MODELING SNOWMELT IN A MOUNTAINOUS RIVER BASIN ON AN EVENT BASIS

A snowmelt model for short time flood forecasting of mixed rain-snowmelt floods in a high alpine watershed has been developed. The model is based on a subdivision of the basin into elevation bands. The energy input into the snowpack is computed by the energy-balance approach, using physically based preset parameters. The internal processes are parameterized by introducing heat and water storage capacities. The state of the snow cover throughout the basin is charactacterized by distinguishing three zones with different melt and drainage conditions. The lowest zone is saturated and runoff-producing. Above it, there is a transition zone of partly soaked snow. In the uppermost portion of the basin, no liquid water is stored in the snow. Snow line, saturation line and dry-snow line form the boundaries between the respective zones. Time variations of snow cover conditions are described by the altitudinal fluctuations of the lines. In performing simulation runs, the three boundary lines are found to follow different patterns during a six-day test period. Due to the prevailing melt conditions, the snow line rises monotonically and is only slightly influenced by different weather conditions. The saturation line and consequently the band width of the soaked zone, however, are controlled by day-to-day and diurnal changes in meteorological variables and exhibit a significant increase on rainy days and pronounced fluctuations during fair weather. The dry-snow line shows minor fluctuations. A sensitivity analysis indicates that the influence of model parameters on simulated melt rates is moderate or small when simulation periods of several days are considered so that parameters may be pre-set without inducing much additional uncertainty. The influence of model parameters on simulated melt and dynamics of basin snow cover conditions is discussed. The snowmelt routine was developed with the intention of starting it during the ablation period. Thus, initial conditions for the above mentioned boundary lines are required. Based on sensitivity analyses, it is found that the elevation of the snowline must be derived from current observations. For saturation line and dry-snow line, simple relations to air temperature are given. Simulation results indicate that the areal extent of the saturated snow cover must be considered, if proper model performance for the first hours after model start is desired. © 1990.