| Supraglacial debris covers have the potential to evaporate large quantities of water derived
from either sub-debris ice melt or precipitation. Currently, knowledge of evaporation and
condensation rates in supraglacial debris is limited due to the difficulty of making direct
measurements. This paper presents eddy covariance and lysimeter measurements of moisture
fluxes made over a 0.2 m debris layer at Miage debris covered glacier, Italian Alps, during the
2013 ablation season. The meteorological data are complimented by reflectometer
measurements of volumetric water fraction in the saturated and vadose zones of the debris
layer. The lysimeters were designed specifically to mimic the debris cover and were
embedded within the debris matrix, level with the surface. Over the ablation season, the latent
heat flux is dominated by evaporation, and the flux magnitude closely follows the
daily cycle of daytime solar heating and night time radiative cooling of debris.
Mean flux values are of the order of 1 kg m-2 day-1, but often higher for short
periods following rainfall. Condensation rates are relatively small and restricted
to night time and humid conditions when the debris-atmosphere vapour pressure
gradient reverses due to relatively warm air overlying cold debris. The reflectometer
measurements provide evidence of vertical water movement through capillary rise in the
upper part of the fine-grained debris layer, just above the saturated horizon, and
demonstrate how debris bulk water content increases after rainfall. The latent heat flux
responds directly to changes in wind speed, indicating that atmospheric turbulence
can penetrate porous upper debris layers to the saturated horizon. Hence, vertical
sorting of debris sediments and antecedent rainfall are important in determining
evaporation rates, in addition to current meteorological conditions. Comparison
of lysimeter measurements with rainfall data provides an estimate that between
45% and 89% of rainfall is evaporated directly back to the atmosphere. Rainfall
evaporation rates increase with debris impermeability and temperature, with highest
rates occurring when a shower falls on hot debris. If these point measurements
are representative of larger scales, evaporation rates of the order of 1000 tonnes
km-2 day-1 are implied, with higher rates following rainfall. This has important
implications for downstream runoff, sub-debris ice melt rates (due to consumption of
evaporative latent heat energy) and, possibly, convective atmospheric processes. |