Regulating Soil And Water Conservation Of Riparian Vegetation Along The Lower Yellow River Based On Hydrological Connectivity

Soil and water loss is an important ecological and environmental problem facing the world today,which has resulted in a series of negative influences on environment,such as land fertility descent,river sediment concentration increase,river pollution,and severely threaten the environment sustainable development.As the last barrier to prevent and intercept water and sand into rivers,riparian vegetation plays an important role in soil and water conservation,thus it is of graet importance to improve the soil and water conservation function of the riparian buffer slope.Therefore,this study was conducted to regulate the soil and water conservation function of the riparian slope along the lower Yellow River based on the artificial simulation rainfall experiments and Flowlength index(FL).FL,which was applied to characterize the potential soil erosion is an indicator can reflect the coupling relationship between topography and vegetation and soil erosion.Firstly,the runoff and sediment yielding characteristics under different rainfall intensities(54 mm/h and 90 mm/h),slope gradients(5°,10°,15°,and 20°),and vegetation covers(0%,15%,and 30%)were analyzed based on the artificial simulation rainfall experiments.Secondly,the responses of hydrological connectivity to slope gradient and vegetation coverage were revealed by means of FL.Thirdly,the relationship between runoff sediment and hydrologic connectivity were explored based on the artificial simulation rainfall experiments and FL.Lastly,soil and water conservation capacity of simulated vegetation patterns was analyzed based on FL,and the optimal vegetation pattern in the study area was constructed to provide the control strategy for the soil and water conservation function of the riparian zone.The main results are shown below:(1)Different vegetation covers under rainfall intensity of 54 and 90mm/h could reduce runoff velocity,and the decelerating effect under rainfall intensity of 90 mm/h was obvious on the high coverage slope(30%).Under different rainfall intensities,changes in total runoff and runoff depth were similar,both decreased gradually with the increasing slope gradient and vegetation cover.Meanwhile,the effect of slope gradient increased gradually with increasing rainfall intensity.On the slope of less than 15°,the erosion sediment yield under rainfall intensity of 54 mm/h increased slowly with increasing slope gradient,however,the effect of slope gradient was not obvious.On the slope more than 15°,the erosion sediment yield increased rapidly and was obviously affected by slope gradient.Under the 90 mm/h rainfall intensity,the erosion sediment yield on the slope of less than 15° increased with increasing slope gradient,and it decreased gradually with increasing slope gradient on the slope of more than 15°.During this process,the erosion sediment yield varied greatly with the increasing slope gradient,and was obviously affected by slope gradient.In the interaction of slope gradient and vegetation cover,the effect of vegetation cover weakened gradually with increasing slope gradient,while the role of slope gradient enhanced gradually and became the dominant factor influencing soil and water loss.(2)Hydrological connectivity was significantly affected by slope gradients on the bare slopes,and this effect was weakened on vegetated slopes.The FL on the bare slope obviously decreased with the increasing slope gradient,and the FL change on the vegetated slope was not obvious with the increasing slope gradient.The vegetation cover exhibited significant effect on the FL.The vegetation cover on slope was the main reason for the difference of hydrological connectivity between slopes.In the interaction of vegetation and topography,soil erosion caused by runoff process was obviously affected by slope gradient,while the role of vegetation cover was obviously greater than that of slope gradient in the soil erosion caused by the confluence process.(3)Under different rainfall intensity,the runoff and sediment yield showed a trend of increasing with increasing FL.When the vegetation covers was reduced from 30% to 15%,the FL increased.In this process,the increase of sediment yield was obvious under 90 mm/h and 54 mm/h rainfall intensity,and the increase of runoff was only obvious under the 90 mm/h rainfall intensity.When the vegetation covers was reduced from 15% to 0%,the FL increased significantly.In this process,the increase of sediment yield was obvious,and the increase of runoff was less.With the increase of FL,the runoff became leveling off,and the sediment yield remained increasing,which indicated that the sand carrying capacity of slope runoff was enhanced with the development of hydrological connectivity between "source and sink".(4)Based on the scenario simulation of vegetation pattern,the vegetation configuration of middle slope-coarse granularity-cluster distribution,which exhibited the smallest FL and the weakest hydrological connectivity,was identified as the optimal vegetation pattern for soil and water conservation.On the scale of the slope,the hydrological connectivity of the optimal vegetation pattern and the actual vegetation pattern decreased with increasing vegetation cover.In comparison with the variation trend of FL on the actual vegetation slope,the FL of optimal vegetation pattern was generally lower than that of the actual vegetation slope.On the riparian scale,the FL of D1?D2?D3 and D4 was greater than 0.03,which belongs to the region with higher probability of soil and water loss.Based on the regulation strategy,optimization solution was designed for the 4 regions to improve the function of soil and water conservation.After regulation,the FL all decreased by more than 60%(FL values decreased from 0.035 km,0.037 km,0.047 km and 0.036 km to 0.008 km,0.009 km,0.019 km and 0.008 km,respectively)when the vegetation cover increased by no more than 10%.The results of this study can provide guidances for the assessment of soil and water conservation function and restoration of degraded riparian zones along the riparian slope of the lower Yellow River.In the future,water and soil loss in riparian slope can be regulated through properly optimizing the spatial distribution of vegetation patches,which ensure the soil-water conservation function of riparian vegetation achieve the best.