Climate effects on intra-annual groundwater level fluctuations in Fennoscandia

Climate strongly influences the regional hydrology. However, the relationship between climate specific seasonal fluctuations and groundwater dynamics is not well defined for Fennoscandia. For the period 1980-2016, the climate in Sweden and Finland is classified as subarctic, with the most southern parts of Sweden classified as humid continental climate (Kottek et al., 2006; Beck et al., 2018). These climate zones have a winter season heavily affecting the water balance. Large quantities of precipitation accumulate as snow in the cold months and is released during a melting period in spring. This distinctive cycle affects the groundwater level. Annual maxima occur during periods characterised by low evapotranspiration (ET) and snowmelt. Groundwater level minima coincide with high ET and precipitation events occurring at negative temperatures (snowfall). Kottek et al. (2006) predict the subarctic climate zone to shift northwards between the classification periods 19511975 and 2001-2025. The climate zones are expected to shift more dramatically in space during the current century, altering the local winter season furthermore (Beck et al., 2018). How this affects the regional hydrology is important for mitigation efforts and management, since high and low water tables occur in conjunction with incidence of water scarcity, floods, and hydrological droughts. This research focusses on the identification of characteristics and driving mechanisms of intra-annual groundwater level fluctuations in Sweden and Finland. Mean timing and duration of gaining (positive water budget) and losing (negative water budget) periods are identified for several groundwater observation sites. The time series consist of biweekly measurements. The time series for two 5-year periods (1980-1984 – "past", 2006-2010 – "present") are correlated to modelled (temperature and precipitation-based) snowmelt, rainy and high ET periods to identify driving mechanism. Changes in the dynamic behaviour between both periods are quantified by a statistical evaluation of the time series. Spatiotemporal analysis show how the climate zones impact local water table dynamics, and how the climate zone shift affects change in groundwater level fluctuations.