Duration of ice cover of inland waters shortens

The duration of the ice cover of Finnish inland waters has shortened, and seasonal winter and spring discharges have increased due to milder winters, according to a Doctoral dissertation.

Johanna Korhonen, Development Manager of the Finnish Environment Institute SYKE, studied the long-term changes and variability of the winter and spring season hydrological regime in Finland.

Her doctoral dissertation will be examined at the University of Helsinki on February 22, said a press release of SYKE.

"The changes in ice cover and the hydrological conditions in winter touch us in many ways, and they are of importance also on a global scale, particularly with a view to the radiation balance of the Earth. The research results can be applied to various planning and research purposes, such as water level regulation and hydroelectric power generation, flood risk management, recreational use of waters and biological and ecological research," said Johanna Korhonen from SYKE.

Climate warming will have a major impact on seasonal winter and spring hydrology in Finland, as well as elsewhere on the boreal snow-driven regions. Research analyses show statistically significant long-term changes in both ice and discharge regime until recent years.

The dissertation studied long-term changes and variability of the winter and spring season hydrological regime based on long-term ice phenology and river discharge time series in Finland. The ice freeze-up has become later, and the ice break-up is earlier, while the duration of the ice cover has shortened. In recent decades, changes have become even more apparent, and the number of extremely early break-ups and late freeze-ups has increased.

"In ice phenology, new records have been set in the 21st century. In the 21st century, the freeze-ups have been later than average about on nine years out of ten. The average was calculated for the period 1961-2000. The break-ups were also earlier than average about on nine years out of ten."

Some of the Northern and Central Finland observation sites had increasing maximum ice thickness trends until 2002, and decreasing trends were observed in the south. The maximum ice thickness time series, updated until the 2017-2018 winter, continue to show decreasing maximum ice thickness trends in the south, whereas no trends were present in Northern and Central Finland. "The ice thickness is also significantly affected by snow conditions, since a thick sub-zero snow cover has an insulating effect. During mild winters, snow also forms 'snow ice'," Korhonen explained.

The river flow trend analysis until 2004 revealed an increase in winter and spring discharges and a trend towards an earlier spring peak discharge. The analysis revealed no changes in the mean annual flow or the spring high flow in general. Similar tendencies have even continued with updated data until the year 2017. These observed seasonal winter and spring hydrological trends are in line with findings of annual and seasonal air temperature and precipitation changes and also with projected climate change scenarios. The results of corresponding research studies conducted in other Nordic Countries show similar trends.