| Riparian areas possess a high level of species diversity,which is meaningful for sustaining a healthy ecosystem.When species degenerated,its ecological function would reduce.Previous studies mainly focused on the effect of river-related process on the distribution of riparian vegetation,both in the longitudinal dimension and lateral dimension.However,in recent years,more and more inland rivers in arid areas have been regulated at home and abroad.River diversion has changed the hydrological process of natural river,reduced the flood frequency,river water flux and river water permanence,result in reduced the disturbance and water supply to riparian plants.The riparian plants mainly rely on groundwater.However,for regulated rivers in arid areas,the effect of water-table depth on riparian species,its threshold,riparian groundwater dynamic process and the spatial distribution of plant composition and diversity in riparian zone are still lack of in deep study.This paper systematically explored the controlling factors of dynamic process of groundwater in riparian zone and its influence on groundwater attribute heterogeneity.We demonstrated and revealed the relationship between the lateral and longtitutal zonation of riparian plants and river-groundwater interaction.Gaussian distribution model and general additive model were used to assess the threshold of water-table depth in the riparian zone of the middle and lower reaches of Manasi River.Numerical simulation was used to reveal the characteristics of groundwater dynamic field at regional scale.Finally,we put forward suitable river diversion scheme according to the mechanism of river-groundwater interaction and the statistical relationship between plant distribution and groundwater depth for riparian vegetation protection and restoration.The main contents and conclusions are as follows:(1)The relationship between groundwater attribute and species distribution and the contribution of each environment factors on species are analyzed by canonical correspondence analyses.Results showed that:the water-table depth and TDS were the major environment factors controlled the species distribution.The species distribution was mainly along the direction of groundwater table depth and TDS gradient.When groundwater salt was low,the species mainly changed from hydrophyte to mesophyte to xerophyte along the increase in groundwater direction.When the groundwater salt was high,halophyte species mainly existed.(2)The relationship between water-table depth and plant appearance frequency and diversity index were studied by lognormal distribution model and general additive model.The results showed that given the results of the logarithm distribution model,the groundwater table depth appropriate for herbs was 1–1.5 m,while that for shrubs was 2–4m,and groundwater table depth less than 6 m satisfies the growth requirements of major species;species diversity peaked at the groundwater table depth of 2-3 m,3-5 m,and2.0-4.0 m for herbs,shrubs and all of the species,respectively;the frequence in appearance of Phragmites communis and Tamarix chinensis was not as sensitive to depth to water table.To reconstruct the riparian zone,Phragmites communis and Tamarix chinensis could be planted in a groundwater table depth of less than 3 m and 2-5 m,respectively.(3)The development,controlling factors of groundwater flow system and the formation and evolution of hydrochemical types in regional scale are analyzed based on the methods of geological analysis,numerical simulation and hydrochemical isotope analysis.The results showed that:under the control of geomorphology,text structure and lithologic structure,a local-regional nested groundwater flow system was developed within the depth of 400 m below the riverbed.The groundwater flow velocities are 0.1–1.0 m/d and<0.1 m/day for the local groundwater flow system and regional groundwater flow system respectively.The primary groundwater hydro-chemical type takes on apparent horizontal and vertical zoning characteristics,and the TDS of the groundwater evidently increases along the direction of groundwater flow,driven by hydrodynamic processes.(4)The hydrodynamic process of riparian groundwater and the driven factors of spatial-temporal change of river-groundwater relationship along the Manasi River was analysed by means of data collection,geomorphic analysis,numerical simulation and field dynamic observation.The results show that:Geomorphic configurations,lithological structure,river diversion and upland groundwater level could affect and alter the hydrodynamic process of riparian groundwater and river-groundwater relationship.Hydrological process of riparian groundwater was different as river-groundwater relationship changed,which induced riparian groundwater attribute heterogeneity.The basic types of river groundwater relationship could be divided into five basic types,which are:Type 1:the river and groundwater were disconnected during water conveyance and lost flux exchange when water delivery ceased.Type 2:the river discharge groundwater independent to water conveyance.Type 3:river recharge groundwater during the period of highest water flow and discharge groundwater during the period of low water flow;when water delivery stopped,the groundwater discharge river or groundwater discharge occurred at the riverbed.The extent of groundwater strong fluctant zone was 0-120 m away from the river while the groundwater was not affected by river when the distance from the riverbank was more than 500-650 m.Type 4:the river recharge groundwater during water delivery,when water delivery stopped,no groundwater discharge took place.The extent of groundwater fluctant zone was 0-30 m away from the river.Type 5:no river conveyance throughout the year,where the groundwater discharge river or groundwater discharge occurred at the riverbed.