., 1979). Analyses of environmental isotopes in connection with measurements of the electric conductivity can be used for constructing a discharge model for a glacierized basin. Such a model helps to estimate discharge portions and mean residence times. ACKNOWLEDGEMENTS Gratitude is expressed to the Osterreichische Akademie der Wissenschaften, Vienna, for their support, and to all voluntary hands for their assistance. Special gratitude is

6 34 W. Ambach et al. expressed to the members of the Institut fur Météorologie und Geophysik der Universitat Innsbruck for submission of the ablation data (Figs 1-3, parts (a) and (b)). REFERENCES Ambach, W., Eisner, H. & Url, M. (1973) Tritium activity variations in runoff from an Alpine glacier. In: Symposium on the Hydrology of Glaciers (Cambridge, UK, September 1969), pp IAHS Publ. no. 95. Ambach, W., Eisner, H., Elsasser, M., Loschhorn, U., Moser, H., Rauert, W. & Stichler, W. (1976) Deuterium, tritium and grossbeta-activity investigations on Alpine glaciers (Oetztal Alps). J. Glaciol. 17 (77), Ambach, W., Loschhorn, U., Moser, H., Rauert, W. & Stichler, W. (1978) Deuterium-Tritiumgehalte im Gletscherabfluss und Witterungsverlauf. 14. Tagung fur alpine Météorologie in Rauris, Sept Arbeiten aus der Zentralanstalt fur Météorologie und Geodynamik in Wien, Heft 31, 58/1-58/10. Behrens, H., Bergmann, H., Moser, H., Rauert, W., Stichler, W., Ambach, W., Eisner, H. & Pessel, K. (1975) Study of the discharge of Alpine glaciers by means of environmental isotopes and dye tracers. In: Snow and Ice (Proc. Moscow Symp., August 1971), pp IAHS Publ. no Behrens, H., Moser, H., Orter, H., Rauert, W., Stichler, W., Ambach, W. & Kirchlechner, P. (1979) Models for the runoff from a glaciated catchment area using measurements of environmental isotope contents. In: Isotope Hydrology 1978, vol. II, pp IAEA Publ. no Collins, D. N. (1979) Quantitative determination of the subglacial hydrology of two Alpine glaciers. J. Glaciol. 23 (89), Dincer, T., Martinec, J., Payne, B. R. & Yen, C. K. (1974) Variation of the tritium and oxygen-18 content in precipitation and snowpack in a representative basin in Czechoslovakia. In: Isotopes Techniques in Groundwater Hydrology, vol. 1, pp IAEA, Vienna. Loschhorn, U., Ambach, W., Moser, H. & Stichler, W. (1977) Modellmassige Bestimmung von hydrologischen Verweilzeiten in einem vergletscherten Einzugegebiet mit Hilfe von Messungen des Deuterium- und Tritlumgehaltes. Z. Gletscherk. Glazialgeol. XII (2), Martinec, J. (1972) Tritium und Sauerstoff-18 bei Abflussuntersuchungen in reprasentativen Einzugsgebieten. Gas, Wasser, Abwasser Nr. 6, 1-8. Moser, H. & Ambach, W. (1977) Glacial-hydrological investigations in the Oetztal Alps made between 1968 and Z. Gletscherk. Glazialgeol. XII (1/2), Received 9 June 1981; accepted 1 December 1981.