Spatial Variability Of Streambed Vertical Seepage Fluxes In The Hyporheic Zone Of The Weihe River,Shaanxi Province

The quantitative study on the magnitude and spatial variability of streambed vertical water fluxes in the hyporheic zone has great significance for analyzing the interaction of groundwater and surface water.Furthermore,the differences of erosional and depositional conditions in distinctive stream morphologies can affect the connections betweeen groundwater and surface water and spatial variability of vertical water fluxes in the hyporheic zone.However,so far,there has been little study about streambed vertical water fluxes in different streambed morphologies.Hence,the in situ falling-head permeameter test was applied to measure streambed vertical hydraulic conductivity(Kv)and vertical hydraulic gradient(VHG)at three different stream morphologies(meander bend,anabranching channel and straight stream channel)in the Weihe River and one large tributary(the Beiluo River)in Shaarnxi Province.The vertical water fluxes through a streambed were determined using Darcian flux calculations based on the measurements of streambed Kv and VHG.Meanwhile,vertical sediment temperature profiles were measured with a verticalprobe located at different streambed depths along with in situ Darcy measurements at very close locations.The vertical water fluxes through a streambed were also determined using one-dimensional steady convective-conductive heat transport equation based on the measurements of vertical sediment temperature profiles.Hence,Darcian flux calculations and vertical sediment temperature profiles were used to assess the pattern and magnitude of hyporheic water exchange at three different stream morphologies.Then,we also observed and analyzed the influences of stream morphologies,streambed topography,streambed Kv and VHG,water depth,sediment physical property and the patterns of groundwater/surface-water interaction on the spatial variability of vertical water fluxes.The main conclusions of the study can be shown below.(1)In the Weihe River and its tributary,the direction and magnitude of vertical water fluxes based on the Darcian flux calculations and vertical sediment temperature profiles have obvious different spatial distribution.The values of vertical upward water fluxes based on the Darcian flux calculations and vertical sediment temperature profiles range from-4.1 mm/d to-427.6 mm/d,and from-20.5 mm/d to-132.6 mm/d at the tributary of the Weihe River,respectively.The values of vertical upward-water fluxes based on the Darcian flux calculations and vertical sediment temperature profiles range from-0.2 mm/d to-1915 mm/d,and from-67.4 mm/d to-321.7 mm/d in the Weihe River.The values of the upward fluxes derived from Darcian flux calculations show greater variability than the values of the upward fluxes derived from vertical sediment temperature profiles.(2)Despite the differences of flux direction and magnitude,flux directions based on vertical temperature profiles are in good agreement with results from Darcian flux calculations at the meander bend of the Weihe River and at the anabranching channel of its tributary.Also,Kruskal-Wallis tests show no significant differences between the estimated upward fluxes based on the two methods at each site of the tributary.Furthermore,the upward fluxes based on the two methods both show similar spatial distributions with streambed vertical hydraulic conductivity(Kv)or vertical hydraulic gradient(VHG)in the same channel.Generally,higher fluxes occurred at certain locations with higher streambed Kv or where a higher VHG were observed.(3)The upward fluxes based on the two methods show similar spatial distributions on the streambed at the same types of test channels(except for the anabranching channel).At the meander bend,higher upward water fluxes were observed from the center of the channel towards the erosional bank in the Weihe River or near the bank in the tributary.At the straight channel,higher upward water fluxes occurred near the center of the channel.(4)Streambed topography can cause variations of flux direction at the same channel.The magnitude of vertical water fluxes would not be affected by water depth,but affected by the physical property of streambed sediments(streambed grain size distribution and sediment structure).Also,at each test channel,upward flux can co-exist with downward flux at the same channel based on each method.This significant spatial variability of flux directions may be the combination of the regional-scale groundwater flow and local small-scale upward/downward flux induced by strambed topography in the hyporheic zone that work together.The sediment physical property can influence the streamed permeability and thus influence the magnitude of vertical water fluxes.The grain-size distributions of the sediments at each test channel are a major control in the spatial distribution of streambed Kv and vertical seepage flux.Furthermore,the differences in the sediment structure can also influence the magnitude of streambed Kv and vertical seepage flux.For the Weihe River,the streambed at the straight channel was mainly covered by fine silt and clay,streambed Kv values are all less than 0.4 m/d;in situ permeameter tests at the anabranching channel were carried out on a sandy streambed that was covered by a continuous clogging layer,and Kv values for the upper layer and lower layer are mostly lower than 0.9 m/d and 0.4 m/d,respectively;streambed sediment at the meander bend has higher contents of coarse-grained sand,the Kv values for the upper layer and the lower layer mostly range from 3.03 m/d to 34.9 m/d,and from 0.22 m/d to 21.03 m/d,respectively.The values of streambed Kv and vertical seepage flux at the meander bend are the highest among the three channels of the Weihe River.(5)Generally,at the straight channel and the anabranching channel of the Weihe River,the streambed Kv in the upper sediment layer was much lower than that in the sediment of the lower layer.At the meander bend of the Weihe River and one straight channel of its tributary,the streambed Kv in the upper sediment layer was much higher than that in the lower sediment layer.At the straight channel of the tributary,the average value and median value of streambed Kv for the upper layer are respectively 8.72 m/d and 7.51 m/d,and the average value and median value of streambed Kv for the lower layer are respectively 1.22 m/d and 0.43 m/d.At the straight channel of the Weihe River,the average value and median value of streambed Kv for the upper layer are respevtively 0.06 m/d and 0.04 m/d,and the average value and median value of streambed Kv for the lower layer are respectively 0.12 m/d and 0.10 m/d.At the anabranching channel of the Weihe River,the average value and median value of streambed Kv for the upper layer are respevtively 0.36 m/d and 0.05 m/d,and the average value and median value of streambed Kv for the lower layer are respectively 0.37 m/d and 0.13 m/d.At meander bend of the Weihe River,the average value and median value of streambed Kv for the upper layer are respevtively 14.98 m/d and 15.4 m/d,and the average value and median value of streambed Kv for the lower layer are respectively 3.76 m/d and 1.69 m/d.For each test site,the main effect in the difference between streambed Kv values from the upper layer and lower layer is due to difference in their sediment grain size distribution,which is mainly affected by the flow patterns of groundwater/surface-water interaction.However,local water upward and downward flow can relocate fine materials within two connected sediment layers and can even change their pore size,and eventually change their original streambed Kv.Hence,the flow patterns of groundwater/surface-water interaction and local upward/downward flow can vary sediment physical property and Kv in the upper layer and the lower layer of sediments,and make vertical water fluxes vary accordingly.