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Human influences threaten environmental flows directly or indirectly through groundwater abstraction. In alluvial geological settings, these may affect the contributions from groundwater-sustaining streamflow during dry summer months. The Dreisam River valley in southwest Germany represents a typical case where recurrent hydrological drought events between 2015 and 2022 have led to interruptions of longitudinal connectivity in the stream network. When and where vertical connectivity changes and where the streambed dries out have therefore become important questions. To help answer them, zero water level (ZWL) occurrences were previously measured at 20 locations in the river network during the drought of 2020, but they revealed high variability. This study therefore aimed to develop a methodology that allows the connectivity to be assessed along the entire stream network, i.e. by employing a numerical groundwater model to obtain the spatial distribution of the exchange flow between groundwater and surface water along the river. A reference model simulation for the period 2010–2022 assumed near-natural conditions. Stress test scenario model runs then imposed either an altered recharge regime or a set of introduced groundwater abstraction wells or both. To gain confidence in the model, ZWL patterns are compared to observations of dry riverbed locations in 2020, and the model generally reproduces the observed relative drying. Modelled exchange flows of the stress tests were then compared against the reference simulation. A set of specific metrics combining longitudinal and vertical connectivity is introduced for this task. The results of the stress test model runs show stronger changes of vertical connectivity in response to groundwater (GW) abstraction than to the imposed recharge stress. Reaches are identified where the effects of the stresses are particularly strong. Nevertheless, these results have to be interpreted within the limits of model realism and uncertainty. For more model realism, a number of improvements will be needed such as a higher-resolution parametrization of the riverbed's hydraulic conductivities and better coupling to contributions from hillslopes; for a quantification of the uncertainties, a systematic sensitivity analysis would be required. The study introduces a framework for modelling stress tests and metrics for surface water–groundwater interaction that can be transferred to other cases. It also suggests that even if not all influences can be modelled, the approach may help inform a resilient management of water resources under multiple stresses. [ABSTRACT FROM AUTHOR]

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]

Groundwater is the main source of freshwater and maintains streamflow during drought. Potential future groundwater and baseflow drought hazards depend on systems' sensitivity to altered recharge conditions. We performed groundwater model experiments using three different generic scenarios to estimate the groundwater- and baseflow drought sensitivity to changes in recharge. The scenarios stem from a stakeholder co-design process that specifically followed the idea of altering known drought events from the past, i.e. asking whether altered recharge could have made a particular event worse. Across Germany groundwater responses to the scenarios are highly heterogeneous with groundwater heads in the North more sensitive to long-term recharge and in the Central German Uplands to short-term recharge variations. Baseflow droughts are generally more sensitive to intra-annual dynamics and baseflow responses to the scenarios are smaller compared to the groundwater heads. The groundwater drought recovery time is mainly driven by the hydrogeological conditions with slow (fast) recovery in the porous (fractured rock) aquifers. In general, a seasonal shift of recharge (i.e. less summer recharge and more winter recharge) will therefore have low effects on groundwater and baseflow drought severity. A lengthening of dry spells might cause much stronger responses, especially in regions with slow groundwater response to precipitation. As climate models suggest such directional changes for Germany in the future, the results of the stress tests suggest that groundwater resources in Germany may not decrease in general, but water management may need to consider the potential for more severe groundwater droughts in the large porous aquifers following prolonged meteorological droughts. [ABSTRACT FROM AUTHOR]