Analysis of long-term stable isotopic composition in German precipitation

- We analysed long-term data sets of isotopic composition in German precipitation.
- Annual isotopic averages vary according to the latitude and elevation.
- The continentality effect explains geographical gradients in isotopic composition.
- Most of the tested sites show increasing trends of isotope contents over time.

SummaryStable water isotopes in precipitation (18O and 2H) have been frequently used as environmental tracers to understand processes and timescales in hydrology and climate research. Capturing changes of isotopic composition over time and investigating long-term processes requires long-term data set analysis. In Germany, we have one of the world's densest national networks for long-term isotopic analysis of precipitation covering up to 36 years of time series at 28 locations. These data were used to identify the average as well as the temporal evolution of isotopic composition in German precipitation and how it is related to meteorological and geographical parameters. We found that individual Local Meteoric Water Lines (LMWL) and the long-term averages of δ18O and δ2H depend on latitude and elevation. More variable isotopic compositions and more enriched averages were found at the coast compared to more stable compositions and depleted averages in the South, South-East and at higher elevations. This continentality effect was strongly influenced by seasonal isotope-temperature dependencies. Removing the seasonality and looking at the changes over time compared to long-term averages indicate similar patterns for temperatures and 18O at some locations. We concluded that temperature and isotopes are in equilibrium in inland air masses only. The trend in temperature evolution was consistent on the national level, and temperature increases were observed in almost all stations. In contrast, temporal patterns of 18O revealed different patterns and increases were only observed in 20 out of 28 locations. Therefore, changes in isotopic composition in precipitation are not only influenced by large scale processes (i.e. temperature) but also by local factors which need to be further investigated.