Rainfall on an existing snowpack is a fairly common occurrence in the spring. The snowpack is usually able to absorb some of the rain initially and thus to reduce the quantity of runoff at first. In case of prolonged and heavy rainfall, however, or if the snowpack is already thoroughly wet, as is customary in spring once rainfall sets in, additional melting can augment runoff.
Critical conditions can occur in the autumn as well, as in October 2011, when numerous roads and railway lines were flooded after heavy rainfall coincided with the melting of recently fallen snow. The flood of October 2011 clearly revealed gaps in scientific understanding of how heavy precipitation affects the snowpack, which impedes the ability of forecasters to foresee such events accurately. For this reason SLF researchers have undertaken a study examining more than 1000 historical rain-on-snow events with the help of data collected by the IMIS weather stations and precipitation measurements recorded by MeteoSwiss. They used the data to identify where and when rain had fallen significantly on a snowpack during the last 16 years. The information also allowed the researchers, with the aid of the SNOWPACK model, to examine snowpack processes, such as melting, changes in the snowpack, and water infiltration into the ground for the identified events. They were thus able to determine the influence exerted by the snowpack on the runoff formation.
Improving flood forecasting
The results show that in most of the events that were examined the largest portion of runoff quantity originated from rain. In individual cases, however, as much as 70% of the runoff was attributable to melting snow. The following factors were found to augment the runoff formation: persistent rainfall, homogeneous snowpack properties at the start of the event, high air temperatures and wind speeds, and snowpack attributes capable of fostering the early onset of runoff produced by the snow. Rain falling on a snowpack does not necessarily bring about a critical situation. Runoff formation depends on numerous processes that take place on different spatial scales. Properties of the snowpack such as its thickness, layering and moisture, are instrumental in determining whether it retains rain or increases runoff formation as it melts.
The results of this study are helping to refine the models produced by the SLF's snow-hydrological service and thus to provide a clearer picture of imminent rain-on-snow events. These improvements are being applied directly in the operational flood forecasts of the National Environmental Agency.