Configuration of the Drainage System of Midtdalsbreen, Norway, as Indicated by Dye-tracing Experiments

During the summers of 1987 and 1988, 15 dye-tracer tests from a total of eight injection points were conducted in the ablation area of Midtdalsbreen, a northern outlet of Hardangerj0kulen, southern Norway. The spatial and temporal patterns of water discharge, shapes of the dye-return curves, through-flow velocities, dye-recovery rates, dispersivities, and velocity/ discharge relationships suggest the existence of distinct catchments beneath the eastern and western halves of the glacier which are characterized by different types of drainage sytem. Experiments on the eastern side were associated with high melt-water discharges and produced short-lived and highly peaked dye-return curves, fast through-flow velocities, high dye-recovery rates , low dispersivity values which decreased through the melt season, and a velocity/ discharge relationship with an exponent of 1.0. Experiments on the western side were associa ted with low melt-water discharges and produced flat, extended dye-return curves which often displayed secondary peaks, slow through-flow velocities, low dye-recovery rates , high dispersivity values which increased during the melt season, and a velocity/ discharge relationship with an exponent of 0.6. Compariso n of observed through-flow velocities with values calculated theoretically using various hypothetical drainage-system structures suggests that water flows in a major sinuous conduit beneath the eastern half of the glacie r and in a system of linked cavities beneath the western half. A model for the seasonal evolution of the whole drainage network is postulated which has important implications for temporal VarIatIOns in subglacial water pressures and glacier-sliding velocity.