Effects Of Shallow Groundwater Depth On Vegetative Filter Strips Retarding Transport Of Nonpoint Source Pollution (Nitrogen And Phosphorus):Experiments And VFSMOD-W Modeling

Agricultural Non-point Source Pollution severely threaten ecological environments nd water resources,and restricts the sustainable development of the agriculture.Agricultural Non-point Source Pollution is extremely hard in management because of the uncertainty of emission time and frequency,the complexity of the occurrence mechanism,the spatial differentiation of the contamination and the difficulty of the simulation and control.Therefore,how to control agricultural non-point source pollution scientifically and effectively has become an important scientific problem that need to be solved urgently in China.Vegetative filter strips?VFS?,defined as vegetative areas situated downslope of agricultural lands,has been recommended by the US Department of Agriculture as the Best Management Practice?BMP?to control the non-point source pollution.In China,the research and application of VFS in riparian zone are still in the exploratory stage,and the effects of shallow groundwater depth on VFS retarding transport of non-point source pollution still need further research,which provides reference for the effective control of agricultural non-point source pollution.In this study,the typical non-point source pollution area in Jianghan Plain was chosen as a study area.In order to investigate the transport mechanism of runoff,sediment,and non-point source of nitrogen and phosphorus pollutants in VFS under the condition of shallow groundwater table,this study conducted the controlled flume experiments and the field test of the VFS in the typical area.The combination of the VFSMOD-W model and simple transport model of phosphorus and nitrogen was used to simulate the VFS performance for retarding the transport of runoff,sediment,and non-point source of nitrogen and phosphorus pollutants.With this study,we can develop the VFS configuration with the effect of highly effective pollutant control,which can be applied to Han River Basin,and then provides reference for scientific management and pollution restoration of comprehensive prevention and control of agricultural non-point source pollution.The results are listed as following:?1?The controlled flume experiments were used to quantify the effect of a shallow groundwater table?the depths were 0.08 m,0.22 m,and 0.36 m,respectively?on VFS efficiency for retarding the transport of runoff,sediment,phosphorus,nitrogen and bromide.Combined with the bromide tracer method,we want to analyze both the similarities and differences in the transport behavior of phosphorus,nitrogen and bromide in the surface runoff and subsurface lateral flow,and illustrate the transport mechanisms of phosphorus,nitrogen and bromide in the subsurface lateral flow.The study found that the deeper groundwater table,the smaller vegetation spacing,the lower slope and the smaller rainfall intensity were more favorable for the VFS efficiency for retarding the transport of runoff,sediment,phosphorus,nitrogen and bromide.The retention efficiencies varied greatly from 27%,62%,39%,21%and 33%to 96%,98%,96%,95%,and 94%for surface runoff,sediment,phosphorus,nitrogen,and bromide,respectively.The groundwater depths?0.08 m,0.22 m,and0.36 m?distinctly influenced the hydrologic responses,and the normalized surface runoff outflow rate and the normalized subsurface lateral flow rate fitted well?R²=0.95?.A power equation was used to describe the relationship among the sediment yields,runoff volume,and peak flow rate for each runoff treatment(Sed=6E+06·?Q·Q_p?^0.93).Multiple linear regression models were developed for quantifying the TP,TN,and bromide trapping efficiency of the VFS,the predictive equations were based on the runoff retention efficiencies and sediment retention efficiencies(TP_ret=54.5+0.76·Q_ret–0.32·Sed_ret,R²=0.98;Br^-_ret=-12.4+0.73·Q_ret–0.40·Sed_ret,R²=0.95;TN_ret=-49.33+0.34·Q_ret+1.06·Sed_ret,R²=0.95).In surface runoff outflow,the average normalized concentration of TP and Br^-were less than 1,which decreased with the decrease of groundwater depth and grass spacing.In addation,the average normalized concentration of TP and Br^-increased with the increase of slope and rainfall intensity.However,the normalized TN concentration decrease as a power function?C/C₀=a×t^b?,and the normalized concentration?1.The transport of phosphorus,bromide,and nitrogen in the subsurface lateral flow exhibited breakthrough behaviors during each runoff experiment,and the normalized concentration in the increasing limbs can be described by power equations?C/C₀=W×t^E?.The power equations described the adsorption,desorption,and/or interception of nitrogen,phosphorus and bromine by soil,organic acids secreted by the plant roots,or iron oxides.?2?Establishing the VFS study area in the typical non-point source pollution area Maozui Town in Tongshun Rver Basin.The study analyzed the effects of the filter length on VFS efficiency for retarding the transport of runoff,sediment,and nitrogen with the presence of groundwater table?the depth>1.8 m?,and the transport mechanism of nitrogen.The retention efficiencies varied greatly from 16.47%and 21.17%to 81.44%and 87.58%for surface runoff and sediment,respectively.The groundwater table depth were>1.8 m during the experiments,so the effects of groundwater table depth on VFS retarding surface runoff was smaller than the length of the VFS and the inflow rate.The retention efficiencies varied greatly from 24.70%,14.77%,10.51%,and 8.10%to 88.47%,87.73%,82.97%,and 