Assessing 20th Century Climate Change Impacts on Surface–Groundwater Interactions in Southern Estonia, Northeastern Europe

Recent years have highlighted Europe’s increasing vulnerability to climate change through the rising frequency and severity of extreme hydrological events, such as droughts and floods. In addition, winter warming and gradual decline in snow cover depth and duration have occurred in northern Europe since the middle of the 20^th century. These changes not only affect surface waters directly but also alter groundwater recharge patterns and disrupt the balance between surface and subsurface hydrological systems.

To support effective water resource management under future climate conditions, it is essential to understand how projected climatic changes will influence surface–groundwater interactions. However, predictions of future dynamics are not possible without first analysing how past and ongoing climate changes have already affected key components of the hydrological regime, including groundwater recharge, runoff generation, and their relative contributions to total flow.

In Estonia, this need is being addressed through the development of coupled surface–groundwater models in five pilot areas across the country, as part of the LIFE-SIP AdaptEST project. The overarching goal is to increase the readiness and adaptive capacity of regional and local authorities in Estonia to respond to the impacts of climate change.

The objective of this study was to assess the impact of climate change during the 20^th century on surface–groundwater interactions in Southern Estonia. A hydrological model, PRMS (Precipitation-Runoff Modeling System), was applied to a small pilot catchment in Southern Estonia characterized by a high baseflow component and pronounced surface–groundwater interaction. The model was calibrated and validated for the period 1952–2017 using measured hydro-meteorological data. Model performance, evaluated using the Kling–Gupta efficiency, ranged between 0.56 and 0.77, indicating a satisfactory representation of hydrological processes and surface–groundwater interactions. The interpretation of the model results shows how changes in climatic parameters (air temperature, precipitation amounts) in the past have brought about parallel changes in river runoff regime (e.g. timing of low-flow and high-flow periods) as well as in baseflow and groundwater recharge. The results indicate that the applied surface water modelling approach provides a suitable basis for coupling with groundwater models and for future climate change impact assessments in Estonia.

This study has been funded by the project LIFE21-IPC-EE-LIFE-SIP AdaptEST/101069566 "Implementation of national climate change adaptation activities in Estonia”.