Development of an ice and snow melt model for long-term analysis of water resources from highly glacierised basins

In this thesis, glacier surface melt over an ablation season on Haut Glacier d'Aroila, an alpine glacier in the western Pennine Alps, Switzerland, is investigated with particular emphasis on its modelling. An enhanced temperature index (ETI) glacier melt model is presented, in which the classical dependency on temperature is extended by considering global radiation and albedo. The Standard approach has been recently improved by addition of a potential direct radiation term. Here, this approach is taken a step further by the inclusion of albedo, which controls the amount of global radiation available for conversion to melt energy at the snow or ice surface. An additive form of the temperature index model is proposed, aiming at clearly separatingthe two important contributions to melt energy, namely the longwave radiation and turbulentfluxes in one term and the shortwave radiation in the other. An extensive field campaign was conducted in the ablation season 2001 to collect the data necessary to the model development. Five automatic weather stations were located along two intersecting transects, both in the accumulation and ablation area, between May and September 2001. They provided a picture of the spatial variability of melt rates and of the meteorologicaland surface factors Controlling it both along the glacier centreline (variability related to elevation) and along a contour line (variabilitydue to topographiefactors other than elevation). One of the five sites, the central one, was designed to be the model development and calibration site. Ablation data for the model Validation were collected using both an UltrasonicDepth Gauge (UDG) at the central site and 26 ablation stakes distributed across the glacier. The set of ablation stakes provided a spatially distributed set of ablation data independent from that of the UDG at the central site, where continuoushourly ablation data were available. A physically-based energy balancemodel was used at the five sites on the glacier to compute hourlymelt rates to assess the Performance of the enhancedtemperature index model. Hourly melt rates simulated by the energy balance model were preferred to direct measurementsof ablation because the latter are affected by significant errors at hourly time resolution. In addition, energy balance simulations provide continuousablationdata that are available at any location wheregoodquality meteorological observations are recorded. Energy balance melt rates were validated against both the UDG readings and the ablation measurementsat a stake. It is found that the.agreement between the energy balance simulations and the measurementsis very good for the period in which reliable density measurementswere available. The three sites at the same elevation showed remarkably different patterns of modelled melt rate due to the effect of slope and aspect, thus revealing that elevation alone is an insufficient predictor of spatial melt variations. The new melt model was developed at the central site, where the empirical coefficients were calibrated. Themodel Performance was then evaluatedindependently by running the model at each of the remaining four sites and comparingpredictedhourly melt rates with melt rates calculated by the energy balance model. At each site, the model was also compared to previousversions of the temperature index method. At the point Scale, the availabilityof high quality input data allowed to evaluate the model Performance independentlyfrom errors introducedby parameterisatiori and extrapolationof the meteorological input data. The results show that the enhanced temperature index model is capable of aecounting for about 90 % of the surface melt rate Variation at all the stations tested. Particularly, including albedo enables the model to capture the major increases in the surface melt rate due to snow metamorphism and the transition from a snow to an ice surface in the ablation area. Additionally, the more physically-based melt process representationof the enhanced temperature index model reduces the over-sensitivity to temperature fluctuations in comparison to simplerformulationsof the temperature index approach. The