Search

Gross gains and losses of stream water and the consequent hydrologic turnover may modify the composition of stream water and drive in‐stream ecological functioning. We evaluated over 500 breakthrough curves of conservative tracer additions to analyze the channel water balance resulting in gross gains and losses, net exchange, and hydrologic turnover. During the hydrological year 2019, seven tracer experiments had been carried out in six first‐order streams along 400 m study reaches. All streams are located in the Holtemme catchment (Central Germany) with three each dominated by forested and agricultural land use. Four of the six streams were characterized by net‐losing conditions. The overall median of gross exchange was five times higher than net exchange. On average, subsurface gains replaced 50% of the original stream water over less than one kilometer of stream length. We even observed cases where over 95% of the stream water turned over within 100 m. Gross exchange was relatively higher in forested than in agricultural streams. Patterns of exchange in the forested streams persisted spatially and were temporally independent of streamflow, whereas in the agricultural ones, variable spatial patterns and streamflow dependence occurred. Overall, moderate flow coincided with highest relative gross exchange. Our results support previous findings that in‐stream solute concentrations could heavily depend on location and magnitude of gains and losses. Gross exchange embodies a permanent but variable control of downstream solute concentrations interacting with the signal of biogeochemical activity. We highlight the importance to include reach‐scale hydrological processes in studies on nutrient spiraling. The vitality of stream ecosystems largely relies on the exchange of water between surface and groundwater. This comprises all gains and losses of stream water from and to the subsurface and is referred to as gross exchange. We investigated gross exchange for six headwater streams in the Holtemme catchment (Central Germany) during the hydrological year 2019. By applying salt tracer experiments we calculated the extent of exchange. Consistently, the investigated stream reaches lost more water than they gained. On average, half of the stream water was replaced by newly added groundwater along less than one km of stream length and, in few cases, almost the entire volume was exchanged within 100 m distance. Streams surrounded by forest exchanged more water than streams in agricultural landscapes. The location and direction of exchange remained similar in the forested streams, but varied temporarily for the agricultural streams. We could show that groundwater represents an important volume of our streams and that the true gross exchange can easily be underestimated if only the sum of gains and losses is measured. Therefore, solute concentrations can be strongly modified by gross exchange, which is important to better understand the transport of solutes in streams. In over 90% of the cases, gross exchange equals five times the net exchange, which impacts interpretations of nutrient uptakeGross exchange and hydrologic turnover show spatiotemporal patterns persisting over discharge at forested, but not at agricultural sitesModerate discharge exhibits the highest relative gross exchange In over 90% of the cases, gross exchange equals five times the net exchange, which impacts interpretations of nutrient uptake Gross exchange and hydrologic turnover show spatiotemporal patterns persisting over discharge at forested, but not at agricultural sites Moderate discharge exhibits the highest relative gross exchange

During the flow of stream water from losing reaches through aquifer sediments, aerobic and anaerobic respiration (denitrification) can deplete dissolved oxygen and nitrate (NO3−), impacting water quality in the floodplain and downstream gaining reaches. Such processes, which vary in time with short and long‐term changes in stream flow and temperature, need to be assessed at the stream corridor scale to fully capture their effects on net turnover, but this has rarely been done. To address this gap, we combine a fully‐integrated 3D transient numerical flow model with temperature‐dependent reactive transport along advective subsurface flow paths to assess aerobic and anaerobic respiration dynamics at the stream corridor scale in a predominantly losing stream. Our results suggest that given carbon availability (as an electron donor), complete NO3−removal occurred further away from the stream after complete oxygen depletion and was relatively insensitive to variations in temperature and transit‐times. Conversely, transit‐times and oxygen concentrations constrained nitrate removal along short hyporheic flow paths. Even under limited carbon availability and low‐temperatures, NO3−removal fractions (RNO3) will be greater at locations further from the stream than along shorter hyporheic flow paths (RNO3= 0.4 and RNO3= 0.1, respectively). With increasing temperature, the relative effects of stream flow and solute concentrations on biogeochemical turnover and the redox zonation around the stream decreased. The study highlights the importance of seasonal variations of stream flow and temperature for water quality at the stream‐corridor scale. It also provides an adaptive framework to assess and quantify reach‐scale biogeochemical turnover around dynamic streams. Nitrate pollution is a widespread problem in many catchments with intense agricultural activities. Denitrification is a redox process that removes nitrate from the aquatic system via its transformation to nitrogen gas. Denitrification is difficult to assess at larger scales since it depends on multiple factors, such as solute concentrations, temperature variations, and also the time that water resides in the subsurface, where reactions can take place. To evaluate how these factors can influence denitrification, we employed a coupled modeling approach representing the riparian zone of a 4thorder stream in central Germany. We found that temperature variations strongly regulate the process and that during the winter the aerobic (oxygen rich) zone around the stream expands, which further inhibits denitrification in the near stream groundwater. However, even in the winter denitrification occurs, but at larger distance from the stream where oxygen has been depleted sufficiently. With increasing temperature, the influence of other factors on denitrification and on the redox zonation around the stream decreases. Coupled numerical models can provide further insights into the occurrence and interrelations of the multiple processes controlling water quality patterns in river corridors. Fully‐integrated 3D transient flow model coupled to a temperature‐dependent flow path‐reaction model to assess riparian solutes turnoverTurnover mainly controlled by high temperature in summer; in winter solute concentrations and transit‐times variations become more importantAssessments of riparian turnover restricted to near stream areas might overlook processes affecting overall downstream water quality Fully‐integrated 3D transient flow model coupled to a temperature‐dependent flow path‐reaction model to assess riparian solutes turnover Turnover mainly controlled by high temperature in summer; in winter solute concentrations and transit‐times variations become more important Assessments of riparian turnover restricted to near stream areas might overlook processes affecting overall downstream water quality