Response Mechanism Of Groundwater Recession Law To Sponge City Construction

Groundwater recession law of river flood under changing environment is the key and difficult point of hydrological process simulation and prediction.In recent years,with the rapid urbanization process,a large number of impervious pavements have blocked the hydraulic connection between atmospheric water,surface water and groundwater,and changed the formation law of runoff and the disaster pregnant process of flood and drought,resulting in the coexistence and frequent occurrence of urban flood and drought disasters.The Sponge city is a kind of urban stormwater management concept,its purpose is to solve the urban water problem,Its purpose is to solve the urban water problem and balance the ecological relationship between people and water.The representative measures are to use permeable pavement measures to replace the original impervious land surface and gray building group,and play the role of vegetation,soil,river and lake water system in the accumulation,infiltration,purification and slow release of urban rainwater runoff,and restore groundwater recharge.Pingxiang City in Jiangxi Province is one of the first batch of sponge city construction pilot projects in China.The evaluation of the impact of sponge city reconstruction measures on groundwater recession law can provide scientific basis for groundwater regulation and management.In this paper,the distributed urban hydrodynamics model is improved.Taking the flood recession of Wufeng River Basin in sponge city of Pingxiang City as the research object,base flow segmentation,scenario simulation and orthogonal test sensitivity analysis are used to evaluate the influence of sponge city construction on groundwater recession process,and reveal the cause analysis of groundwater recession law on sponge city construction.The main results of this study were shown as follows:(1)Taking the groundwater recession process in the Wufeng River Basin of Pingxiang City as the research object,and based on the actual recession process,the runoff components of each flood are divided,the runoff segmentation method and groundwater recession curve model are optimized,and the groundwater recession law before and after the construction of sponge city is analyzed.The results show that:after the construction of sponge City,the time history tendency rate of standard recession curve,peak discharge and total runoff of the basin decreased by 60%,9.2%and 43.6%,respectively.The underground runoff and its proportion to the total runoff increased by44.6%and 60.4%respectively,and the discharge at the base flow separation point increased by 65.2%and ahead of time,Based on the discharge,the relative error of annual mean value and variation coefficient of base flow before and after the construction of sponge city by Pettitt method is the smallest,with the average error of1.7%and 5%compared with other nonparametric testing methods.Set pair analysis has the largest connection degree,which is a suitable non parameter test method for checking the base flow separation point in the process of flood recession in urbanization basin,the simulation results of rational function regression curve model are close to the measured values in mean value and coefficient of variation,NSE and consistency index D are the largest,which are 82.1%and 94.914%,respectively;nrmse and cumulative relative errors are the smallest,which are 13%and 106.4 respectively.The results show that the deviation between the simulation results of the rational function regression curve model and the measured value is small,and the accuracy and credibility are high;the NSE between the simulation value and the measured value of the coupling rational function regression curve and the distributed urban hydrodynamics model is between0.57 and 0.99,with an average value of 0.83,indicating that the improved distributed urban hydrological and hydrodynamic model has a good simulation effect,and can be used for basin groundwater simulation research.(2)Taking the influence of sponge city construction on groundwater recession law as the research object,a 24-hour rainstorm scenario with different return periods(5years,10 years,20 years,50 years and 100 years)and rain pattern coefficient(r=0.3,r=0.5 and r=0.7)was designed,and the groundwater recession process before and after the construction of sponge city was simulated by improved distributed urban hydrodynamics model.The results show that:after the construction of sponge City,the runoff coefficient of design rainstorm scenario set is reduced,and the decrease range is39.7%to 52%,with an average decrease of 46.2%;the initial discharge and recession time of underground runoff increase from 1.6%to 297.8%and 3 to 51.7%respectively,with an average increase of 96.3%and 23%respectively Before the construction,the decrease range is between 0.6%and 33%,with an average decrease of 13.5%,the total runoff and the groundwater withdrawal rate of the return section of the flood are reduced by 13.4%to 57.6%and 1.6 to 45%respectively,with an average decrease of34.7%and 26%respectively.The underground runoff and its proportion in the total runoff increase from 11.5%to 268.8%and 12.8 to 66.6%respectively,3%and 34.9%respectively,compared with before construction,the spatial mean and runoff coefficient of underground net rainfall increased by 4.5?46%and 24.4%on average.The spatial variation coefficient of net underground rainfall decreased by 4.9?63.7%with an average decrease of 37%,indicating that the net underground rainfall increased significantly compared with other areas,which was close to the natural agricultural landform in the upper and middle reaches of the basin The spatial heterogeneity of underground net rainfall decreased.The reason is that the source reduction,process blocking and terminal regulation and storage functions of sponge facilities reduce the surface runoff generation capacity,enhance the underground runoff generation capacity,and slow down the confluence process.(3)Taking the response of groundwater recession law to sponge city as the research object,a 24-hour design rainstorm process with return period of 5 years,rainfall pattern coefficient of r=0.3,r=0.5 and r=0.7 was selected,Based on the hydrological and hydrodynamic parameters of sponge city before and after the construction of sponge City,the sensitivity scheme of 3 factors and 5 levels was constructed by orthogonal test method,and the groundwater recession process of field flood under different factors and level combinations was simulated by improved distributed urban hydrodynamics model.The results show that:the initial discharge and runoff of groundwater in Wufeng River Basin are positively correlated with CN value and stable seepage rate when CN value is small,negative correlation when CN value is large,insensitive to stable seepage rate?3 mm h^-1,and no significant change in surface roughness,spatial mean of underground net rainfall,stable infiltration rate and surface roughness are positively correlated,but have no significant change in CN value.One of the main reasons for the decrease of flood peak discharge,the slowing down of groundwater recession rate,and the increase of spatial mean value of underground net rainfall and the proportion of underground runoff to total runoff in Wufeng River Basin are caused by the functions of source reduction,process blocking and terminal regulation and storage of sponge facilities.The reason is that the function of sponge facilities reduces the CN value,increases the steady infiltration rate and increases the surface roughness.Among them,the sensitivity of groundwater initial discharge,groundwater runoff and groundwater net rainfall to the decrease of CN value and the increase of steady infiltration rate are higher,and the change of CN value and stable seepage rate is the main driving factor of groundwater recession law.