Variability Of Hydraulic Conductivity And Water Exchange In The Hyporheic Zone At A Natural Channel Bend

Hyporheic zone has an irreplaceable role in mintaining the stability of stream ecosystem,by the exchange of material and energy between stream water and ground water namely hyporheic exchange.Water exchange and hydraulic conductivity in the hyporheic zone are critical for the re lationship and quality of stream and ground water.The aim of this study is to demonstrate the variability of water exchange and hydraulic conductivity in the hyporheic zone temporally and spatially.It has an important ecological significance of assessment,development and utilization reasonably of water resources,and the fate of stream life.The field experiments were conducted at the bends and their upstream and downstream of Beiluo River.The vertical hydraulic conductivity and hydraulic gradient were determined by the in-situ falling-head permeameter tests,and then hyporheic water fluxes were calculated through the Darcy's law.And empirical equations were adjusted by the particle analysis method.The study results were summarized as follows:(1)The streambed sediments vertical hydraulic conductivity suggested that significant variability,especially in July 2015.Changes in vertical hydraulic conductivity were as follows temporally:July 2015>July 2016>January 2016,that is,vertical hydraulic conductivity in summer was higher than that in winter;upper reaches>lower reach;the center of the channel>the left bank>the right bank.The vertical hydraulic conductivity decreased firstly and then increased in the flow of the stream water,and higher hydraulic conductivity transferred from the deposition bank of the upper reaches to the erosion bank of the lower bank in January 2016.(2)The downward fluxes were one of the main water exchange pattern,with the occurring of certain upward fluxes at the center of the channel and its vicinity.The variations of water exchange capacity in the hyporheic zone were as follows temporally:July 2015>January 2016>July 2016;the large water exchange capacity occurred in the center of the channel and the changes in hyporheic water exchange was different and complex in each study period.Hyporheic water exchange capacity had an increasing trend in the flow direction of stream water.(3)The composition and distribution of streambed sediment is key factors controlling vertical hydraulic conductivity.There were more sand and less slit or clay,corresponding to larger median grain size in summer than winter.That resulted in the variations of vertical hydraulic conductivity in time and space.(4)Hyporheic water exchange showed significant change across the channel bend due to two variables,vertical hydraulic conductivity and hydraulic gradient.And most large values of vertical hydraulic conductivity were found at the upwelling area,conversely,small values of vertical hydraulic conductivity occurred at the downwelling area.