Snowmelt runoff modeling in a balsam fir forest with a variable source area simulator (VSAS2)

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Abstract
The variable source area model of storm flow generation VSAS2 has been adapted to simulate snowmelt runoff in the Lac Laflamme forested basin, 80 km north of Quebec City. The model has been adapted to accept different water inputs according to topography, to factor the effect of lake water storage in the discharge hydrograph and to process 3 months of hourly data. To ensure convergence of the mathematical solution, the explicit form of flow equations included in VSAS2 has been transformed to use an iterative approach. Lysimeter measurements as well as outputs from the snowmelt temperature index model SNOW‐17 were used as input data in the simulation. Results showed that low flows in early spring were well reproduced by the physically based model. However, all seasonal (1985, 1986 and 1987) peak flows were underestimated when all water input was considered to infiltrate. Observations on the site showed that the natural topographical drainage network was augmented in the spring by superficial concrete frost, pipe throughflow at the organic‐mineral soil interface and snowpack basal ice layers. It was therefore necessary to use an infiltrability index to improve the model. Because it was closely related to the development of an impervious layer at the soil surface, the relative proportion of bare ground areas on the basin was also related to the rising of streamflow hydrographs. Thus values of snow cover areal extent were taken from SNOW‐17 outputs and used as an index to the prevailing conditions of soil infiltrability on the basin. Results from VS AS2 were greatly improved, supporting the hypothesis of an important surface flow contribution during late snowmelt. The study underlined the necessity of further field investigations on the dynamics of infiltration processes into frozen soil.