Climate change effects on the annual cycle in shallow subarctic groundwater over 50 years

Groundwater-level seasonality (regime) is shifting under climate change, with direct implications for drought and flood risks, water supply reliability, agriculture, and ecosystem functioning. These shifts are accentuated in shallow, fast-responding aquifers, particularly in boreal settings where the recharge- and evapotranspiration-constrained freezing winters are changing. To characterize subarctic groundwater regimes and assess their past development, we analyzed 50-year groundwater level records from 53 monitoring stations across Finland (covering >300,000 km²). After method evaluation, we classified regimes using partitioning around medoids (PAM) clustering based on Pearson correlation distances, supported by principal component analysis (PCA) of normalized monthly groundwater levels to summarize seasonal variability.

The clustering identified four groundwater regimes that align primarily with a southwest–northeast gradient of frost-season intensity. Comparing two periods (1975–1999 vs. 2000–2024) revealed the regimes to have migrated northeastward, toward colder regions. This is locally seen as higher winter groundwater levels, lower summer levels, earlier spring recharge peak, and prolonged summer low season. Regime expression also varied with aquifer size: within the 0.01–70 km² range, larger aquifers exhibited lagged seasonal responses, consistent with longer flow paths and greater storage. The results implicate that the selected approach effectively displays the spatially evolving groundwater dynamics, and highlight the importance of long-term environmental monitoring for effective decision-making and preparedness for shortages in water availability in the changing climate.