Study On The Impact Of Water Level Changes In The Hanjiang-to-weihe River Valley Water Diversion Project On Water Bodies And Soil Environment

The Hanjiang-to-Weihe River Valley Water Diversion Project is a large inter-basin water transfer project in Shaanxi Province,whose main function is to intercept and transfer water resources from the Han River to the Guanzhong region in order to alleviate the contradiction between water scarcity and economic development in the receiving areas along the route.Many studies have found that the periodic changes in water levels after the reservoir is built can cause massive die-offs of plant communities in the reservoir zone under the stress of multiple factors such as flooding and oxygen deprivation,and reverse succession of higher to lower plants can occur.The process of these changes will be accompanied by changes in the cycling process of inorganic elements such as nitrogen and phosphorus as well as organic substances such as dissolved organic carbon and organic nitrogen,making the reservoir water quality worse and eutrophication of the water body occurring.As a result,the reservoir water ecosystem and the ecosystem of the fading zone face a series of ecological and environmental problems.Moreover,the changes that occur in the ecological environment of reservoirs in different regions and the risk of being polluted are unique.It is therefore necessary to gain an in-depth understanding of the impact of the construction and operation of the reservoirs of the Hanjiang-to-Weihe River Valley Water Diversion Project on the reservoir water ecology and the wetland ecosystems in the fading zone,so as to provide a scientific basis for the project management department to formulate targeted environmental management strategies.The main findings of this study are as follows.(1)In September 2020,a space-for-time approach was used to compare the differences in water quality,plant communities and plankton between the Huangjinxia Reservoir and Shiquan Reservoir to predict changes in the ecosystem after the completion of the Huangjinxia Reservoir.The results show that after the establishment of the dam,water quality will be significantly altered due to changes in hydrological conditions and changes in environmental indicators.The dominant algae in the natural water column of the Huangjinxia reservoir are Diatom.After the construction of the reservoir,the physicochemical properties of the water column will change significantly,causing the dominant algae to change gradually from Diatom to Cyanobacteria,green algae and methanogens.After the construction of the reservoir,the species and biomass of small flea-like zooplankton such as branchiopods and copepods will also increase significantly due to the change in the physicochemical properties of the water body.(2)In June 2021,the physical and chemical properties of soils along the Sanhekou Reservoir and the distribution patterns of inorganic phosphorus were investigated as baseline information for the soils of the Sanhekou Reservoir,and the inorganic phosphorus fluxes stored in the soils along the Sanhekou Reservoir were predicted.The results show that all soils along the Sanhekou Reservoir have the same distribution pattern of phosphorus,with Ca-P being the most abundant,Fe-P and Al-P being more abundant and not very different,and O-P being the least abundant.Based on the distribution of soil phosphorus morphology,the Bio-P flux in the Sanhekou reservoir area was estimated,and the phosphorus reservoir capacity of the whole reservoir area could reach 2.12×10~4 t.Combined with the construction planning and phosphorus capacity of the Sanhekou reservoir,the prediction was made that if all the released phosphorus was kept in the water body,a release rate of 23%would make the reservoir water quality lower than the environmental quality standard of surface water category III.(3)The results showed that the physical and chemical properties of soil planted with Cynodon dactylon,Alternanthera philoxeroides and Calamus varied considerably depending on the growth characteristics of the plants at a flooding depth of 1 m or less.In general,the Cynodon dactylon is still able to use nutrients such as nitrogen and phosphorus when submerged in shallow water and has a good capacity to adsorb these nutrients,and the soil with the root is heavily depleted of nutrients when it falls dry.Caladium,as a water-holding plant,grows well when flooded by 1 m,but after it has fallen dry,growth is inhibited by the lack of water.Alternanthera philoxeroides may die and degrade when inundated with 1 m of water,resulting in elevated nutrients such as nitrogen and phosphorus,but after it has dried out it may regenerate and consume large amounts of nutrients.Soil microbial load increases with increasing water depth when subjected to inundation within 1 m of water depth.After one inundation-fall-dry cycle,the structure of the soil microbial communities of non-flooded and flooded soils diverged considerably at the generic level.The above studies show that there is a risk of ecological deterioration of the water and WLFZ after the dam is built,due to the hydrological conditions of the water body and the rise in water level.At the same time,the physical and chemical properties and the survival of the soil in the flooding environments indicate that calamus and dogwood are suitable for planting within the 1 m inundation zone to prevent ecological degradation of the floodplain.