Subglacial hydrology plays a key role in many glaciological processes, including ice dynamics via the modulation of basal sliding. Owing to the lack of an overarching theory, however, a variety of model approximations exist to represent the subglacial drainage system. The Subglacial Hydrology Model Intercomparison Project (SHMIP) provides a set of synthetic experiments to compare existing and future models. We present the results from 13 participating models with a focus on effective pressure and discharge. For many applications (e.g. steady states and annual variations, low input scenarios) a simple model, such as an inefficient-system-only model, a flowline or lumped model, or a porous-layer model provides results comparable to those of more complex models. However, when studying short term (e.g. diurnal) variations of the water pressure, the use of a two-dimensional model incorporating physical representations of both efficient and inefficient drainage systems yields results that are significantly different from those of simpler models and should be preferentially applied. The results also emphasise the role of water storage in the response of water pressure to transient recharge. Finally, we find that the localisation of moulins has a limited impact except in regions of sparse moulin density.

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Assessing the potential for black carbon (BC) and dust deposition to reduce albedo and accelerate glacier melt is of interest in Washington because snow and glacier melt are an important source of water resources, and glaciers are retreating. In August 2012 on Snow Dome, Mount Olympus, Washington, we measured snow surface spectral albedo and collected surface snow samples and a 7 m ice core. The snow and ice samples were analyzed for iron (Fe, used as a dust proxy) via inductively coupled plasma sector field mass spectrometry, total impurity content gravimetrically, BC using a single-particle soot photometer (SP2), and charcoal through microscopy. In the 2012 summer surface snow, BC (54 ± 50 μg/L), Fe (367±236 μg/L) and gravimetric impurity (35 ± 18 mg/L) concentrations were spatially variable, and measured broadband albedo varied between 0.67-0.74. BC and dust concentrations in the ice core 2011 summer horizon were a magnitude higher (BC = 3120 μg/L, Fe = 22000 μg/L, and gravimetric impurity = 1870 mg/L), corresponding to a modeled broadband albedo of 0.45 based on the measured BC and gravimetric impurity concentrations. The Big Hump forest fire is the likely source for the higher concentrations. Modeling constrained by measurements indicates that the all-sky 12 h daily mean radiative forcings in summer 2012 and 2011 range between 37-53 W m À2 and 112-149 W m À2 , respectively, with the greater forcings in 2011 corresponding to a 29-38 mm/d enhancement in snowmelt. The timing of the forest fire impurity deposition is coincident with an increase in observed discharge in the Hoh River, highlighting the potential for BC and dust deposition on glaciers from forest fires to accelerate melt.

In the Swiss Alps, climatic changes have not only caused glacier retreat, but also likely increased sedimentation downstream of glaciers. This material either originates from below the glacier or from periglacial environments, which are exposed as glaciers retreat, and often consist of easily erodible sediment. Griesgletscher's catchment in the Swiss Alps was examined to quantify erosion in the proglacial area, possible hydrological drivers and contributions of the sub-and periglacial sources. Digital elevation models, created from annual aerial photographs, were subtracted to determine annual volume changes in the proglacial area from 1986 to 2014. These data show a strong increase in proglacial erosion in the decade prior to 2012, coincident with increasing proglacial area size. However, examination of the gradient between discharge and sediment evacuation, and modeled sediment transport, could suggest that the proglacial area began to stabilize and sediment supply is limited. The large influx of sediment into the proglacial reservoir, which is roughly 2.5 times greater than the amount of sediment eroded from the proglacial area, demonstrates the importance of subglacial erosion to the catchment's sediment budget. Although far more sediment originates subglacially, erosion rates in the proglacial area are over 50 times greater than the rest of the catchment. In turn, both sub-and periglacial processes, in addition to constraining sediment supply, must be considered for assessing future sediment dynamics as glacier area shrinks and proglacial areas grow.

Sediment discharge from glaciers impacts downstream aquatic habitats, hydropower operations, and river infrastructure. Since discharge of subglacial sediment will evolve in response to glacier retreat, estimating future subglacial sediment dynamics is of great relevance. To develop tools and methods to better constrain the responsible processes, we present a till dynamics model that accounts for limited sediment access coupled to a subglacial hydrology model to describe the evolution of a subglacial till layer over a glacier's longitudinal profile in one dimension. Synthetic simulations examining the effects of changing hydrology highlight the importance of properly constraining both the erosion of underlying bedrock and the subglacial sediment connectivity. This is because changes in hydrology alter the timing of peak sediment discharge but only marginally affect total sediment discharge once the subglacial sediment reservoir is exhausted. Model simulations for real‐world glaciers yield insights into the distribution of sediment along the glacier bed, including locations where sediments are deposited or exhausted. Comparison between model results and field data shows that total and peak sediment discharge, as well as interannual variability, can be captured with acceptable skill for periods ranging from hours to decades. The results from this model show that modeling subglacial sediment transport on decadal to subdaily scales is possible but requires processes such as bedrock erosion and sediment connectivity to be considered.