Seasonal reorganization of subglacial drainage inferred from measurements in boreholes

Abstract

Abstract:The eect of the formation of a major subglacial drainage channel on the behaviour of the subglacial drainage system of Haut Glacier d'Arolla, Switzerland, was investigated using measurements of borehole water level and the electrical conductivity and turbidity of basal meltwaters. Electrical conductivity pro®les were also measured within borehole water columns to identify the water sources driving water level changes, and to determine patterns of water circulation in boreholes. Prior to channel formation, boreholes showed idiosyncratic and poorly coordinated behaviour. Diurnal water level¯uctuations were small and driven by supraglacial/englacial water inputs, even when boreholes were connected to a subglacial drainage system. This system appeared to consist of hydraulically impermeable patches interspersed with storage spaces, and transmitted a very low water ux. Drainage reorganization, which occurred around 31 July, 1993, in response to rapidly rising meltwater and rainfall inputs, seems to have involved the creation of a connection between an incipient channel and a wellestablished channelized system located further down-glacier. Once a major channel existed within the area of the borehole array, borehole water level¯uctuations were forced by discharge-related changes in channel water pressure, although a diversity of responses was observed. These included (i) synchronous, (ii) damped and lagged, (iii) inverse, and (iv) alternating inverse/lagged responses. Synchronous responses occurred in boreholes connected directly to the channel, while damped and lagged responses occurred in boreholes connected to it by a more resistive drainage system. Pressure variations within the channel resulted in diurnal transfer of mechanical support for the ice overburden between connected and unconnected areas of the bed, producing inverse and alternating patterns of water level response. #

Oscillatory subglacial drainage in the absence of surface melt

“…Similar behaviour, with nearby boreholes displaying dissimilar but diurnally varying pressures, has been recorded at a number of other glaciers (e.g. Gordon et al, 1998;Fudge et al, 2005Fudge et al, , 2008.…”

A double continuum hydrological model for glacier applications

“…As for the end-ofwinter configuration, the IDS water head is more sensitive to the leakage length scale than is the EPL length. Gordon et al (1998) 70 m at the opening of the efficient drainage system, in good agreement with the modelled drop for leakage length scale ϕ ≥ 10 m. Figure 10 compares the modelled EPL extent for ϕ = 10 and ϕ = 20 m to the reconstructed channel system at the end of the 1990 melt season (Sharp et al, 1993). This comparison shows a good agreement between the extent of the modelled active EPL and the channel system observed at the end of summer for both values of ϕ.…”

Recent Advances in Our Understanding of the Role of Meltwater in the Greenland Ice Sheet System

“…Their results suggest that the moulins directly fed an efficient channelised component of the drainage system while the boreholes, by contrast, monitored a hydraulically inefficient drainage system. This juxtaposition of efficient subglacial channels draining waters between hydraulically inefficient areas of the bed mimics conditions observed in alpine glacial systems [59,68]. A contrary conclusion was reached by a study which found minimal subglacial water pressure variations in a borehole 34 km from the ice margin, under ice~830 m thick [69], which the authors interpreted as indicative of a lack of channelised drainage extending inland beneath deep ice.…”