Model Choice Impacts the Quantification of Seasonal Hyporheic Exchange Depths and Fluxes

Direction and depths of hyporheic exchange fluxes at the groundwater - surface water interface are drivers of biogeochemical processes influencing nutrient cycling and water quality. Model concepts on the dynamic relationship between hyporheic exchange fluxes and exchange depth are typically based on the assumption of a linear relationship between both measures. Here, we quantify seasonal and episodic variations of hyporheic exchange fluxes and hyporheic exchange depths with methods of heat tracing. Numerically (FLUX-BOT) and analytically (VFLUX; method based on temperature amplitude dampening developed by Hatch et al., 2006, https://doi.org/10.1029/2005wr004787) working program scripts were used to solve the one-dimensional conduction-advection-dispersion equation and compute hyporheic flux rates from three vertical sediment water temperature profiles recorded continuously in a small low mountain creek between 2011 and 2017. By comparing the behavior of two differing water temperature-based modeling approaches, dissimilarities in the sensitivity to sediment thermal properties were identified. Besides the differences in parameter responsivity differences in how the temperature data is incorporated, explain deviating behavior of the models regarding exchange flux and depth calculations. We show that the vertical extension of hyporheic exchange depth has a distinctive seasonal pattern over seven years, which differs according to the positioning in the riffle-pool-sequence. Surface water levels, groundwater levels and stream discharges show significant correlations with both flux direction and hyporheic zone extension. Temperature derived flux data allowed for establishing a significant relationship between hyporheic exchange fluxes and hyporheic exchange depths.