Study On The Effect Of Vegetation Slope Pattern On Soil And Water Loss

In this research, on the basis of the field trials and the indoors analysis, systematic researches were carried out to study the effects of vegetation pattern on erosion on the natural grassland slope. Considered the spatial structure, degree of fragmentation and heterogeneity, the artificial simulating rainfall experiment and the runoff scouring experiment were made on multifarious patterns such as random pattern, stripe pattern, congregate pattern (top, middle and bottom), and net concentration. Through analysis the main conclusions of the study were obtained as followed:1. Under the simulating rainfall condition, effects on slope soil erosion of same vegetation area varied with the different vegetation pattern. Random pattern has the largest accumulating infiltration and the best effect on slope runoff reduction, the congregate pattern (in the bottom of the slope) is the best one to decrease sediment yield.The sequence of average infiltration rate and cumulate infiltration can get as: random pattern>stripe pattern> congregate pattern (on the middle of the slope)> congregate pattern (on the bottom of the slope)> congregate pattern (on the top of the slope). Every slope of different vegetation patterns reaches a steady infiltration rate at about 0.5 mm/min in 10 minute. The sequence of outflow time of 5 different vegetation patterns can get as: congregate pattern (on the bottom of the slope)>random pattern>stripe pattern>congregate pattern (on the top of the slope)>congregate pattern (on the middle of the slope). By the increase of the rain intensity, the outflow time of each vegetation pattern tend to be shorter. The sequence of cumulate runoff yield: congregate pattern (on the top of the slope)> congregate pattern (on the bottom of the slope)> congregate pattern (on the middle of the slope)> stripe pattern> random pattern. The sequence of cumulate sediments yield and average sediment concentration: stripe pattern> congregate pattern (on the middle of the slope)> congregate pattern (on the top of the slope)> random pattern> congregate pattern (on the bottom of the slope). A slow decline trend of the sediment concentration in the slope flow can get from the rainfall experiment.2. Different vegetation pattern has different effects on runoff and sediment yield reduction under different scouring flux. Under 16L/min flux the congregate pattern (on the middle of the slope) and congregate pattern (on the top of the slope) has the least runoff yield and while under 20L/min and 24L/min the congregate pattern (on the middle of the slope) and net pattern do. Under the three kinds of scouring flux, the runoff yield of congregate pattern (on the bottom of the slope) and random pattern reached the largest relatively. Towards the scouring flux, the sediment yield on the slope of five different vegetation patterns present critical value. Under 16L/min flux the congregate patterns (on the bottom of the slope) has the least sediment yield and while under 20L/min and 24L/min the net pattern do.3. The ability of runoff and sediment subtraction reduced by the decrease of the vegetation area on the slope, and there was a break on this reducing process. Result from the experiments, the effect of vegetation on slope soil and water conservation has close relation with not only the rain and flow intensity but also the space position. In general, the same area vegetation on the bottom has the stronger effect on runoff andsediment decrease than on the top. The vegetation located at the bottom of the slope, which lack of the effect of blockage on the flow came from the upside, hence which has the weaker runoff decrease effect. With the decrease of vegetation cover, the action of vegetation to runoff and sediment yield decrease not evenly. There was a break on this reducing process. The break point of runoff and sediment decrease effect of the congregate pattern on the top of the slope augmented by the increase of rain intensity, while the break point of the congregate pattern on the bottom of the slope keep steady at about 60%~80%.4. Landscape heterogeneity is not important to runoff of the slope, but important to the sediment yield .It is less than 10%change to the runoff of the slope in the pattern of bareground patch in grass or the grass patch in bareground .But to the sediment yield of the slope, the grass with bareground patch increases 1-6 times sediment yield than the grass without bareground patch .The bareground with grass patch decreases 10.33%-36.25% sediment yield than the bareground with no grass patch.5. The effects of fragment degree of vegetation pattern on slope runoff subtraction varied under different rainfall intensities. Under low rainfall intensity, the more fragment degree the vegetation pattern is, the less the runoff of slope is. Under high rainfall intensity, it is opposition. The extreme value phenomenon appeared in the different scouring flow on the decrease of vegetation .Before the scouring flux reaching the critical flux , the more fragment degree the vegetation pattern is , the less the runoff of the slope is . But when the scouring flow reaching the critical value, there were little difference between the two patterns. To sediment yield reduction on the slope, the big fragment degree of vegetation pattern on slope has more effect than the small one.