Simplifying properties of glacier runoff resulting from Ice-albedo feedback

The instantaneous rate of glacier melt depends strongly on the surface albedo. For example, snowfall increases surface albedo and reduces melt rate, while dark impurities deposited on the surface enhances melt. We discuss two interesting consequences of this feedback, which lead to simplifications in models describing the runoff of glacierised catchments. 

 

We investigated the interannual variability of modelled streamflow on two Himalayan catchments. On an excess-precipitation year, glaciers receives more snow, which reduces melt. In contrast, on a precipitation-deficient year, glaciers have less snowcover and they produce more meltwater. This behaviour makes the annual runoff contribution from the glaciers in any given catchment to be either weakly sensitive or insensitive to the interannual variability in precipitation. Further, this characteristic is shown to be independent of the climate setting of the glacier, or the models used. A general linear-response expression for the streamflow response to climatic perturbations is proposed, where the glacierised parts respond to temperature variability and the non-glacierised parts respond to precipitation variability. This simple expression reproduces several well known characteristics of the variability of the runoff of glacierised catchments, e.g., glacier-compensation effect, buffering effect, peak-water effect etc. 

 

The melt enhancing effects of dark supraglacial impurities lead to the formation of tiny cylindrical cryoconite holes that are commonly seen on glacier surfaces across the globe. Their contribution to glacier mass balance and runoff generation is debated. We build an idealised model of the evolution of these holes on sunny days, and show that the holes of a given diameter reach a steady depth, which scales linearly with the diameter. The predicted depth-diameter scaling is consistent with available global-scale observations. This scaling imply that the total melt contribution of the holes to glacier runoff is likely to be negligible, and that the formation of these holes provides an effective mechanism for restricting excess melt when supraglacial impurities are present.