An investigation of anthropogenic influences on hydrologic connectivity using stress tests.

Worldwide, human influences directly and indirectly threaten environmental flows through groundwater (GW) abstraction. This highlights the need to consider GW withdrawals together with climatic changes in future water management plans to maintain water availability in river networks. In alluvial geological contexts, such as the Dreisam River in South-Germany, contributions from GW often sustain streamflow in the summer months. In the specific case of the Dreisam however, several hydrological drought events between 2015 and 2022 lead to interruptions of longitudinal connectivity in the stream network. This raises the question on where and when the stream network is gaining or losing water from the GW and how these vertical connectivity changes influence streambed drying. This study therefore aims to analyse both, how changes in longitudinal and vertical connectivity in the Dreisam valley respond to stresses from climatic variations of recharge and to anthropogenic water abstractions from the hydrological system. As GW-SW interaction is difficult to measure, numerical groundwater modeling was used to obtain a spatial distribution of the exchange flow between GW and SW. The results show in which stream reaches, the connection between GW and SW ceases during dry conditions. Changes of vertical connectivity due to GW abstraction were found to be stronger than due to recharge stress on short timescales. A combined analysis of vertical and longitudinal connectivity depicts local points along the stream network, where the effect of GW abstraction on temporal drying dynamics is likely particularly strong. These results have to be interpreted within the limits of model reality and uncertainty. Simulated zero water levels were in good agreement with measured zero water levels at 50% of measurement locations, whereof the majority was in the valley bottom. Future work needs to improve coupling to upstream contributions and bedrock aquifers along the hillslopes of the valley. Overall, our findings highlight the value of a combined analysis of different dimensions of hydrologic connectivity for the evaluation of model results. Approaches that better distinguish locations affected by natural and anthropogenic drivers of hydrologic drought and streamflow intermittency deserve further development and are needed for application on different spatial and temporal scales. [ABSTRACT FROM AUTHOR]