Recent trends of groundwater temperatures in Austria

8 Climate change is one if not the most pressing challenge modern society faces. Increasing temperatures are observed 9 all over the planet and the impact of climate change on the hydrogeological cycle has long been shown. However, so 10 far we have insufficient knowledge on the influence of atmospheric warming on shallow groundwater temperatures. 11 While some studies analyse the implication climate change has on selected wells, large scale studies are so far lacking. 12 Here we focus on the combined impact of climate change in the atmosphere and local hydrogeological conditions on 13 groundwater temperatures in 229 wells in Austria, which have in part been observed since 1964. A linear analysis finds 14 a temperature change of + 0.8 ± 1.0 K in the years from 1994 to 2013. In the same timeframe surface air temperatures 15 in Austria increased by 0.72 ± 0.04 K displaying a much smaller variety. However, most of the extreme changes in 16 groundwater temperatures can be linked to local hydrogeological conditions. Correlation between groundwater 17 temperatures and nearby surface air temperatures was additionally analysed. They vary greatly with correlation 18 coefficients of -0.36 in central Linz to 0.80 outside of Graz. In contrast the correlation of nationwide groundwater 19 temperatures and surface air temperatures is high with a correlation coefficient of 0.83. All of these findings indicate 20 that while atmospheric climate change can be observed in nationwide groundwater temperatures, individual wells are 21 often primarily dominated by local hydrogeological conditions. In addition to the linear temperature trend a step-wise 22 model was also applied that identifies climate regime shifts, which have been observed globally in the late 70s, 80s, 23 and 90s. Hinting again at the influence of local conditions, at most 20% of all wells show these climate regime shifts. 24 However, we were able to identify an additional shift in 2007 which was observed by 33% of all wells. Overall the 25 step-wise representation gives a slightly more accurate picture of observed temperatures than the linear trend. 26