Quantitative Determination of the Subglacial Hydrology of Two Alpine Glaciers

Abstract

ABSTRACT. Two components of discharge through the internal hydrological systems of Alpine glaciers were separa ted on the basis of chemical composition of water. Some surface melt waters reta in low solute contents after fl owing without delay through conduits in whi ch no chemical enrichment occurs, whereas those flowing slowly at the glacier b ed h ave increased io ni c concentrations. A simple mixing m odel was used to investigate te mporal variations in the quantities ofw a le r routed through each of the two sub-sys tems. Electrical conductivity was taken as an indicator of melt-water composition and was monitored fo r p e riods during the summer ablation season of 1975 at Gornergletscher and of 1977 at Findelengletscher. At both g laciers, conduct ivity of melt waters varied diurna ll y inversely with discha rge flu ctuations, depending o n the proportion of tota l discharge routed through the two sub-syste ms. TOlal discharge and the flow component routed rapid ly through conduits within the glacier, a large proportion (50-80% ) of total discharge, exhibited in-phase rhythmi c diurnal hydrogra phs at the two glaciers. Distinctive subglacial hyd rologi cal reg imes are contrasted. At Findelengletschcr, the hydrographs of total discharge and of subglacial chemically enriched flow were in phase. At Gornergletscher, the subglacial hydrogra ph occurred with reverse asymm etry and out of phase. A possible interpretation is that water was tempo ra rily stored in basal cavities during high total discharge. During the night, stored water was released, contributing much of the total discharge at times of low fl ow. RESUME . D etermination quantitative de l'hydrologie sous-glaciaire de deux glaciers alpins. Les deux composantes du debit a trave rs le reseau hydrologique interne de glaciers alpins ont ete distinguees sur la base de la composition chimique des eaux. Quelques eaux de fusion d e surface n'ont qu'un e fa ible teneur e n matiere dissoute a pres avoir coule sa ns reta rd dans des cheneaux ou il n'y a pas d'enrichissement chimique, tandis que celles qui ont coule lentement le long du lit du glacier ont accru leur concentration en ions. Un. simple modele de melange a servi pour rechercher les va riations dans le temps des quantites d'eau empruntant l' un des deux sous-resea u x. La conductivite electrique fut choisie comme indi cateur de la composition de I'eau d e fusion et fut surveillee pour des p eriod es de la sa ison es tiva le d'a bla tion d e 1975 a u Gornergletscher et de 1977 a u Findelengle tscher. Sur les d eux glaciers, la conductivite des eaux de fusion variait journelle m e nt en raison inverse des debits selon la proportio n du debit total transitant da ns les deux sous-reseaux. Le debit tota l et la co mposa nte de l'ecoul ement empruntan t rapidement le reseau intra-glaciaire, une large part (50-80% ) du d e b it tota l, montre pour les deux glaciers des hydrogrammes quotidiens rythmes e n phase. L es regimes hydro logiqu es sous-glaciaires distincts sont opposes. Au Findelengletscher, les ...

Spatial and Temporal Variations in Electrical Conductivity in a Pro-Glacial Stream System

“…1200 the margins of a trapezium than by a straight line, such is the range of solute concentrations which may be associated with a given discharge (Collins, 1979). The Tsidjiore Nouve data, in contrast, display a relatively well-defined pattern, indicating a more conservative relationship between variations in stream-flow and electrical conductivity than that reported for the Gornera.…”

“…There were ablation-related, diurnal discharge cycles, particularly after 10 July, and approximately inverse diurnal cycles in electrical conductivity similar to those observed in other pro-glacial streams (Oerter and others, 1978;Collins, 1979) are also clearly discernible. Nevertheless, it is evident that the diurnal rhythm was mISSIng or heavily damped throughout most of June and early July as a consequence of a high residual winter snow-cover on the glacier and additional summer snow-fall events causing flow recession periods from 16 to 19 June , from 24 to 27 June .…”

“…Hourly instantaneous monitoring at the water intake Figure 2 shows the time series of hourly instantaneous discharge and electrical conductivity measured at the water intake from 7 June to 30 July elsewhere in the Alps during the summer ablation period (Behrens and others, 1971;Collins, 1979). There were ablation-related, diurnal discharge cycles, particularly after 10 July, and approximately inverse diurnal cycles in electrical conductivity similar to those observed in other pro-glacial streams (Oerter and others, 1978;Collins, 1979) are also clearly discernible.…”

“…Collins (1979) termed these "englacial" and "subglacial" flow contributions respectively, and formalized a simple two-component mIXmg model to explain temporal variations in the quantity and quality of bulk melt waters emerging from a glacier. Raiswell (1984) Previous studies of the electrical conductivity of pro-glacial streams have invariably concentrated on temporal variations observed at single sites.…”

“…Several researchers have noted that glacial basal ice and waters may be solute rich others, 1973, 1978;Vivian and Zumstein, 1973;Souchez and Lorrain, 1975) and that sllpraglacial melt wa ter has a low solute con centra tion (Collins, 1979). Collins (1979) and Oerter and others (1978) have also 108 spatio-temporelles. Les resultats montrent I'existence de cycles diurnes it la fois dans la conductivite electrique et le debit d<!…”