How the snow that controls the spring flood is measured

The Swedish mountain region is vast: 165,000 square kilometres, accounting for 40 per cent of the country’s total area. It is also here that the heaviest snowfall occurs in winter; snow that melts in spring and flows into the rivers in the valleys, where a large proportion of Sweden’s hydroelectric power is generated.

The most power producing river is Lule River. At the end of March, when the snow cover is stable and often at its highest level, hydrologists head out to measuring points along fixed lines in the landscape marked by ten posts. Here, they check the snow depth using a snow tube and take samples to determine the weight of the snow, which reveals how much water it contains.

“Roughly half of all precipitation in our forecast area falls as snow. We still use the same method as in the 1940s. This allows us to compare year on year and track trends over time,” says Susanne Nyström, hydrologist at Vattenfall.

The method is simple but robust – and thanks to the long time series, it is a very valuable indicator of how spring water flows may develop.

Helicopter measurements provide the bigger picture 

To complement the manual measurements, radar measurements from a helicopter are also used. An antenna is mounted beneath the helicopter, and around 400 kilometres of fixed routes are measured within the forecast area. The travel time of the radio waves through the snow is affected by how deep and densely packed the snow is.

“The measurements from the helicopter are a supplement to the manual ones. Together, the two methods provide a strong picture of the snow conditions in the mountains,” says Susanne Nyström.

In the Swedish mountains, hydrologists check the snow depth using a snow tube and take samples to determine the weight of the snow.

A dense data network in the mountains

To obtain a comprehensive picture of weather and snow conditions, Vattenfall has around ten of its own Hydmet stations located in the mountain regions. The stations measure temperature, precipitation, wind and water levels, and report hourly values daily via satellite communication. 

“The stations provide data from areas where no one else is currently measuring. This makes a huge difference when we’re trying to understand the bigger picture,” says Susanne Nyström.

The data is incorporated into Vattenfall’s hydrological model for the Lule River.

From the mountains to the forecast – how the data is used 

Once the snow measurement data has been collected, it is compared with Vattenfall’s hydrological model, which is updated daily with precipitation and temperature data. The model is used to assess how the spring flood might develop.

“The snow measurements help us to see how much water is stored in the snow. It’s an important check on the model’s calculations,” says Susanne Nyström.

In the long term, the next-generation model is expected to be able to incorporate the data from the snow measurements directly – something that could provide more accurate forecasts.

The variations of winter are evident in the small details

Formal snow measurements form the basis – but sometimes the differences between winters are only noticeable to the naked eye. Susanne tells us about a hut on the Norwegian border that she often passes in winter.

“Some years it’s almost completely covered in snow, other years it stands almost bare. That says a lot about how different winters can be and what the wind does to the snow cover,” says Susanne Nyström.

The small hut on the Norwegian border shows how much winter conditions can vary. Some years it’s almost completely covered in snow, other years it stands almost bare.

A solid foundation for spring

As the spring flood approaches, the measurements have already been compiled, analysed and incorporated into the planning. This is when the work become most tangible – in the decisions on how the reservoirs are to be regulated, how risks are to be managed and how hydropower production is to take place during the intense spring period.

“Estimating how much snow there is and, consequently, how large the spring flood might become is a central part of the hydrological work. This is often where the annual planning begins,” says Susanne Nyström.