Long-Term Changes in Groundwater Levels in the Białowieża Forest Under Climate Change

Global warming can lead to changes in groundwater levels in forested areas. Groundwater is the primary water source for ecosystems, so changes in groundwater levels, if directional and constant, can cause changes in vegetation and habitat character. In Białowieża National Park, a significant decline in the water table was observed at the beginning of the 20th century. The question therefore arose as to whether the changes that occurred at that time were permanent and whether the negative trend continued in such a clear direction in the following years. And whether the large water reservoir, which was built near the northern boundary of the Białowieża Forest, had an impact on the groundwater level in the forest areas. The study is based on measurements from 1985-2005 and 2022- 2023, taken in the same monitoring wells. Complete data was collected from 21 monitoring wells, of which seven each represent swamp, moist and fresh habitats. An analysis of groundwater levels between 1985 and 2005 showed an average decline of 0.08 m/10years in swamp habitats, 0.11 m/10years in moist habitats and 0.21 m/10years in fresh habitats. The prediction for the year 2023 showed a significantly greater decreasing groundwater level than the measurements. The measurements in 2022 and 2023 showed that the trend of falling water levels has slowed down in almost the entire study area, with water levels in recent years being similar to those at the beginning of the century. This was also confirmed by comparing years with similar precipitation: 2022 with 1986, 2002, 2004 and 2023 with 1999. This is due to the higher precipitation after 2005. In the period 2006-2023, precipitation in the hydrological years was on average 60 mm higher than in the period 1985-2005. Despite the clear trend towards rising air temperatures, the higher precipitation compensated for the higher evapotranspiration. However, one area showed a systematic decrease in water levels, this occurred at the watershed of the two largest rivers in the Białowieża Forest. It can be indicated that watershed areas are most vulnerable to lowering the groundwater level due to climatic heating.