Subglacial and subaerial fluvial sediment transport capacity respond differently to water discharge variations

Abstract. Sediment transport capacity in both subaerial and subglacial channels depends on the shear stress exerted across the channel bottom, which varies with water velocity and channel width. In subaerial channels, water discharge variations are accommodated by flow depth and width changes, along with water velocity. However, in subglacial channels, water is pressurized by the ice above, and they grow in response to frictional heating of water flowing through them. As a result, water discharge changes mainly result in velocity variations, as the channel geometry evolves slowly (over days). Here, we present formulations of sediment transport capacity in different channel types and apply subglacial and subaerial hydraulics models to hydrographs from an Alpine glacier and the Greenland Ice. Numerical experiments show that the changing channel size results in sediment transport capacity peaking before the maximum water discharge. This hysteresis in channel size causes a highly variable relationship between sediment and water discharge in a transport-limited subglacial system. The results also indicate that high subglacial sediment transport capacities can occur across a wide range of water discharges. A second set of numerical experiments shows that subglacial sediment transport is highly non-linear with respect to water discharge, creating more variability in sediment transport capacity. Yet, results and formulations of subglacial sediment transport capacity show that its variability can approach that of subaerial systems when subglacial channel size is in equilibrium with water discharge. The implications of these findings are discussed in the context of sediment discharge from glaciers with different hydro-climatic forcings. We also discuss the impact of different assumptions of channel behavior on sediment transport capacity. These findings can improve the interpretation of sediment discharge records in glacierized catchments.