Integrated Terrestrial Simulations over Europe: Groundwater-Atmosphere Feedbacks with Altered Water Tables Including the Effect of Recent Droughts

Abstract

The groundwater flow is an essential part of the terrestrial system encompassing the atmosphere, land surface, and subsurface. In past research, there was usually no focus on water in the subsurface because climate models worked with simplified free drainage assumptions. However, recent research has shown that the groundwater table has a vital memory function, especially in hydrometeorological extremes. Droughts are remembered for long timescales, but surpluses in groundwater can also mitigate current water deficits. Groundwater can also interact in feedback loops altering the water and energy cycle. <br /> Suited to investigate these research questions in a modeling environment is the Terrestrial Systems Modeling Platform (TSMP). TSMP couples an atmospheric, land surface and subsurface model to simulate the whole water cycle from the bedrock to the cloud top. Three studies exploring groundwater interactions utilizing TSMP over Europe are combined in this thesis. First, groundwater memory and predictability are investigated by combining three states of recent droughts with past atmospheric boundary conditions leading to a model ensemble with varying drought initial conditions. The ensemble was simulated for one year resembling atmospheric uncertainty and natural variability. The results show the increased probability of ongoing drought conditions and the dominant influence of the initial condition on the timescale of one year over atmospheric forcing. <br /> Secondly, the results of the drought ensembles are compared to the original realization of the years not influenced by drought conditions. The comparison reveals changes in the energy cycle with more available energy at the surface. Together with changes in cloud properties, the results indicate a drought feedback loop where the persisting water deficits contribute to higher and thinner clouds leading to increased incoming shortwave radiation at the ground. <br /> Lastly, potential feedback processes between groundwater and precipitation are further investigated, showing that a climatology with a shallower water table due to changed parameters also influences rainfall at the continental scale. This feedback connecting groundwater and precipitation makes calibration impossible in a fully coupled system. Furthermore, the feedback highlights the potential influence of altered water tables due to climate change in the future. <br /> The results of this thesis highlight the importance of incorporating groundwater in climate models, especially in hydrometeorological extremes. Significant interactions are observed between all components of the terrestrial system, which would otherwise be overlooked. While including a complex groundwater representation is connected to additional computational costs, feedback processes strongly influencing atmospheric processes are essential for climate projections. Further improvement of the physical representation of numerical models and increased resolutions might additionally emphasize the connections in the future.