ABSTRACT:

Underflow, or the downstream flow of water in the shallow subsurface parallel to a stream, is important for the transport of water and solutes through river corridors and is driven by hydrostatic controls such as hydraulic conductivity and volumetric subsurface capacity. However, these controls rely on point measurements and thus the actual volume of underflow is difficult to quantify. In this study, we take advantage of bedrock outcrops paired with reach and segment scale water balances to quantify underflow. From the water balances, we learn that reach and segment scale balances can have differing interpretations due to the existence of underflow. Additionally, we suggest that fluxes, geometries, and timescales are important for fully understanding the role of underflow and present a high-level review of methods that can be used to address all three components to better quantify underflow in future studies.

ABSTRACT:

Supplementary information for surveyed reaches, model mesh quality outputs, RSF inputs, p-values for Kruskal-Wallis tests, and percent differences for exchange flux and percent upwelling particles. Supplementary information is associated with, "Testing Hidden Assumptions of Representativeness in Reach-Scale Studies of Hyporheic Exchange" by Becker, Ward, Herzog, and Wondzell, 2022.