The exchange of water and solutes between the stream and groundwater along a stream network affects the source composition of discharge and solute load in the stream. To date, this hydrologic turnover has only been analyzed with respect to the exchange of water. In this study, we extend the concept of hydrologic turnover to solutes and analyze the effects of different hydrologic conditions on the spatial patterns and magnitude of exchange fluxes. Based on a coupled stream‐groundwater model built in MODFLOW using the Streamflow‐routing package, we simulated stream‐groundwater exchange along a stream of 30 km in length and evaluated the evolution of stream water source composition under different precipitation and streamflow conditions. Results show that even for highly variable hydrologic conditions, the direction of stream‐groundwater exchange (loss/gain) remained unchanged in consistently gaining or losing reaches, but changed in interspersed transitional reaches. The comparison between the source composition of discharge and nitrate load in stream water revealed a decoupling of discharge and solute load contribution from groundwater to the stream, as zones of high water gains do not necessarily coincide with zones of high solute concentrations. Overall, nearly 80% of total groundwater contributions to the outflow of water from the catchment were generated over only 20% of the total stream length. Our research highlights the importance of distinguishing water and solute load contributions to streams. This in turn implies that to reduce groundwater‐borne nutrient loads to streams, measures need to focus on the specific reaches with highest load contributions.

Stagnation points have been found to be useful in characterizing groundwater flow regimes in 2D domains. However, knowledge on stagnation points in 3D basins is limited. In this study, we first derive a transient solution for 3D Tóthian basin flow under spatially undulating and periodically changing water table and examine the occurrence of stagnation points or pseudo‐stagnation points, which constitute stagnation lines or pseudo‐stagnation lines in the 3D domain. Then, we propose a method of delineating 3D groundwater flow systems using stagnation or pseudo‐stagnation lines for the first time, and successfully apply it to 3D synthetic basins with either steady‐state or transient groundwater flow. In 3D basins with periodically fluctuating water tables, transient stagnation or pseudo‐stagnation lines are controlled by the combination of hydraulic diffusivity and the period of water table fluctuation, both of which determine the dimensionless response time, and their variations reflect the evolution of transient groundwater flow systems and control the tortuosity of particle trajectories. This study enhances the efficiency of dividing groundwater flow systems in 3D basins and also improves the knowledge on how stagnation or pseudo‐stagnation points influence streamlines and trajectories.

Accurate quantification of groundwater evapotranspiration (ET_g) is essential for reliable water resource assessment. Existing methods for estimating ET_g from water table fluctuation largely rely on one-dimensional simplifications that neglect transient groundwater flow. However, in areas with shallow water table and topographic relief, where transient groundwater flow often occurs, the validity and accuracy of this simplification remain inadequately evaluated. In this study, we used HYDRUS-2D to construct a 50 m-long sandy hillslope with a 0.05 gradient to investigate ET_g based on the water table fluctuation (WTF) method under transient groundwater flow conditions. The results indicate that periodic evapotranspiration generates water table fluctuations along the hillslope that exhibit amplitude attenuation and temporal phase lag, features not captured by 1D models. Ignoring transient groundwater flow leads to a systematic underestimation of ET_g by up to 85% in sandy soil near the topographic lows. Furthermore, we found that both the decoupling depth and the extinction depth are significantly amplified by lateral groundwater flow, by up to 66% and 51%, respectively, compared with 1D estimates derived from the Shah method. These findings highlight the importance of incorporating transient flow processes into ET_g estimation to improve the accuracy of water balance assessments and ecohydrological predictions, particularly in areas with shallow water tables and topographic relief.

The exchange of water between streams and groundwater plays an important role for hydrologic and biogeochemical processes. Along a stream the composition of stream water is modified by sequential losses of stream water with the current in-stream chemical signature to the subsurface and gains of water with another signature from the subsurface. This process has been termed hydrologic turnover. To date, most studies on hydrologic turnover have been focused on small stream networks. Moreover, the influence of hydrologic conditions on hydrologic turnover has not been systematically investigated. Taking the lower Selke River in central Germany as an example, we evaluated the evolution of stream-groundwater exchange and the source composition of stream water under different precipitation and stream discharge conditions, based on a coupled stream-groundwater model built in MODFLOW using the Streamflow-routing (SFR1) package. The results show that the stream reaches could be classified into three types: permanently gaining reaches, permanently losing reaches, and transitional reaches. Transitional reaches range from losing condition at higher stream discharge or lower precipitation to gaining condition at lower stream discharge or higher precipitation. In the lower Selke River with a length of 30 km, transitional reaches account for nearly 30% of the total river length in the studied period from 2011 to 2018. Regardless of dry or wet hydrologic condition, nearly 80% of the total groundwater contribution to stream discharge at the catchment outlet were generated over 20% of the total river length. This indicates diffuse groundwater pollution such as from agricultural nitrate may enter the stream network predominantly at a few distinct reaches. Our analysis can help to prioritize areas in a catchment where reduction of diffuse groundwater pollution would have the highest impact on improving stream water quality.

Stagnation points have been found to be useful in characterizing groundwater flow regimes in 2D or 3D domains. However, in 3D basins with complicated water table undulation and/or fluctuation, knowledge on stagnation points is limited. In this study, we first derived transient solution of basinal flow under spatially undulating and periodically changing water table in 3D Tóthian basins and examined the occurrence of stagnation points. Based on the analysis of groundwater flow systems distribution in simple 3D basins, we extend the method of delineating groundwater flow systems in 2D profiles using stagnation points to delineating 3D groundwater flow systems using stagnation or pseudostagnation lines, which consist of a series of stagnation or pseudostagnation points. This novel approach was successfully applied to 3D synthetic basins with more complex water table configuration (with undulations in all directions). Based on the transient hydraulic head solution in 3D Tóthian basins with a periodically fluctuating water table, it was found that the evolution of (pseudo)stagnation lines are controlled by the combination of hydraulic diffusivity and the period of the water table fluctuation, both of which determine the dimensionless response time. The change in shape of (pseudo)stagnation lines, induced by spatial undulations and/or temporal fluctuations of the water table, also reflects the variation of groundwater flow systems in penetration depth and horizontal range. The method proposed here improves the efficiency of partitioning groundwater flow systems in 3D domains, and our analytical study of (pseudo)stagnation lines partially fills the knowledge gap between stagnation points and stagnation zones.