Quantifying Vertical Hyporheic Exchange and hyporheic residence time in thalweg paths of meandering streams characterized by multiple riffle-pool sequences morphology

Abstract. Riffle-pool sequences in the thalweg paths of meandering streams are of pivotal importance to the hyporheic exchange pattern in a fluvial network, but the complex hydrodynamic, morphological, and sedimentary features of riverbed sediments increase the difficulties associated with vertical hyporheic exchange (VHE) quantification. This study applied depth-dependent radon (²²²Rn) and diel temperature variations to quantify VHE and residence time (t_r). The study was conducted in four different hyporheic areas with riffle-pool sequences in the third-order Ghezel-Ozan River, located in north-west Iran. The mean values of temperature-derived VHE (VHE_T) and radon-derived VHE (VHE_Rn) were 0.67±0.32 m/day and 0.63±0.36 m/day, respectively. Due to effects of sediment bed heterogeneity on temperature variation and ²²²Rn activity at downwelling and upwelling points, there were discrepancies between radon-derived (tr_Rn) and temperature-derived residence time (tr_T), with mean values of 2.11±1.17 days and 1.87±1.26 days, respectively. The value of tr_T was well within uncertainty boundaries at a 95 percent confidence interval (p<0.05) and was lower than tr_Rn at the downwelling points. The analysis of vertical diel temperature, radon and electrical conductivity variations revealed subsurface water exchange to be greatly affected by larger scale regional flow. The comparison between VHE_T and VHE_Rn with VHE obtained from PHAST model simulation (VHE_PHAST) revealed a higher correlation between VHE_T and VHE_PHAST (R²=0.96) than with VHE_Rn (R²=0.76). Furthermore, vertical hydraulic conductivity (K_v) of the sediment-bed materials, calculated in situ by the permeameter test, indicated not only that K_v was up to 21 % higher in areas dominated by upward movement than at downwelling points, but also principle component analysis (PCA) demonstrated the dependence of K_v on porosity, VHE, and %sand of the stream-bed materials. This study provides evidence that vertical flux in the hyporheic zone is mainly affected by stream sinuosity and regional subsurface flow, and that the temperature method is more suitable than radon activity to quantify hyporheic exchange patterns.