(5)The vertical and horizontal zonation characteristics of species composition and diversity were studied by means of quadrat survey and field investigation.The results showed that:In the view of longitudinal dimension,the species composition was significantly different and herbs changed mainly from xerophyte to hygrophyte to mesophyte to xerophyte to halophyte while shrubs changed mainly from xerophyte to mesophyte to mesophyte and xerophyte to Halophyte with the alteration of the river-groundwater relationship.hygrophyte herbs with a high level of diversity were found at the potential sink position of local groundwater flow system.Species changed from hygrophyte herbs and mesophyte shrubs to mainly halophytes species with low species diversity were found at the potential sink position of regional groundwater flow system.Hygrophyte and mesophyte herb species and mesophyte and xerophyte shrub species were mainly grow where existed river diversion and groundwater discharge from upland with a high level of diversity.The species met water stress and degenerated to mesophyte and xerophyte plants with low species diversity as upland groundwater drawdown.The species degraded to halophytes species with low species diversity when the river diversion stopped all year round.In the view of lateral dimension,When the riparian zone had groundwater discharge and river water delivery,hygrophyte herb species were mainly located 0-120 m away from the river edge,mesophyte herb species were mainly located at 250-500 m away from the river and mesophyte and xerophyte-semixerophyte herb species were mainly located near the upland.When the riparian zone had groundwater discharge but no water delivery,hygrophyte herb species were mainly located 0-30 m away from the river edge and halophyte herb species were located in the remaining areas.When the riparian zone had river water delivery but did not have groundwater discharge,hygrophyte herb species were mainly located near the river edge and xerophyte-semixerophyte herb species were located in the remaining areas.The shrub species had no obvious differences in the lateral dimension.Mesophyte and xerophyte-semixerophyte shrub species were mainly in the riparian zone with river water delivery while halophyte shrub species mainly existed in the riparian zone without river water delivery.(6)Field dynamic observation and numerical simulation were used to define the boundary of riparian zone.The results show that the extent of riparian zone depends on the relationship between river and groundwater.When the river and groundwater was disconnected,the riparian zone is controlled by the river,and the boundary of the riparian zone is the physical boundary within the range of 0-1000 meters included the first and second terraces.When the riparian zone is controlled by the river-groundwater interaction,the extent of riparian zone depended on the amount and duration of water diversion.Under the existing water conveyance mode,the riparian zone ranges from 0 m to 250 m.When the groundwater recharges the river all year round,the riparian zone is controlled by groundwater and is not affected by the river flow regime.The range of riparian zone is unlimited or 0-30 meters away from the river bank.(7)The three-dimensional saturated groundwater flow model in regional scale and the profile two-dimensional saturated-unsaturated groundwater flow model of river-riparian groundwater system are constructed to simulate the change in riparian water-table depth under different water diversion schemes based on the aim of protection and restoration of riparian plants.The results showed that when the river seepage increased from 8000*10~4m~3/a to 8500*10~4m~3/a,9000*10~4m~3/a,10000*10~4and 2000 m~3/a,the average water-table depth within groundwater overflow zones decreases by 0.5 m,0.8 m,1.3 m,and 3.6 m respectively.The decrease of water-table depth in overflow zone played an important role in maintaining the growth of wetland herbaceous plants and the stability of wetland area.If the river stage maintained at 2 meters,the water-table depth within 0-100 meters away from the river can be effectively reduced by continuous water diversion for 200 days,which is conducive to the restoration of degraded xerophytic herbs and xerophytic shrubs to hygrophytic herbs,mesophytic herbs and mesophytic shrubs. |
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