Experimental Study On The Influence Of Soil Unsaturated Zone On Groundwater Recharge By Rain Garden Concentrated Infiltration

To scientifically and reasonably store rainwater for groundwater recharge is of great significance for alleviating the pressure of urban drainage and creating a natural and harmonious water smart city.In the process of infiltrating rainwater with rain garden to recharge groundwater,the soil unsaturated zone is an important medium connecting surface water and groundwater.In this paper,a monitoring experiment was conducted using contralized infiltration for recharging groundwater with a rain garden in Yangzhou City,Jiangsu Province.Firstly,we studied the effect of contralized infiltration of rain garden on groundwater recharge under the influence of different depths of unsaturated soil layer,and the HYDRUS-1D model was applied to simulate the amount of groundwater recharge under different influence factors.Secondly,we analyzed the difference between soil water holding capacity and groundwater recharge as affected by different unsaturated soil zone characteristics.The research achievements are as follows:(1)In the process of contralized infiltration of rainwater in rain garden,the overall distribution of soil moisture in unsaturated area was controlled by soil texture,rainfall characteristics,infiltration amount of rain garden,initial soil moisture,meteorological condition and thickness of unsaturated soil layer.The moderate rain can wet the soil in the unsaturated zone with depth of lm,and the heavy rain can supply the soil water deficiency in the unsaturated zone with depth of 2.3m.The effect of secondary rainfall on recharge of groundwater varied greatly,which was influenced by the characteristics of rainfall and the depth of unsaturated soil layer.(2)During the 9-month measurement period,the groundwater level rose by approximately 2.5m.The rain garden used in the experiment had effectively recharged the groundwater.After the rain garden was used to effectively collect rainwater runoff,the rainfall runoff with small rain intensity can wet the soil in unsaturated zones or produce a slight recharge effect on the groundwater.Heavy rain and dense rain peaks could fully supplement soil moisture in the unsaturated zone.The recharge amount of soil moisture stored in the unsaturated zone with depth of 2.3m was about 7.3 times of that recharged to groundwater.About 12%of the rainwater runoff infiltrated into the unsaturated soil zone continued to infiltrate downward after rainfall event(3)The process of groundwater receiving infiltration exhibited a hysteresis feature when the depth of unsaturated soil layer exceeded 3.5m and rainstorm or torrential rain occurred,the time lag of groundwater recharge was generally more than 12h;when the depth of unsaturated soil layer exceeded 3m,common moderate rain in the study area was not able to effectively recharge groundwater;when the depth of unsaturated soil layer was less than 2.5m,moderate rain could effectively recharge groundwater.The time lag of groundwater recharge was shortened from more than 12h to 4h when the depth of unsaturated soil layer was reduced from 3.5m to 2m(4)The simulation results of HYDRUS-1D model indicated that the thickness of soil layer,the depth of plant rhizosphere and the initial soil moisture in the unsaturated zone of rain garden had different effects on groundwater recharge:(?)when the thickness of unsaturated soil layer was less than the rhizosphere depth,the groundwater recharge highly responded to the contralized infiltration of different amount of rainwater and to the groundwater evaporation;when the thickness of unsaturated soil layer was larger than the rhizosphere depth and less than the ultimate buried depth,the thickness of unsaturated soil layer increased,which weakened the groundwater evaporation and increased the groundwater recharge to a certain extent;when the thickness of unsaturated soil layer was larger than the ultimate buried depth,evident differences could be observed between the potential and actual recharge of groundwater by contralized infiltration over quantity and temporal scales.Groundwater receiving infiltration was delayed.With the increase of groundwater depth from 0.8m to 5m,the centralized infiltration recharge coefficient decreased from 0.64 to 0.2.The simulated the recharge coefficient was higher than the actual situation.(?)The rhizosphere depth of plants in the rain garden had little effect on the groundwater recharge by contralized infiltration.(?)Under the long term effect,the amount of groundwater recharge of rain garden by contralized infiltration groundwater had a positively linear correlation with the initial soil moisture;The amount of groundwater recharge in the process of secondary rainfall increased exponentially with the increase of initial soil moistureUsing rain garden to contralizedly infiltrate rainwater runoff,and giving full play to the regulation role of unsaturated soil area in this process can realize the scientific management of rainwater resources and effectively increase the storage capacity of groundwater.