Diurnal variability of subglacial drainage conditions as revealed by tracer experiments

The morphology of the drainage system of Unteraargletscher, Switzerland, and the diurnal variability of drainage conditions were investigated by conducting a series of tracer tests over a number of discharge cycles during the ablation season 2000. Dye injections into a moulin were repeated at intervals of a few hours and were accompanied by simultaneous measurements of discharge of supraglacial meltwater draining into the moulin and bulk runoff in the proglacial stream. Fast transit velocities in conjunction with low dispersion values suggest that the tracer was routed through a hydraulically efficient, channelized drainage system. However, detailed analyses reveal a large diurnal variability in terms of transit velocity and dispersion coefficient. This finding underlines the difficulty of detecting a possible drainage system evolution based on single tracer tests conducted at coarse temporal intervals. Furthermore, the obtained velocity-discharge relationships display pronounced hysteresis. We suggest that the evolution of the cross-sectional area of an ice-walled conduit and the modulation of inflow at the junction of a tributary moulin to a main subglacial channel are responsible for the observed behavior.