Recent trends in hydroclimate and groundwater levels in a region with seasonal frost cover

In cold climate, the main annual recharge event is spring snowmelt. Projections of groundwater storage hence depend on winter hydroclimate. Winter recharge is a factor of rain, melt and ground frost. Increases in winter rain and melt have already been observed. However, due to complex processes involved in ground frost formation and its natural patchy coverage, the effect of ground frost on recharge is poorly understood. The objective of the study is to improve the understanding of the connection between groundwater storage and hydroclimate in Sweden and Finland, a region with temperate and cold climate, from a seasonal frost cover perspective. In the study, annual, frost-free season and frost season temperature and precipitation trends in the climate regions are compared between one global and two regional reanalysis datasets. Effective precipitation and wet day frequency trends between 1980 and 2010 are compared to groundwater level trends for the same period. Trends are calculated using the Mann-Kendall test and Sen's slope. The results show that the region is generally experiencing warming and increased precipitation. These trends are strongest in the frost season. Further, the global dataset systematically underestimates median temperatures and precipitation compared to the regional datasets. Effective precipitation trends are generally weak, and indicate wetting in the frost season and drying in the frost-free season. Wet day frequency trends are decreasing significantly on the regional scale in Finland, but with nearly no trends in Sweden. Groundwater levels in southern Finland and southeastern Sweden are declining significantly, while groundwater levels in southwestern Sweden are significantly rising. The results indicate that groundwater trends in southern Finland are linked to wet day frequency and effective precipitation trends in the frost-free season, and that trends in the frost season have no impact on annual groundwater level trends. In southern Sweden, groundwater level trends appear linked to annual and frost-free season trends in effective precipitation, though insignificantly. These trends may be explained by century-scale trends in precipitation drought indices identified by previous studies.

Automated summary

The study reveals that the climate regions of Sweden and Finland are experiencing warming and increased precipitation, with the strongest trends observed in the frost season. The global dataset underestimates median temperatures and precipitation compared to the regional datasets, and effective precipitation trends show wetting in the frost season and drying in the frost-free season. Additionally, wet day frequency trends are decreasing significantly in Finland on a regional scale, but with nearly no trends in Sweden.