Recent trends of groundwater temperatures in Bavaria, Germany

Surface temperature variations have been well shown to transfer their thermal signature into the subsurface. This continuous heat transfer manifests in altered thermal conditions in the subsurface where temperature variations over a long lapse of time are more pronounced than shorter ones. Hence, repeated temperature depth profiles allow to investigate the effects of recent climate change on the subsurface. In this study we present recent temperature trends in more than 40 observation wells in Bavaria, Germany. Temperature depth profiles have been quarterly measured for one year between 1992-1994 and measurements have been repeated two times in 2019. The quarterly measurements reveal that the periodic seasonal temperature signal dampens to around 0.1 K at a depth of 15 m below ground surface. This implies that temperature variations below this depth can be used as climate archives as they store the temperature history of multiple years. The measurements span a time period of almost 30 years which is the most common period of reference for deriving climate normals according to the World Meteorological Organization. Therefore, the findings of recent subsurface temperature variations are assessed versus and complemented by 22 air temperature stations. Preliminary results show, that the linear regression of the annual mean air temperature since 1990 yields a slope of 0.35 ± 0.11 K 10a^-1. In the subsurface, median temperature differences of the respective baselines from 1992-94 period and 2019 are 0.26, 0.13 and 0.07 K 10a^-1 at 20, 40 and 60 m depth below surface, accordingly. Despite the common magnitude and continuous downward decrease, subsurface temperature differences exhibit a much higher variance compared to air temperature changes. This is due to local effects, such as varying thermal conductivities of the subsurface, latent heat transport caused by evapotranspiration, lateral and vertical groundwater flow, and anthropogenic influences. Our contribution will feature a comparison of this temperature change in response to recent atmospheric climate change in Bavaria and link these results with perceptions gained by similar investigations on local scale in other European regions.