Impact of losing and gaining streamflow conditions on hyporheic exchange fluxes induced by dune‐shaped bed forms

Abstract

The exchange of water between the surface and subsurface environments plays a crucial role in hydrological, biogeochemical, and ecological processes. The exchange of water is driven by the local morphology of the streambed (hyporheic exchange) and the regional forcing of a large-scale hydraulic gradient, which results in losing or gaining flow conditions. We measured the effects of losing and gaining flow conditions on hyporheic exchange fluxes in a sandy streambed using a novel laboratory flume system (640 cm long and 30 cm wide) under a combination of average overlying velocities and losing/gaining fluxes. Hyporheic exchange fluxes were analyzed based on a new conceptual framework. This combination of experimental observations and modeling revealed that hyporheic exchange fluxes under losing and gaining flow conditions are similar. Because interfacial transport increases proportionally to the square of the overlying velocity and linearly with increasing fluxes of losing and gaining conditions in the sand bed, the hyporheic exchange flux becomes smaller when the losing or gaining flux increases. Thus, losing and gaining flow conditions become the dominant mechanisms of water exchange at a certain flux, which depends on the competitive interaction between the overlying velocity in the stream and the losing/ gaining fluxes.

Impact of watershed topography on hyporheic exchange

“…One of the most emblematic situations in which hyporheic and regional flows interact occurs when the river is gaining, i.e., it is fed by the aquifer. In this situation, the underlying groundwater flow system obstructs the penetration of stream water into the sediments, limiting the extent of the hyporheic zone and the magnitude of hyporheic flow paths [26,27,28,29]. In or- 60 der to analyse the structure of the hyporheic processes embedded within larger groundwater systems it is therefore necessary to describe how the flow originating from the complex surrounding aquifer impacts exchange flux in the hyporheic corridor [30].…”

“…However, the role of the hyporheic zone in this interaction has not been examined in detail [33,34,35,36,37]. Conversely, some modelling and experimental approaches have been developed specifically to investigate the impact of large-scale riveraquifer interactions on the main properties of local exchange at various scales 75 [28, 29,31,38].…”

“…Laboratory studies [29] 80 performed at the small scale of river bedforms, showing that increasing losing and gaining stream flow conditions reduce hyporheic fluxes, for a simple case of interaction with small geomorphological structures. Other analyses were conducted at larger scales.…”

“…All the mentioned studies have provided some insights into the interactions between hyporheic exchange and groundwater flow at dif-100 ferent scales. However, a clear and complete evaluation of how hyporheic fluxes interact with groundwater at the watershed scale (generally 10-1000 km in linear extent) and are affected by the landscape structure is still missing [12,29].…”

Visualizing Hyporheic Flow Through Bedforms Using Dye Experiments and Simulation

“…This approach can be used for research as well as teaching purposes, as flow visualization is commonly used to test hypotheses about governing processes, and can also be used to calculate material fluxes and mass balances, for example hyporheic exchange fluxes between the stream and sediment bed 21 . Dye tracer methods similar to those described here have been used to determine the effects of streambed morphology, sediment heterogeneity, groundwater discharge, and recharge on hyporheic exchange, as well as to assess related processes such as porewater flows induced by waves [39][40][41][42] .…”