Studying the influence of groundwater representations on land surface-atmosphere feedbacks during the European heat wave in 2003

The impact of 3D groundwater dynamics as part of the hydrologic cycle is rarely considered in regional climate simulation experiments. However, there exists a spatial and temporal connection between groundwater and soil moisture near the land surface, which can influence the land surface-atmosphere feedbacks during heat waves. This study assesses the sensitivity of bedrock-to-atmosphere simulations to groundwater representations at the continental scale during the European heat wave 2003 using an integrated fully coupled soil-vegetation-atmosphere model. The analysis is based on the comparison of two groundwater configurations: (1) 3D physics-based variably saturated groundwater dynamics and (2) a 1D free drainage (FD) approach. Furthermore, two different subsurface hydrofacies distributions (HFD) account for the uncertainty of the subsurface hydraulic characteristics, and ensemble simulations address the uncertainty arising from different surface-subsurface initial conditions. The results show that the groundwater representation significantly impacts land surface-atmosphere processes. Differences between the two groundwater configurations follow subsurface patterns, and the largest differences are observed for shallow water table depths. While the physics-based setup is less sensitive to the HFD, the parameterized FD simulations are highly sensitive to the hydraulic characteristics of the subsurface. An analysis of variance shows that both, the groundwater configuration and the HFD, induce variability across all compartments with decreasing impact from the subsurface to the atmosphere, while the initial condition has only a minor impact.