| With the implementation of the large-scale "Grain for Green" project and the rapid development of the economy in the Loess Hilly and Gully Region,food security and living environment problems caused by the shortage of high-quality farmland and urban land has seriously affected the sustainable development of local society and became a hot spot of social concern.To develop the local land potential,a large-scale Gully Land Consolidation(GLC)project has been carried out in the Loess Hilly and Gully Region.In view of the water system imbalance disasters such as control engineering piping,uneven settlement of newly built land and salinization in the process of GLC,we selected different typical GLC gully watersheds as the research objects in this study and investigated the response process of gully water system balance to typical GLC based on the "Watershed self-response theory" and combined with field investigation and indoor physical and mathematical model simulation.Then,we used the corresponding results to demonstrate the safety regulation and application of water system imbalance disasters in the GLC watershed.We have achieved the following main results:(1)Perfection of " Watershed self-response theory ".The GLC project in Loess Hilly and Gully Region is an important part of watershed water system regulation.According to the " Watershed self-response theory ",the elements in the watershed system are interrelated and moving,and the goal of movement is to pursue the balance of the system.The balance is relative,and the imbalance is absolute,when the system is affected by external factors and the system balance is destroyed,the watershed system will automatically develop in the direction of establishing a new balance.This study shows that the water system in the watershed is balanced on average for years,when the water system elements are disturbed,such as land remediation,cutting slopes,burying gullies and other human activities,the water system balance is broken,and the water system in the watershed will automatically adjust to adapt to the balance.In the new adjustment process,without reasonable regulation,there will be a series of disasters caused by water system imbalance,such as water outflow point higher when cutting high and steep slopes,piping and salinization of gully control projects caused by landfill.In this study,the hydrodynamic elements in the process of balanced movement of water system are simulated and regulated by building indoor physical and mathematical simulation model,and applied in practice,thus we perfected the mutual feedback mechanism between water system change and GLC in the " Watershed selfresponse theory ".(2)Impacts of line-type GLC on water system balance.According to the indoor solid model based on " Watershed self-response theory ",under the conditions of simulating the upper engineering loess landfill of 0.1 m,the lower coarse sand landfill of 0.9 m,the groundwater depth of 0.6 m,and the total rainfall of 120 mm,we can conclude that compared with the undisturbed gully with bare slope,the gully with bare slope terrace,vegetation terrace,straw mulching terrace and 60% GLC with bare slope could reduce surface runoff by 25.78%,45.51%,62.40% and 42.1% on average,respectively.Indicating that with the increase of the proportion of gully soil and water conservation measures,the transformation of surface runoff in the gully water system decreases,while the transformation ratio of soil water and groundwater increases.Under the same simulated gully and rainfall conditions,with the increase of rainfall intensity from 45 mm/h to 120 mm/h with the step of 15 mm/h,the groundwater conversion of the undisturbed gully with bare slope,the gully with bare slope terrace,vegetation terrace,straw mulching terrace decreased by 27.2%-53.3%,3.9%-13.7%,and 27.9%-33.3% respectively.However,the recharge amount of groundwater in 60%GLC with bare slope has little change,which indicated that the GLC could significantly intercept storm runoff and transform it into gully groundwater.(3)Impacts of line-type GLC on water system balance under more indoor conditions that difficult to achieve.Based on the simulation results of indoor solid model,we constructed,calibrated,and verified the HYDRUS-3D and Visual MODFLOW models for investigating the impacts of line-type GLC on water system balance under heavy rain intensity and thickened loess landfill,which were difficult to carry out in indoor tests.The results showed that under the conditions of simulating the lower coarse sand landfill of 0.9 m,the groundwater depth of 0.6 m,and the total rainfall of 120 mm,when the rainfall intensity increased from 30 mm/h to 150 mm/h,the average groundwater level under the GLC gully decreases by 6.24%,and when the upper engineering loess landfill increased from 0.1 m to 0.4 m,the average groundwater level decreased by 13.62%.Which indicated that the increase of upper engineering loess landfill weakened the groundwater transformation more than the increase of rainfall intensity.Therefore,in the GLC gully with deep loess landfill,it is necessary to regulate the water system,for increasing the transformation of groundwater and avoid the disaster caused by long-term accumulation of surface runoff.(4)Impacts of basin-type GLC on water system balance.We used hydrological analogy,satellite monitoring images,and the establishment of Visual MODFLOW model for the impacts of basin-type GLC on groundwater to investigate the impacts of basin-type GLC on water system balance in Yan’an New Area.The results showed that due to the land surface compaction and the reduction of the relative height difference between the surface compaction and the slope,the maximum peak discharge at the outlet of Qiaoergou watershed where Yan’an New Area located is 6.16-9.24 m/s under the condition of daily rainfall of 40-60 mm,and the average total amount of surface runoff after the rainfall is 3.04 times that before the GLC.Therefore,it is necessary to pay special attention to the risk of excessive surface runoff caused by the basin-type GLC.At the same time,due to the continuous soil and water conservation,urban greening,artificial irrigation,gully landfill and other reasons,the volume moisture content of surface soil in Yan’an New Area increased from 0.102 to 0.163.The numerical model simulation showed that the groundwater distribution in the excavation area was reduced and in the filled area was concentrated.And there is a concentrated area of high and steep slopes with a height of about 100 m around the basin-type GLC watershed,where groundwater activities are frequent and there is a high probability of water system imbalance disasters.Setting blind ditch for groundwater drainage in the concentrated area of high and steep slope could reduce the groundwater level by about26 m and could reduce the negative impact caused by frequent groundwater activities.(5)Regulation and prevention of water system imbalance disaster in GLC watershed.Aiming at the difficulty in measuring gully erosion caused by the imbalance of water system in the GLC watershed,we developed a method for measuring the characteristic parameters of gully erosion by using satellite images,which could achieve 97.4% accuracy in measuring the loess incised gully and 91.1% accuracy in measuring the amount of breaking regulation dam in line-type GLC gully.This method could meet the needs of disaster investigation in GLC.The results of indoor test and simulation showed that,under the same rainfall intensity,the design flood standard of gully regulation dam can be improved 65.6% by 60% proportion of GLC.At the same time,we optimized the design of preventing salinization and piping with blind ditch for groundwater discharge.And the demonstration application showed that it can reduce soil moisture by 46.81% and the maximum soil conductivity decreased by 15.41μs/cm,and the effect of preventing salinization and piping erosion was excellent.Under the condition of rainstorm with daily rainfall of 120 mm,the intact rate of GLC had increased by more than 80% in the watershed with dike consolidation and flood protection project,which showed that our research results can effectively control and prevent the water system imbalance disaster in the GLC area. |
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