Local head differences and topography-driven groundwater flow to robustly identify gaining and losing streams

Interactions between groundwater (GW) and surface water (SW) play a pivotal role in influencing water quantity, quality, and associated biogeochemical and ecological processes in stream networks. Understanding the spatial pattern of gaining and losing rivers is crucial for managing water resources at catchment scale. Each method to identify losing and gaining rivers, from point to reach to catchment scale, has distinct advantages and limitations. These limitations can potentially be mitigated by combining different approaches.In this study, we combined local information from hydraulic head differences between GW and SW with the regional information derived from topography-driven groundwater flow to robustly identify and characterize the spatial pattern of gaining and losing rivers in two study areas located at Central Germany –the Bode catchment and Free State of Thuringia. Central Germany has faced a drought period in the last five years, which has impacted groundwater levels. To evaluate local head differences, we compared the measured averaged groundwater levels (GWLs) and estimated surface levels (SWLs). The GWL data were obtained from 49 and 826 groundwater monitoring wells within a 1500 m distance from rivers in the Bode catchment and Thuringia, respectively. We developed a method for estimating SWLs across river networks by correcting a coarse DEM (25 m) based on the river bed elevations and river water depths recorded at gauging stations and river network topology. Uncertainties of SWL were also estimated and considered in the classification of gaining and losing rivers. Topography-driven discharge (gaining rivers) and recharge (losing rivers) areas are derived from groundwater upward and downward flow directions according to a 3D spectral solution.The analysis of head differences reveals a widespread occurrence of losing rivers. However, when combining the losing and gaining classifications from topographical-driven groundwater flow with the classifications from head differences, the fraction of river segments having the same classification from both methods is relatively low (around 50% in both study areas). Many river segments showed contradictory classifications from the two methods, with a notable observation being that rivers have losing classifications from head differences but gaining classifications from topographic analyses. Specifically, 41% of river segments in Thuringia and 7 out of 9 (78%) in the Bode catchment fall into this category. This mismatch typically occurred in urban and mining areas, indicating anthropogenically lowered GWLs.By combining local and regional scale methods, our study contributes to a more robust representation of patterns of gaining and losing rivers. Our analysis reveals the prominence of losing rivers despite the topographical classification of a gaining river highlights the anthropogenic impacts on local groundwater levels.