Combining Local Head Differences and Topography‐Driven Groundwater Flow Reveals Gaining and Losing Patterns in Stream Networks

Abstract The exchange between surface water (SW) and groundwater (GW) influences water availability and ecosystems in stream networks. Assessing GW‐SW interactions can be based on various methods at different scales, such as point scale (e.g., local head differences, temperature profiles), reach scale (e.g., environmental tracers, water mass balance), and catchment scale (topographical‐driven groundwater flow), which all have distinct advantages and limitations. In this study, we combined the analysis of local hydraulic head differences with regional topographical‐driven groundwater flow to robustly reveal gaining and losing stream patterns in two study regions in Central Germany (Bode catchment and Free State of Thuringia). To evaluate local hydraulic gradients, we developed a method for estimating surface water levels across stream networks by modifying surface elevations from a coarse digital elevation model (25 m) and compared these to measured groundwater levels. Our results reveal prevalent occurrences of losing streams. Numerous stream locations are characterized by mismatching classifications from the two methods providing additional insights for understanding water cycles. The most notable discrepancy is the classification as losing based on head differences and gaining from topographic analyses accounting for 37% and 47% of the stream locations in Thuringia and in Bode catchment. This mismatch indicates anthropogenically lowered groundwater levels, typically occurring in urban and mining areas in the study areas. Our approach, combining local hydraulic head analysis and topographical‐driven groundwater flow enhances the understanding of gaining and losing stream patterns at catchment scale, revealing widespread occurrences of losing streams and highlighting the significance of anthropogenic influences on water cycles.