Coupled Long‐Term Simulation of Reach‐Scale Water and Heat Fluxes Across the River‐Groundwater Interface for Retrieving Hyporheic Residence Times and Temperature Dynamics

Abstract

Flow patterns in conjunction with seasonal and diurnal temperature variations control ecological and biogeochemical conditions in hyporheic sediments. In particular, hyporheic temperatures have a great impact on many temperature‐sensitive microbial processes. In this study, we used 3‐D coupled water flow and heat transport simulations applying the HydroGeoSphere code in combination with high‐resolution observations of hydraulic heads and temperatures to quantify reach‐scale water and heat flux across the river‐groundwater interface and hyporheic temperature dynamics of a lowland gravel bed river. The model was calibrated in order to constrain estimates of the most sensitive model parameters. The magnitude and variations of the simulated temperatures matched the observed ones, with an average mean absolute error of 0.7°C and an average Nash Sutcliffe efficiency of 0.87. Our results indicate that nonsubmerged streambed structures such as gravel bars cause substantial thermal heterogeneity within the saturated sediment at the reach scale. Individual hyporheic flow path temperatures strongly depend on the flow path residence time, flow path depth, river, and groundwater temperature. Variations in individual hyporheic flow path temperatures were up to 7.9°C, significantly higher than the daily average (2.8°C), but still lower than the average seasonal hyporheic temperature difference (19.2°C). The distribution between flow path temperatures and residence times follows a power law relationship with exponent of about 0.37. Based on this empirical relation, we further estimated the influence of hyporheic flow path residence time and temperature on oxygen consumption which was found to partly increase by up to 29% in simulations.

Beyond Classical Observations in Hydrogeology: The Advantages of Including Exchange Flux, Temperature, Tracer Concentration, Residence Time, and Soil Moisture Observations in Groundwater Model Calibration

“…Consequently, the model was not compared to flux observations, and the data worth of observations of T for the reproduction of water fluxes was not assessed. Munz et al () noted that the calibrated mean horizontal hydraulic conductivity was 1.6 times higher than measured values. Like Munz et al (), Karan et al () also calibrated K rb of a 3‐D river‐aquifer model against observations of H and T based on automated calibration with a multivariate objective function using PEST.…”

“…Munz et al () noted that the calibrated mean horizontal hydraulic conductivity was 1.6 times higher than measured values. Like Munz et al (), Karan et al () also calibrated K rb of a 3‐D river‐aquifer model against observations of H and T based on automated calibration with a multivariate objective function using PEST. Trying to limit the confounding nature of T observations for the reproduction of fluxes, Karan et al () employed an exceptionally high‐resolution measurement network of 400 temperature sensors within a 10 × 10 × 0.5 m riverbed section and calibrated three structurally different models.…”

“…Munz et al () conducted a sophisticated water flow and heat transport study of a real‐world river‐aquifer system using a 3‐D IFM model, calibrated against a large number of observations of H and T. The authors calibrated multiple thermal and hydraulic parameters, including separate heterogeneous K rb ‐ and K aq ‐fields, using the automated, weighted multivariate objective function calibration routine implemented in PEST. An extensive global sensitivity analysis based on 780 model runs was carried out prior to model calibration using Morris' elementary effects screening method (Morris, ).…”

“…The calibrated model was able to reproduce the observed H and T well. Munz et al () used the model to investigate the temperature distributions within the hyporheic zone and the aquifer and found that the water temperature is strongly dependent on the RT of water in the hyporheic zone. The purpose of the model was not to analyze the magnitude of water fluxes.…”

The efficacy of riparian tree cover as a climate change adaptation tool is affected by hydromorphological alterations

“…Surface‐sub surface flow exchange may also affect stream temperatures spatially and temporally depending on the rate of exchange (Schmidt, Bayer‐Raich, & Schirmer, ; Birkel et al, ). For example, channel sections with conductive floodplain aquifers and porous streambed topography may have their stream temperatures moderated by enhanced rates of hyporheic exchange (Cardenas, Wilson, & Zlotnik, ; Munz, Oswald, & Schmidt, ; Poole, Stanford, Running, & Frissell, ; Woessner, ).…”