Seasonal, spatially distributed modelling of accumulation and melting of snow for computing runoff in a long-term, large-basin water balance model

In mountainous regions of mid latitudes. the accumulation and melting of snow plays an important role for the seasonal water balance. These processes not only exhibit a strong seasonality, but also a high spatial variability. which has to be accounted for when establishing distributed water balances in alpine environments. A methodology was developed for seasonal, spatially distributed modelling of accumulation and melting of snow and was embedded in a water balance model that uses only monthly values of precipitation and air temperature as meteorological input data. Hence. this methodology can also be applied in regions with limited data availability. The model uses a conceptual approach with a spatial resolution of a 1 km x 1 km raster. Snow accumulation is computed from temperature and precipitation data. Snowmelt is computed with a temperature-index approach. A direct application of these simple concepts using re, monthly inputs would not yield satisfying results. Therefore, precipitation is disaggregated into rainfall and snowfall by using a transition range considering temporal variations of temperature within a month and the mean deviation of temperature on days with and without precipitation. For modelling snowmelt, two different approaches were tested to incorporate variable temperatures within a month. The model was applied for the whole of Austria (84000 km(2)) and simulated runoff was compared with observed runoff at 135 gauges for a 30 year period. The model performs best in high and medium mountainous catchments. Lower model performances are achieved in lowland catchments. where the contribution of snowmelt to river runoff decreases. It can be concluded that modelling accumulation and melting of snow in mountainous areas with monthly data yields good results if a temporal disaggregation of precipitation and temperature is applied. Copyright (C) 2006 John Wiley & Sons, Ltd.