Temporal switching between englacial and subglacial drainage pathways: Dye tracer evidence from the Haut Glacier d'Arolla, Switzerland

Thirty-four dye tracing experiments conducted from a moulin (m3Cf), located 1.5 km from the snout of the Haut Glacier d'Arolla, Switzerland, during the summers of 1989-1991 reveal two distinct drainage paths between the moulin and the glacier snout. In July, drainage appears to be predominantly via a hydraulically efficient channelised drainage system which results in dye breakthrough curves with velocities of >0.3 m s(-1) and dispersivities of <10 m. In August, drainage is via a more hydraulically inefficient distributed system which results either in velocities of <0.2 m s(-1) and dispersivities >10 m or in zero dye recovery. One injection conducted in mid July 1991 produced a double peaked breakthrough curve, indicating simultaneous drainage through both systems. The observed behaviour is opposite to that displayed by injections made at over 50 other moulins on the glacier, in which channelised flow replaces flow through a distributed system as the melt season progresses. It can be explained if moulin m3Cf intersects an englacial conduit which conveys water rapidly to a major subglacial channel in July when the distributed system is poorly developed and water pressures are high, but which is abandoned later in the year when increased glacier bed separation has expanded the drainage capacity of the distributed system. Dilution and retardation of dye within this enlarged distributed system can account for the frequency of zero recoveries from the August dye injections.