Study On Spatiotemporal Variation Of Water Exchange In The Hyporheic Zone Of Manas River

In the northwest Inland areas of China,rainfall is rare,evaporation is strong,and the conversion of rivers and groundwater is frequent.As an important link between rivers and groundwater,the hyporheic zone has unique important functions such as hydrological regulation,environmental buffer and ecological protection.The study on the spatial-temporal change of water exchange in the hyporheic zone provides data support for scientific management and sustainable development of the river-groundwater system in the Manas River basin.Meanwhile,the internal hydrodynamic and temperature fields drive the mixing of river water and groundwater as well as the transfer and transformation of energy,which become the basis for the in-depth hydrogeochemical study of the hyporheic zone.This article selects the manas river overflows with typical river downstream high space-time resolution section as the test point,through field in-situ test and laboratory test,based on the hydraulic gradient method with temperature gradient method comparative analysis research of manas river before and after the flood,the hyporheic zone with spatial and temporal variation characteristics of water exchange,reveal the hyporheic zone before and after the flood with the internal thermal migration rule,different sex and applicability of the two methods are discussed.The main research results are as follows:(1)The water exchange capacity of the hyporheic zone in the non-flood period calculated by the hydraulic gradient method is 0.05?0.95m~3/m~2/d,and the water exchange capacity in the flood period is 0.92?4.75m~3/m~2/d.The range of water exchange capacity in non-flood period calculated by temperature gradient method was 0.05?3.46m~3/m~2/d,and that in flood period was0.11?17.31m~3/m~2/d.The results of hydraulic gradient method and temperature gradient method are consistent in the direction of water exchange in the hyporheic zone.The water exchange volume calculated by hydraulic gradient method in non-flood period is obviously lower than that calculated by temperature gradient method on the whole.The water exchange volume calculated by hydraulic gradient method in flood period increases rapidly in a short time.Compared with the water exchange volume calculated by temperature gradient method,hydraulic gradient method is more sensitive to the change of water level.(2)In the non-flood stage,the river water level is relatively stable,the shallow water head of the riverbed is larger than the deep water head,and the flow direction is overall downward.With the arrival of the flood,the river level rises rapidly in a short time,and the rise amplitude is larger than the groundwater level,and the rise amplitude of the groundwater level has a certain lag.Compared with the non-flood period,the riverbed temperature in the flood period decreases as a whole,and the temperature variation is smaller,and the riverbed temperature changes more uniformly as a whole.The central temperature is lower than that on both sides of the river,and the central temperature variation is smaller than that on both sides of the river.During the whole monitoring period,the riverbed surface and below 30cm is the layer with strong temperature change,the riverbed below 30cm?50cm is the layer with weak temperature change,and the riverbed below 50cm?100cm is the layer with stable temperature.(3)The comparative analysis was made between the left bank(sedimentary bank)of the riverbed section and the right bank(eroded bank)of the riverbed section.In the whole monitoring depth,the exchange capacity of eroded bank water was significantly higher than that of sedimentary bank water;the change of water quantity in the center of the river was more stable than that of the two Banks;water exchange in the hyporheic zone was positively correlated with the river level to some extent.The exchange intensity of the subsurface hyporheic zone fluctuates greatly and the water exchange capacity of the hyporheic zone is relatively stable with the increase of vertical depth.There is heterogeneity in the water exchange in the hyporheic zone and the vertical direction does not change monotonously with the increase of bed depth.(4)As a traditional water exchange calculation method,the hydraulic gradient method is widely used to calculate the water exchange of the hyporheic zone under the condition of accurate determination of river bed permeability coefficient and vertical hydraulic gradient.As a new and accurate calculation method of water exchange,the temperature gradient method is more suitable especially in the strong and weak variation layers of river bed temperature,where the amplitude ratio phase difference changes obviously.Based on hydraulic gradient method,the volume of water exchange in the hyporheic zone usually increases with the increase of river level and river bed permeability coefficient.Based on the temperature gradient method,the volume of water exchange in the hyporheic zone decreases with the increase of the volume heat capacity of water and the thermal dispersion and thermal conductivity of riverbed sediments,and increases with the increase of the porosity and volume heat capacity of riverbed sediments.The sensitivity of water exchange parameters is the volumetric heat capacity of water>Sediment particle volume heat capacity>Fluvial sediment porosity>Thermal conductivity>Thermal dispersion of sediment particles in riverbed.