82.81%for TN,NH₄⁺,NO₃^-,and NO₂^-,respectively.The decreased of concentration of different forms of nitrogen in the inflow and outflow of surface runoff in the VFS can be described as a power function.NO₃^-was the main form of nitrogen in surface runoff outflow,and the content of particulate nitrogen is greatly reduced.The concentration analysis results of different forms of nitrogen in the soil samples showed that under the condition of one experiment scale?1 h?,the main factor affecting the concentration variation was the surface runoff infiltration,and the concentration of total nitrogen decreased with deeper soil depth.The soil nitrate nitrogen and ammonium nitrogen leached with the migration of infiltration water.Within one week after the experiment,the main factors affecting the concentration variation were the chemical reaction inside the soil and the migration and diffusion of soil water.During that period,the concentration of TN and NO₃-N in surface and deep soil increased,while the concentration of NH₃-N decreased but not obvious.The increase of nitrate nitrogen content in surface soil may be related to the nitrification of ammonium nitrogen and atmospheric nitrogen deposition,while the increase of nitrate nitrogen content in deep layer is mainly caused by the downward migration of nitrate nitrogen.Within one month after the experiment,the main factors affecting the concentration variation were plant absorption and utilization and the microbial action?e.g.denitrification and mineralization?.The analysis results of nitrogen concentration in the aboveground part of plant showed that The ratio of the N uptake load by the aboveground part of the plant to the influent load were?45.2±0.3?%and?55.5±0.5?%for the experiment plots of VFS1and VFS2,respectively.Therefore,the aboveground part of the selected plant had a certain positive effect on the enrichment of N from non-point source pollution.?3?The VFSMOD-W?Vegetative Filter Strips Model?model was used to simulate the hydrology and sediment transport in the VFS with the existence of shallow groundwater table.Combing the VFSMOD-W model and the simple empirical transport model,the objective of this chapter was to simulate the phosphorus and nitrogen transport.The important input VFSMOD-W parameters were also calibrated.The global sensitivity and uncertainty analyses of the VFSMOD-W were conducted by combining with Simlab model.The calibrated VFSMOD-W parameters obtained by inverse modeling were within cceptable ranges of measured values.The VFSMOD-W model performed fairly well in predicting the surface runoff and sediment transport.The goodness-of-fit indicators of hydrology,sediment,and P simulations showed that the C_eff were greater than 0.900 for each quantity except RDR and CSF,these good predictions in runoff and sediment also resulted in good prediction of DP and DN transport(C_eff>0.900).The global sensitivity analysis results of Morris method showed that with the existence of the shallow groundwater table,the groundwater table depth?GTD?affects the composition patterns of important inpout parameters controlling the efficiency of the VFS.For the controlled flume experiments,the GTD was shallow?<0.5 m?,so the GTD gained the importance interacting with VKS for the hydrological component.In addition,OS and FWIDTH had a certain impact on surface runoff output results.In the case of the sediment component,the GTD and VKS also interact strongly to control sedimentation,the Sediment grain size(d_p),FWIDTH,OS,and VN also gain important on the outputs.For the field VFS experiments,the GTD was deep?>1.8 m?,VKS was therefore the most important parameter for the surface runoff output results,followed by VL,FWIDTH,OS,and GTD.For the sediment component,the SDR were mainly controlled by the d_p,VL,FWIDTH,VKS,OS,and GTD.The global uncertainty analysis for the selected important input parameters were obtained from the method of extended FAST.The uncertainty of the results can represent the probability of exceeding the expected value or the design value.For example,Under the condition of surface runoff flow rate generated from the source area of the field VFS experiments,if a 80%reduction?RDR<0.20?of surface runoff was desired,for the VFS plot with 15 m long,the probability of RDR<0.20 was 0.61,while the probability was smaller than 0.10 for RDR<0.20 for the VFS plot with 5 m long,so the VFS plot with 5m long did not meet the design requirements.Based on the good performance of the VFSMOD-W model,the manager can use this model to design the VFS and formulate the optimal management schemes for different VFS?such as changing the length of VFS and density of vegetation?to control the pollution of nitrogen and phosphorus non-point source pollutants in the basin and reduce the threat of non-point source pollution on the water quality of the Han River.The study conducted the laboratory runoff experiments and the field vegetative filter strips tests in the typical non-point source pollution area Maozui Town in Tongshun Rver Basin to evaluate the effect of a shallow groundwater table on VFS efficiency for retarding the runoff and non-point source pollutants.The influence mechanism of shallow?depth<0.5 m?and deep?depth>1.8m?groundwater tables on the VFS raterding transport non-point source pollutants was clarified preliminarily.Combined with the VFSMOD-W and Simlab model,the analysis results found that the depth of the groundwater table affected the composition mode of the important sensitive factors controlling the efficiency of the VFS,which is helpful to formulate BMP for different VFS.