Mountain hydrology: improving flow predictions by taking better account of snow

In some French regions, such as the Alps, Pyrenees or the Massif Central, river flows are affected by the snowpack. Having built up over winter, snow leads to a significant increase in flow during the spring snowmelt that can lead to high water and floods. Incorporating snow into hydrological models is therefore crucial to creating accurate simulations and predictions for river flows in these regions.

With this in mind, a model capable of changing snowfall into snowmelt was developed in 2010 by Irstea 1. It uses easy to access data (precipitation and temperatures) and calculates only two parameters: speed of melting in relation to temperatures (the hotter it is, the more quickly it melts) and energy available in the snowpack (the colder the snow, the more energy, heat, required to melt it).

Combined with traditional hydrological models that convert rain into flow rates for rivers, the CemaNeige model provides an effective tool, specifically adapted to regions affected by snow. "Particularly reliable, simple and flexible, the module quickly became operational as it was integrated into our GRP hydrological models used regularly by flood prediction services (SPC) who develop the Vigicrues warning maps," clarifies Guillaume Thirel, hydrology specialist at Irstea's Antony center.

An improved version of CemaNeige

Although efficient, it nevertheless presented limitations that scientists have now overcome2. The first version of CemaNeige used an estimate for snow cover that was not compared to actual data. For this reason, it could not accurately represent the amount of snow in the river basin3. This gave scientists the idea of using MODIS satellite data, which supplies daily images of snow cover over 500 m2. "By comparing flows calculated by the model to real flows (measured in the river) along with snow cover simulations by CemaNeige to those provided by the satellite images, we were able to improve the quality of snow representation in our model and make it more realistic," explains the researcher. The comparison needed to include a further component: the variability of snowmelt across a river basin. "Although snow may fall fairly uniformly over a river basin, melting is generally more varied; quicker on southern watersheds, for example. To improve our model, we had to include the mathematical relationship [ratio] that exists between the speed of snow-melt and the proportion of snow cover in the river basin," concludes Guillaume Thirel.

Now that snow is better taken into account, the results of the project will soon become operational. By the end of 2018, the improvements to CemaNeige should be incorporated into the flood prediction models used by SPC for regions affected by snow.

However, the project had another significant result: tested against a series of historical precipitation and temperature measurements, the new version of CemaNeige provided flow values that were closer to actual flows measured during the same historical period. This proven reliability is particularly interesting as it means the model is especially relevant when studying the impact of climate change on river flows in mountain regions, along with two other key challenges for these areas: availability of water resources and the likelihood of flooding (duration, intensity)

Irstea at the EGU 2018 conference

From April 8-13, during its annual general assembly, the European Geosciences Union (EGU) will bring together in Vienna, Austria 15,000 researchers specializing in hydrology from all over the world. Irstea will also be attending. For this new edition, Guillaume Thirel will organize a session dedicated to mountain hydrology. The latest international developments in snow modeling and observation for river basin hydrology will be presented.

For more information

1- Research conducted as part of the dissertation of Audrey Valéry (2010).
2- Research conducted as part of the dissertation of Philippe Riboust (2018), co-directed by Guillaume Thirel (Irstea) and Nicolas Le Moine and Pierre Ribstein (UPMC).
3- A river basin is an area limited by the highest topographical lines, which drains each drop of water it receives towards an outlet, river or the sea.