TY  - JOUR
A1  - Jähkel, A.
A1  - Graeber, D.
A1  - Fleckenstein, J. H.
A1  - Schmidt, C.
T1  - Hydrologic Turnover Matters — Gross Gains and Losses of Six First‐Order Streams Across Contrasting Landscapes and Flow Regimes
Y1  - 2022-07-19
VL  - 58
IS  - 7
JF  - Water Resources Research
DO  - 10.1029/2022WR032129
PB  - 
N2  - 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.
N2  - Plain Language Summary: 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.
N2  - Key Points: 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.
UR  - http://resolver.sub.uni-goettingen.de/purl?gldocs-11858/10428
ER  -