Modeling the diurnal variation of tracer transit velocity through a subglacial channel

[1] Tracer injections into a moulin connecting to a subglacial channel at Unteraargletscher were repeated at intervals of about two hours over two diurnal discharge cycles in August and September 2000. The pronounced hysteresis in the relationships between tracer transit velocity and proglacial discharge may be explained by adjustments of the channel cross section in response to discharge variations or alternatively by modulation of inflow from the tributary moulin into a main subglacial channel. To test these hypotheses, we employ a physically based, time-dependent model of channelized subglacial drainage. In a series of numerical experiments, we explore the general characteristics of each mechanism before the model is applied to diurnal discharge variations measured during the tracer tests at Unteraargletscher. We found that the ability of a Röthlisberger channel to adjust its size to the prevailing hydraulic conditions can contribute to the hysteresis in the velocity-discharge relationships. However, using plausible parameter values, R channel hydraulics alone is not satisfactory in reproducing the magnitude of observed transit velocities. The model further confirms that retardation of the tracer in the tributary moulin due to inflow modulation has major effects on transit velocities both in magnitude and timing. With appropriate assumptions on channel sinuosity and moulin size, a combination of both mechanisms can be found that is capable of reproducing the observed transit velocities.