Temperature variations of runoff from highly-glacierised alpine basins
, MSc by research thesis, University of Salford.
The temperature of meltwater emerging from the portal of an alpine glacier is usually around 1°C, irrespective of discharge, but warming with distance downstream is influenced by both flow as well as the availability of heat energy. Warmer conditions lead to greater energy availability but also to enhanced icemelt and hence larger quantities of water in proglacial streams. The greater the discharge, the higher the velocity at which meltwater flows and hence the shorter the time available for water to be heated over a fixed distance from the portal. At the diurnal timescale, water temperature rises in the morning before reducing as the amount of water flowing increases in early afternoon. Initially, increasing energy availability is adequate to raise water temperature but fails to keep pace with the rising volume of water in the channel. As a consequence, water temperature declines with continuing increase in discharge. At the seasonal timescale, ice-melt contributions to runoff suppress water temperatures in summer after a spring maximum, giving a distinctive seasonal temperature regime in rivers draining from highly glacierised basins. Highest water temperatures occur at relatively low flows in April and May and then subsequently as icemelt discharge increases with higher air temperatures and rising transient snow line, water temperatures decrease, mean monthly water temperatures being maintained at about the same level between July and September. A paradox appears therefore in that when solar radiation and air temperatures are high, stream water temperatures are often reduced. On warm days, the temperature of the tongue of cool water extending downstream from the glacier portal can be lower and the length of the cool plume longer. Records of water temperature and discharge from rivers draining from two large Alpine valley glaciers in Wallis, Switzerland have been examined. Data for the Massa, draining from Grosser Aletschgletscher throughout the period 2003-2011 have been analysed, and similar hourly measurements were undertaken in summer months on the Gornera, draining from Gornergletscher, both series being obtained close to the glacier termini. As air temperatures continue to warm, this paradox suggests that water temperatures in glacier-fed streams will cool before warming as the deglaciation discharge dividend first increases runoff before subsequent decline. This evolutionary pattern of flow and temperature development can be approached using modelling. Different approaches to modelling are analysed and recommendations made to develop a simple temperature index runoff model, coupled with a deterministic model to predict water temperature.
Actions (login required)
||Edit record (repository staff only)