| In recent years,due to the reduction of precipitation,unreasonable irrigation and other factors,the pollution of agricultural land sources has become increasingly serious.Taking agricultural land in Beijing mountainous area as the research object,12 undergrowth vegetation leachate ditches with different configurations(quantity,width,surface vegetation coverage)and a blank control were set up in Yanqing District of Beijing.The characteristics of runoff and sediment yield under typical rainfall and outflow scouring conditions were studied,and their reduction effects on total nitrogen(TN),total phosphorus(TP),ammonia nitrogen(NH4+-N),chemical oxygen demand(COD)and biological oxygen demand(BOD)in water quality were analyzed.In order to provide new ideas for solving the problem of agricultural land sources.The results show that:(1)Under natural rainfall conditions,vegetation infiltration ditches can effectively alleviate runoff and sediment.(a)Among the three types of vegetation,the effect of reducing runoff and sediment is worse when the surface of the undergrowth vegetation leachate is tall fescue turf,while the runoff and sediment reduction rate of the other two types of vegetation leachate is 1.26-1.36 times that of the undergrowth vegetation leachate.B)The width of vegetation leachate has an important influence on the interception efficiency of runoff and sediment.In the runoff plot(C-1)with the width of 1.5 m vegetation leachate,the runoff ancl sediment yield are smaller than those in other runoff plots,showing better sediment reduction ability.C)The complexity of vegetation allocation can effectively enhance the reduction rate of runoff and sediment:under the same width of vegetation leachate,the runoff yield and sediment yield of two vegetation-covered runoff plots are less than that of single vegetation-covered runoff plots on the surface of vegetation leachate ditch,while the difference of sediment yield of three vegetation-covered leachate is not obvious.In addition,the yield is more susceptible to the influence of soil water content in the early stage.(2)Under natural rainfall conditions,the reduction rates of TP by vegetation Leachate under farmland and forest were controlled at 34.78%?97.152,19.68%?57.2%for TN,253.64%?80.90%for NH4+N,16.18%?99.87%for chemical oxygen demand and 7.95%?94.76%for biochemical oxygen demand.Vegetation richness can effectively enhance the control of non-point source pollution.The number and width of undergrowth vegetation infiltration ditches are also important factors affecting the control of non-point source pollution.(3)In the scouring experiment,the sediment yield in each runoff plot increases first and then decreases.In some runoff plots,the sediment yield decreases first and then increases with time,and finally decreases and tends to be stable.With the increase of design discharge,the initial sediment yield time will be relatively ahead of schedule.When the number of vegetation infiltration ditches is large,the sediment generated is less.(a)The initial runoff generation time is related to the designed runoff and the soil moisture content in the early stage.The more abundant the vegetation allocation on the surface of the vegetation leachate,the longer the initial runoff generation time is.Among the three kinds of vegetation,the interception efficiency of greens to runoff is better,and the runoff yield time can reach 9'53 86.The initial runoff yield time of each undergrowth vegetation infiltration ditch is longer at a small flow rate.When the design flow rate increases from 0.48m3/h to 0.72 m3/h,the initial runoff generation time will be shortened by 43-200 seconds.When the design flow rate increases from0.72 m3/h to 0.96 m3/h,the initial runoff generation time will be advanced by 3-231 seconds.B)The ability of vegetation infiltration ditches to intercept sediment runoff is limited,so that part of the sediment can continue to move forward with the flow of water,causing soil erosion.The runoff and sediment reduction rate of vegetation allocation is lower than that of single infiltration ditch.When the interception capacity of undergrowth vegetation leachate reaches a certain limit,it is inferior to the reduction rate at low flow rate.Runoff plots with undergrowth vegetation infiltration ditches can reduce runoff and sediment up to 80%.(4)Vegetation infiltration ditches play an important role in the control of non-point source pollution.When other conditions are fixed,the more the number of vegetation leachate,the wider the width,the better the control effect of surface source pollution.Poa pratensis has higher interception efficiency for nitrogen,phosphorus and microorganisms than fescue and green grass,and the richer the vegetation types are,the better the interception effect for non-point source pollution.(5)Scouring amount has an important influence on the control of non-point source pollution in Undergrowth Vegetation leachate ditches.At a small flow rate,the reduction rate of TP and TN by vegetation leachate covered with green was 98.02%and 44.97%respectively.Under large flow rate,the control amount of TP is only 18.03%,and the reduction rate of TN is 64.56%,but the reduction rate of NH4+N is as high as 99%,which is higher than that of the runoff plots covered by three vegetation leachate.Under large flow rate,the control range of undergrowth vegetation infiltration for TP is 18.03%?100%,for NH4+-N is 20%?99.56%,for TN is 6.7%?64.56%,and for chemical oxygen demand and biochemical oxygen demand is 21.1%?94.86%and 54.03%?100%.(6)Comprehensive analysis shows that the more the number of undergrowth vegetation infiltration ditches,the stronger the effect of intercepting surface runoff and sediment and reducing non-point source pollution when the discharge is small.Surface vegetation richness also affects the role of vegetation leachate to a certain extent.At low flow rate,the runoff plot covered by surface vegetation with Poa pratensis(C-11)has better overall interception efficiency,and the surface vegetation is mixed with each other. |
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