Effect of snowpack removal on energy balance, melt and runoff in a small supraglacial catchment

Modelling melt and runoff from snow- and ice-covered catchments is important for water resource and hazard management and for the scientific study of glacier hydrology, dynamics and hydrochemistry. In this paper, a distributed, physically based model is used to determine the effects of the up-glacier retreat of the snowline on spatial and temporal patterns of melt and water routing across a small (0.11 km(2)) supraglacial catchment on Haut Glacier d'Arolla, Switzerland. The melt model uses energy-balance theory and accounts for the effects of slope angle, slope aspect and shading on the net radiation fluxes, and the effects of atmospheric stability on the turbulent fluxes. The water routing model uses simplified snow and open-channel hydrology theory and accounts for the delaying effects of vertical and horizontal water flow through snow and across ice. The performance of the melt model is tested against hourly measurements of ablation in the catchment. Calculated and measured ablation rates show a high correlation (r(2) = 0.74) but some minor systematic discrepancies in the short term (hours). These probably result from the freezing of surface water at night, the melting of the frozen layer in the morning, and subsurface melting during the afternoon. The performance of the coupled melt/routing model is tested against hourly discharge variations measured in the supraglacial stream at the catchment outlet. Calculated and measured runoff variations show a high correlation (r(2) = 0.62). Five periods of anomalously high measured discharge that were not predicted by the model were associated with moulin overflow events. The radiation and turbulent fluxes contribute c. 86% and c. 14% of the total melt energy respectively. These proportions do not change significantly as the surface turns from snow to ice, because increases in the outgoing shortwave radiation flux (owing to lower albedo) happen to be accompanied by decreases in the incoming shortwave radiation flux (owing to lower solar incidence angles) and increases in the turbulent fluxes (owing to higher air temperatures and vapour pressures). Model sensitivity experiments reveal that the net effect of snow pack removal is to increase daily mean discharges by c. 50%, increase daily maximum discharges by >300%, decrease daily minimum discharges by c. 100%, increase daily discharge amplitudes by >1000%, and decrease the lag between peak melt rates and peak discharges from c. 3 h to c. 50 min. These changes have important implications for the development of subglacial drainage systems. Copyright (C) 2002 John Wiley Sons, Ltd.