Meltwater Temperature in Rivers Draining from Alpine Glaciers

Abstract:

Both air temperature and incoming solar radiation influence the seasonal pattern of snow- and ice-melt in glacierised Alpine basins, so that glacier-fed rivers have distinctive regimes with more than 90% of flow occurring in the months April through October. Snow melt increases discharge slowly in April and May, before the transient snow line starts to rise, exposing glacier ice to melt and leading to flow maxima in late July/early August. Meltwater temperature is inversely related to discharge as well as being positively influenced by energy and heat availability. Close to glacier termini, water temperatures reach maxima in spring, before decreasing as the volume of water being heated increases with rising discharge. Records of meltwater temperature and discharge for rivers draining basins with between 17 and 80% glacierisation in Kantons Bern and Wallis, Switzerland, have been examined, together with measurements of radiation and 2 m air temperatures at stations close to or in the catchment areas, at hourly resolution, within the period 2003-2013. The aims were to characterise seasonal and diurnal patterns of variation of meltwater temperature and to assess impacts of energy availability, discharge, distance downstream to measurement site, and percentage cover of basin with snow and ice on temperature. On a diurnal basis, water temperature increases before discharge rises, reducing as runoff reaches daily peak. Diurnal temperature ranges are greatest during times of relatively low flows in spring. On a seasonal scale, water temperature peaks in spring before the main discharge period. Temperatures remain in relatively limited ranges, and are suppressed during high flows in the main ablation season. Summer reduction in temperature is larger the more highly glacierised the basin and the closer to the glacier terminus. A simple radiation-forced model has been used to assess relationships between discharge, water surface area, flow velocity and length of time of water exposure to energy input