Compounding effect of meandering degree and vegetation density on hyporheic residence time in riparian zone

Hyporheic linkages between riparian zones and stream channels are a key component to riparian and stream system, which greatly influence the solutes transportation and transformation that are fundamental to sustaining biogeochemical cycling, biological community, ecological processes and habitat. The extent to which transportation and transformation processes occur is greatly dependent on the hyporheic residence time (HRT). However, our understanding of how this HRT mechanisms is still poorly understood. To explore the mechanism of HRT in riparian zones, we designed an experimental installment, a two-way flow riparian zone model, to evaluate the rate at which an active hyporheic flow developed. Using sodium chloride (NaCl) as conservative solute tracer, tracer experiment was conducted to explore compounding effects of meandering degree and vegetation density on HRT in riparian zone. As vegetation in riparian zones has the potential to fast the hyporheic exchange, it had resulted in the decreasing of the HRT with vegetation density increasing. But vegetation density had a critical value. When the vegetation density was larger than the critical value, the HRT no longer reduced. Over the entire riparian zone, the influence of morphology had apparent mirror-like effect on the distribution of HRT which depended on the position of meandering topography of riparian zone. The mean HRT of the upstream face was about 1/2 of that of downstream face. What's more, hydraulic, vegetation, morphology and soil features had been identified as the key influencing factors of HRT by using dimensional analysis and their sensitivities had been valued with the method of principal components analysis (PCA). Together, the meandering amplitude of topography and the vegetation space were the most sensitive parameters because their total contribution rate reached 91.07%. Under the action of the both factors, it appeared promotional effects on the HRT of the upstream face of meandering wave while there existed not only the promotional effects but also inhibition effects on the HRT of the downstream face of meandering wave.