Simulation Study On The Regulation Of Forage Vegetation Filter Strips On Rainfall-Runoff Process In Loess Slopes

The forages-type vegetation filter strip is usually arranged in sections along the vertical direction of the slope line.It not only prevents slope runoff,sediment,and pollutants from moving down,but its biomass can also be used as raw materials for feed,fuel,biomass energy,etc.It is a soil and water conservation measure that takes into account both ecological and economic functions.Revealing the rainfall regulation effect and mechanism of the slope pasture vegetation filter strip and establishing a scientific evaluation method is the theoretical basis for implementing this measure and plays an important role in improving its comprehensive benefits.In this study,loess soil was used as the tested soil,and four typical forages?white clover,cocksfoot,crown vetch and birdfoot trefoil?of the Loess Plateau were selected to construct vegetation filter strips,using soil box artificial rainfall-discharge simulation test and the numerical simulation method has carried out a systematic study on the above problems,and initially made the following progress:?1?We explained the improvement effect of the vegetation filter strips on the physical properties of the soil.The pasture vegetation filter strip has a significant impact on the soil physical properties,and the forage species and their growth stages are the main reasons for the differences.For example,the vegetation filter strip?2014-2016?with 3 years of growth has improved soil water holding capacity of the four forage vegetation filter strips,and the soil effective water content increased by 1.7-2.4%;soil residual water content and saturated water content There was no regular change;the soil bulk density of white clover,chicken grass and crown vetch decreased by 2.9-4.4%,and the bulk density of soil treated by birdfoot trefoil increased by 0.7%.Within three years,the soil saturated hydraulic conductivity of the roots of the roots decreased continuously to 1.3 cm h^-1,and the treatment of white clover and crown vetchs increased continuously,with the amplitudes of 1.8 cm h^-1 and 1.1 cm h^-1,respectively.In the 2016 greening period?May?,the soil saturated water conductivity was the highest,4.3cm h^-1.?2?Considering the physical parameters of the soil,the vegetation filter material migration model?VFSMOD?was modified,and the model has high credibility.In the past,the effects of vegetation filtration on the slope water reduction and sediment reduction were analyzed from the perspective of vegetation morphology.The improvement of soil physical properties by vegetation filter strip was not considered.Based on experimental observations and modification of model parameters,a new numerical simulation method for slope reduction and sediment reduction of slope pasture vegetation filter strip considering soil physical properties was constructed.The simulation results are credible,the coefficient of determination is above 0.80,and the Nash coefficient is above 0.85.The sensitivity analysis of PAWN and EET consistently showed that the slope reduction of vegetation filter strip was mainly affected by soil saturated hydraulic conductivity,initial soil water content and vegetation filter strip width.The sensitivity coefficients were 0.67,0.31 and 0.17.The soil saturated hydraulic conductivity,vegetation stalk spacing and vegetation filter strip width,and the sensitivity coefficients are 0.16,0.17 and 0.84.Soil physical property parameters such as saturated hydraulic conductivity have a significant impact on slope runoff and sediment output.?3?The mechanism of rainfall runoff regulation effect on the slope pasture vegetation filter strip was preliminarily revealed,and the corresponding evaluation indicators and methods were developed.The study proposes that the amount of soil water storage after rainfall runoff regulation was defined as the soil water storage loss?Ws?before the regulation?bare land?within a certain period of time.Taking the adjusted sediment output reduction?Sr?as the denominator,the evapotranspiration increment?Wet?,the interception increment?Wint?,the runoff reduction?Wr?and the soil water storage loss?Ws?are numerator,respectively.The evapotranspiration increment reduction ratio?WSet?,the interception incremental reduction ratio?WSint?,the water reduction ratio?WSr?,and the water storage loss reduction ratio?WSs?were obtained.Compared with previous indicators such as WSr,WSs considers the effect of rainfall runoff regulation during rainfall interval.WSs combined soil water storage changes and sediment reduction benefits before and after regulation.The soil water storage changes were determined by soil water supply changes and consumption changes,namely:WSs=WSet+WSint-WSr,to simultaneously mitigate soil erosion and drought and water shortage.Based on the simulation test,it was concluded that for the four kinds of pasture vegetation filter strips and bare soil treatments water storage decreased during the growing season in 2016.Compared with the bare land,the runoff of the four pasture vegetation filter strips decreased by 88.2-144.9 mm in 2016,which is less than the sum of the evapotranspiration increment and the interception increment,resulting in the soil water storage decline of the four forage vegetation filter strips is greater than that of the bare soil.The water storage drop is 16.0-50.8 mm.The WSs of the four forage vegetation filter strips ranged from9.4 to 36.5 m³ t^-1,which means that compared with the bare land,the forage vegetation filter strip consumes 9.4-36.5 m³ of soil water per 1 t of sediment,which is related to the forage species.?4?We established a comprehensive benefit evaluation model for pasture vegetation filter strips that comprehensively consider ecological and economic indicators.The net photosynthetic rate of vegetation leaves in the vegetation filter during the growing season of2014-2016 fluctuated between 6-24?mol CO₂ m^-2 s^-1,and the average value of the grass grass was 17.4?mol CO₂ m^-2 s^-1.With the decrease of soil moisture,photosynthesis of white clover was most stress-induced and non-stomatal stress occurred,which was mainly related to root distribution and soil water use.The average biomass of the growth seasons of the grasses of the cocksfoot,the birdfoot trefoil,the crown vetch and the white clover vegetation were:0.88,0.39,0.35 and 0.30 kg m^-2,respectively.Based on the physiological and ecological characteristics of vegetation in the pasture vegetation filter strip,five types of observation indicators such as slope runoff reduction,slope sediment reduction,soil water storage,biomass and photosynthesis were considered.Taking the ecological benefits of vegetation filter strip as the the main aspect,and economic benefits as a side aspect and then we built a multi-objective evaluation model.The results showed that the comprehensive benefits of white clover,birdfoot trefoil and crown vetch were“good”,and birdfoot trefoil was“poor”.Among the four forage varieties,the ecological benefit of white clover was the best,and the economic benefit of chicken grass was the best.It is suggested that when the forage vegetation filter strip is established on the loess slope,according to the experimental conditions,three forage species,white clover,chicken grass and crown vetch,should be given priority.