Snow model

CemaNeige is a module taking into account snow with 2 parameters that was developed specifically for hydrological modelling. It was set up during the PhD work of Valéry (2010).

CemaNeige allows simulating the evolution of snow cover on a catchment, and estimating its melt. The module was tested with several lumped hydrological models functioning in a simulation mode, including the GR4J model, and with the GRP prediction model functioning at a daily time step (Nicolle, 2010).

The functioning of CemaNeige on a single time step is described in the following:

• First, a discretisation of the basin into five elevation zones with equal areas is realised. This allows taking into account orographic gradients and permits a differenciated evolution of the snow cover regarding the altitude.

• Then, the input data are extrapolated for each elevation zone noted z. The snow module uses as an input only precipitation and temperature data. A transformation of the basin precipitation is done, according to a formula making use of a constant altitude correction factor. The average catchment temperatures are also extrapolated using an altitude gradient that can vary regarding the d day that is considered.
• The snow module then proceeds to the differentiation of liquid and solid precipitation, according to the median altitude of each elevation zone for the considered catchment.
• In each elevation zone, the snow is stored within a conceptual store that represents the snow cover of this elevation zone. This reservoir is only fed by the solid part of precipitation. The snow cover presents a thermic state that allows delaying the triggering of snow melt.
• A transfer function allows calculating the melted water that comes from the snow store and flows, and will then be integrated to the hydrological model, in addition to the liquid part of precipitation. The module starts with calculating the potential melted water (maximal snow quantity that can melt) according to a degree-day method, from the thermic state of the snow cover and from the temperature. The effective melt (the actual quantity of melted snow) depends on several factors: the previously calculated potential melt, the snow-covered surface and the minimal melt speed when the storage is very low. The quantity of melted snow is then subtracted from the snow store for updating it.
• Finally, the quantity of water transmitted to the hydrological model contains the liquid fraction previously determined, to which the melted water is added.

CemaNeige has two free parameters that need to be calibrated:

• CTG, the ponderation coefficient (no dimension) of the snow thermic state, between 0 and 1,
• Kf, the degree-day factor (in mm.°C-1.d-1), generally between 2 and 6 mm.°C-1.d-1.