| The problem of soil and water loss is serious in the Loess Plateau.It is significant to carry out the simulation study of soil,water and nutrient transport with surface runoff in the area.Based on the field simulated rainfall experiment,this paper researched the effect of slope length,vegetation types and vegetation growth stages on water flow dynamics characteristics and loss of soil nutrient.Meanwhile,the relationship between the mixing layer depth,slope length and vegetation factors were established,which can be used to reveal the influence mechanism of different control measures on the slope land erosion and nutrients losses.The main conclusions were shown as follows:(1)The runoff time was decreased gradually with the increase of slope length,which can be described by power function.The runoff and total sediment yield were increased with the increase of slope length,but the unit area runoff rate decreased.When the slope length increased from 5m to 10m,15m and 20m,the total amount of sediment yield was 2.06 times,3.53 times and 4.61 times of 5m,respectively.For the same slope,ammonium nitrogen loss in runoff was the largest,followed by nitrogen and phosphorus.The increasing slope length caused a increase of arnmounium nitrogen and water soluble phosphorus losses in runoff.When the length of slope was increased from 5m to 20m,the loss of ammonium nitrogen,nitrate nitrogen and phosphorus in sediment were increased from 2.9 to 4.7 times,3.6 to 8 times,and 1.47 to 3.35 times,respectively.The average flow velocity in the slope surface was increased linearly with the increase of the slope length,and the increasing trend of runoff depth was described by the power function of slope length.The longer the slope length was,the greater the shear force of the slope water flow,and the more serious the slope erosion was.When the slope length increased from 10m to 15m,the flow regime varied from flowing into the rapids gradually.(2)The vegetation surface runoff,runoff nutrient concentration,nutrient loss were decreased with the continuation of vegetation growth period,and the cumulative runoff in the mature period was decreased by 13.7%to 86.3%than that of the young.The ammonium nitrogen loss amount was reduced by 62.5%to 94%,and water soluble phosphorus loss amount was reduced by 69.5%to 94.1%.According to the different types of plants,the runoff processes of the slope were different significantly.In July,the runoff rates and sediment yield of Vicia villosa roth and Crown Vetch were the smallest,while Bean and Red clover were largest.The cumulative runoff of Bean was about 2.89 times of Vicia villosa roth.The Crownvetch nutrient concentration in runoff was smallest,and the ammonium nitrogen loss of other vegetation was larger about 1.39 to 5.94 times than that of Crownvetch,and phosphorus loss amount was larger about 2.21 to 7.58 times than crownvetch.In the mature period,the runoff rate of beans was higher than other vegetation,and the cumulative runoff was about 6.79 times of crownvetch.With the increase of aboveground and underground biomass per unit area,slope runoff and nutrient loss were gradually decreased.The exponential function can be applied to well reflect the relationships between the aboveground and underground biomass,runoff of ammonium nitrogen and the total amount of water soluble phosphorus loss(R2>0.7).(3)The relationship between the effective mixing depth and the equivalent mixing depth had a power function increasing trend with the slope length,and decreased exponentially with the increase of vegetation underground biomass.The effective mixing depth model had better fitting effect when simulated the water soluble phosphorus runoff loss process(R2>0.95),while the fitting effect for ammonium nitrogen needed to be improved.The equivalent mixing depth model can well match the ammonium nitrogen and phosphorus change process(R2>0.94). |
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