Model choice impacts the quantification of seasonal hyporheic exchange
depths and fluxes
Abstract
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) 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 modelling approaches, dissimilarities in the
sensitivity to sediment thermal properties were identified. These
differences in parameter responsivity 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 between the chosen
models. Surface water levels, groundwater levels and stream discharges
show significant correlations with both flux direction and hyporheic
zone extension. In contrast to the numerical modelling approach,
analytically derived flux data allowed for establishing a significant
relationship between the hydraulic gradient observed at a nearby
groundwater well and simulated hyporheic exchange depths.