Hydrological response of snowpack under rain-on-snow events: a field study

Abstract The hydrological response of rain-on-snow events has been studied on a plot scale at 2640 m altitude in the Austrian Alps. Three artificial rain events with different intensities and durations were simulated over two snow plots on a natural snowpack and the behaviour of emerging outflow was examined. Measurements of meteorological parameters, soil temperature and snowpack properties were also made. The investigations show that the impeding characteristics of the ice layers more than doubled the storage capacity of the snowpack. The speed of water movement was estimated to be about 6 m h −1 when the snow pack was fully saturated. Accelerated metamorphism under saturated conditions and preferential flow paths created owing to uneven snow surface caused by the impact of intense rain over the snow surface, are understood to be responsible for the high speed of water flow. This indicates that heavy rain water moves several times faster than the natural snowmelt under non-rainy conditions. Moreover, under rainy conditions, natural snowmelt also percolates faster along with rain water. Observations of the time of arrival of runoff, t a and time to equilibrium concentration of liquid water in snow, t e , for different rain events indicate that after conditioning of the snowpack, a significant reduction of rain intensity (by half in the present study) is not able to change the distribution of runoff much; fast response of water was also observed under reduced rain-intensity. Another important aspect of the snowpack worth noting was that most of the input appeared as runoff. This stage of snowpack with high conductivity is found to be responsible for the production of high streamflows. Heavy rain either with snowmelt or alone can generate floods under such conditions. The importance of the role of rain water in conditioning the snowpack to yield maximum and fast runoff is due much more to input rather than to rain-induced snow melt.