Snowmelt and rain-on-snow events: is a simplified modeling approach possible?

For many watersheds in Canada, the runoff hydrographs at Spring often represent the maximum annual discharges that are used typically in a statistical analysis to define discharge of given return periods. When the river being analyzed is gauged or if it has similar characteristics to an adjacent river that is gauged, a standard statistical analysis can be completed to come up with the design discharges. Very often however, discharge measurements will not be available or the river will have very different characteristics when compared to the available gauging stations and modeling will therefore need to be used to obtain the hydrological response from the watershed for snowmelt or rain-on-snow conditions.

Modeling of these types of events can however be complex and necessitates data that are not always available for a long period and in a sufficient detailed form. Moreover, it is difficult to define a hyetograph that takes into account both the runoff volume as well as the peak discharge and that could be used with a simplified modeling.

The presentation will describe such a simplified approach that has been used for a small watershed north of Montreal in order to define snowmelt hydrographs of different return periods. The hyetograph derivation was based on detailed statistical analysis of both runoff volumes and discharges at a gauging station nearby, assessing the correlation between these 2 parameters (runoff volume and discharge). The shape of the resulting spring hyetograph was then calibrated against the measured discharges before being applied to the ungauged station.
After a review of historical approaches and important elements to be considered to model snowmelt and rain-on-snow events, the presentation will provide the basic steps of the simplified approach and discuss the results as applied to the case study and a floodplain delineation.

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