| Soil erosion has become a major concern in the world.It not only destroys the soil and water resources,but also reduces soil fertility,restricts crop production,and causes soil nutrient loss,resulting in non-point source pollution.The Loess Plateau is the most serious area of soil erosion in China.The problem of soil erosion has greatly hindered the development of agriculture and the protection of ecological environment in the loess region.In view of this,based on the artificial rainfall simulation experiment,this paper aims to analyze the rainfall,runoff and sediment yield process and spatial variation characteristics of the loess slope.The effects of different rainfall instensity on the initial runoff time,infiltration capacity and runoff and sediment yield on slope were explored.The runoff process of loess slope was simulated and to analyze the changes in the flow rate and water depth of slope surface under different rainfall conditions.The influence of rainfall on the hydrodynamic characteristics of the slope has been revealed.Furthemore,measures of grass cover were putted on the slope.The variation of runoff,sediment and nitrogen phosphorus loss were analyzed under different rainfall intensity,grass cover width and grass pattern.The influences of different grass cover width and pattern on the yield process of runoff,sediment and nitrogen phosphorus loss are clarified.The regulating effects on runoff,sediment and nitrogen phosphorus loss under different measures of grass cover are revealed.And the relationship between runoff,sediment and nitrogen phosphorus loss was build.The regulating effects of grass cover measures on the output of nitrogen and phosphorus in runoff and sediment were quantified.The main research results as follows:(1)It is quantitatively clarified that the increase of slope sediment yield is higher than that of slope runoff yield,and the sediment yield is more sensitive to the change of rain intensity.The stable infiltration of the slope were 0.47 mm/min,0.51 mm/min and 0.53 mm/min,respectively,under the rainfall of 1.0 mm/min,1.5 mm/min and 2.0 mm/min.The relationship between rainfall intensity and the slope initial runoff time showed a power function.With the increase of rainfall intensity,the initial runoff time of slope was decreased gradually.The initial runoff time of rain intensity of 1.5mm/min and 2.0mm/min are 1/2 and 1/5 of the 1.0 mm/min rain intensity,respectively.The effect of rainfall intensity on sediment yield is greater than runoff yield on the slope.The runoff of 2.0mm/min rain is 1.68 times and 1.27 times of 1.0mm/min and 1.5mm/min rain intensity and the sediment is 17.1 times and 5.23 times,respectively.(2)The development of rills and the distribution of flow velocity along the slope under rainfall conditions are clarified.The development of rills caused the average flow velocity of the slope increases.The slope flow velocity tends to increase from the top to the bottom of the slope.With the rainfall intensity increases,the average flow velocity of the slope shows an exponential rise.The average flow velocity of the 2.0 mm/min rainfall was 4.1 times and 3.7 times the rain intensity of 1.0 mm/min and 1.5 mm/min,respectively.The flow velocity along the rill on the slope can be divided into three stages:slow increase stage(7-13m),smooth transition stage(3-7m)and rapid rise stage(0-3m).The flow velocity is concentrated in the rill on slope,and the flow velocity in the main rill is greater than tributary rill.The flow velocity distribution on the slope is gradually disperses with the rain intensity increases,and the flow velocity in the main rill is increases.Under the experiment in this paper,the rills on the slope are mainly developed in the lower part of the slope.With the increase of rain intensity,the number of rills on the slope gradually is increase and the distribution is gradually disperses and more connectivity.There is only one rill on the slope under 1.0 mm/min rainfall,which is located at 0m-1.6m on the slope.There are two rill on the slope under 1.5mm/min rain,which are located at 0m-3m on the slope.It has five rill on the lower slope under 1.5mm/min rain,which are located at the position of 8m-9m in the upper middle part of the slope and 0m-4m in the lower part.When the slope angle is constant,the rill erosion position of the slope surface gradually moves toward to the upper part of the slope with the rainfall intensity increases.(3)The regulation mechanism of the grass width and its spatial arrangement to reduce the runoff and sediment yield on the slope is revealed.At 1m,2m and 3m grass width,the average reduction rate of slope runoff yield was 29%,38%and 49%,respectively,and the reduction rate increased by about 10%.the average reduction rate of sediment yield was 37%.57%and 79%,respectively,the reduction rate is increased by about 20%.The regulation effect of the grass pattern in the lower part of the slope on the runoff and sediment yield is obviously greater than that on upper part.For example of 1m grass width,the slope reduction rate of runoff and sediment by L1 is 40%and 54%,respectively,while the slope of U1 on slope is only 15%and 8%.(4)The regulation mechanism of the grass width and its spatial arrangement to reduce the runoff and sediment yield on the slope for controlling the loss of nitrogen and phosphorus nutrients on slopes is revealed.Runoff and sediment nitrogen and phosphorus loss are decreased with the increase of grass width,and the loss concentration was not significantly different.For the grass with the same coverage width,when the grass was moved from the lower part of the slope to the upper slope,the loss of nitrogen and phosphorus in runoff and sediment are increased.There is a good linear relationship between the cumulative loss of nitrogen and phosphorus and the cumulative runoff and sediment yield.This indicated that the loss rate of nitrogen and phosphorus in the pattern of the lower part slope is the smallest,and the loss is the least.The grass pattern in the lower part slope plays a role in reducing runoff and reducing the loss of nitrogen and phosphorus.The loss of soil nitrogen and phosphorus is mainly based on sediment output under the experiment.The regulation effect of grass cover on runoff nitrogen and phosphorus show:L3>L2>L1>U3>IU2>ML1>IL2>M2>U2>MU1>U1,the average value of reducing runoff nitrogen and phosphorus is 62%,58%,49%,45%,44%,43%,43%,38%,32%,30%,25%,respectively.The regulation effect of grass cover on sediment nitrogen and phosphorus show:L3>L2>U3>IL2>L1>IU2>M2>U2>ML1>MU1>U1,the average value of reducing nitrogen and phosphorus in sediment is 91%,72%,71%,63%,55%,54%,53%,44%,44%,37%,22%,respectively.The regulation effects of nitrogen and phosphorus in the sediment is obviously better than that of the runoff nitrogen and phosphorus.The pattern in the lower part solpe is better made for reducing runoff and sediment yield and the loss of. |
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