Short term variations of tracer transit speed on Alpine glaciers
Abstract. We present and interpret the results of a series of tracer experiments conducted on an Alpine glacier over a diurnal discharge cycle. For these injections, a moulin was used into which an ice marginal lake was draining, providing a relatively constant discharge. Measured tracer transit speeds show two diurnal maxima and minima. These findings are qualitatively different from existing observations from two series of injections at another site using a moulin fed by supraglacial meltwater with a high diurnal variability, which displayed one diurnal maximum and minimum. We use a simple two-component model of the glacier drainage system, comprising a moulin and a channel element, to simulate the measured transit speeds for all three injection series. The model successfully reproduces all the observations and shows that the same underlying processes can produce the qualitatively different behaviour depending on the different moulin input discharge regimes. Using the model, we asses the relative importance of the different measurement parameters, show that frequent measurements of moulin input discharge are indispensable and propose an experiment design to monitor the development of the drainage system over several weeks.