| Land use and vegetation cover have changed significantly after comprehensive implement of “grain for green” in the loess gully plateau region.Soil erosion environment improved essentially at some managed small-basins,which resulted in conservation of water and control of soil loss.However,Gully-head retreat caused by flood is still intense to damage tablelands like Dongzhi,as the most complete tableland,when high intensity storm events happened in the region.Research specially investigating gully-head retreat in the region is hard to found in the past decades years.At present,it is not clear that how land utilization transformation affects gully-head retreat process.Gully-heads on typical grassland and shrub-grass land,naturally restored from abandon farmland,and farmland,as a control,were built to investigate their retreat processes.Artificial simulated rainfall combined with scouring experiments were conducted to simulate the process of gully-head retreat and three dimensional reconstruction based on photos technology was used to monitor gully morphology.Differences were analyzed among soil physical-chemical properties and roots distribution characteristics from gully-heads in the three kinds of lands.Effects of land utilization and tableland gradient and gully-head height were illuminated on runoff and sediment yielding process,contribution rate of sediment from tableland,erosion by hydrodynamic and gravitational force as well as gully-head morphology evolution in the process of gully-head retreat.Mechanism of action was revealed of land utilization on gully-head retreat.It may provide important scientific implication for controlling gully to project tableland and establishment of soil erosion model in the region..The results showed that:Average runoff rates and sediment concentrations varied in ranges of 14.060~14.750L·h-1 and 0.054~0.077 g·mL-1.Benefits of runoff and sediment reduction were17.50%~21.91% and 70.37%~71.93% on grassland,and,15.77%~27.38% and56.58%~62.96% on shrub-grass land.Both runoff rate and sediment concentration increased with rise of tableland gradient and gully-head height,they increased by 4.91% and 41.80% onfarmland,and,21.67% and 45.81% on shrub-grass land in the plots with a tableland gradient of 9 compared to the plots with a gradient of 3,and they also raised by 4.15% and 40.74% on farmland,and,14.86% and 65.00% on shrub-grass land in the plots with a gully-head height of 1.5m compared to the plots with a height of 1.2m.Higher variation coefficients of sediment concentration were found on grassland and shrub-grassland with increases by67.81%~95.31% and 41.16%~155.93% respectively compared to the coefficient on farmland.Sediment from tableland contributed to sediment from plot with a rate ranged from12.70% to 26.31% on farmland,lower rates were found on grassland and shrub-grassland under the same test condition with decreases by 24.76%~43.09% and 52.25%~62.18%.The contribution rate increased with tableland gradient increasing while decreased at higher gully-head height condition.The rate was 1.36 times higher on farmland and 1.19 times on shrub-grass land in the plots with a tableland gradient of 9 compared to the plots with a gradient of 3,nevertheless,it accounted for only 65.77% and 71.82% on farmland and shrub-grass land in the plots with a gully-head height of 1.5 m in the contribution rate of the plots with a gully-head height of 1.2 m.Runoff near gully-head was found to be turbulent and rapid generally during all of tests.Shear stress,stream power and unit stream power as well as section energy varied in ranges of5.70~9.77 N·m-2,2.45~5.48 W·m-2 and 0.0230~0.0920 m·s-1 as well as 0.0212~0.0299 m on farmland,each parameter on grassland and shrub-grass land decreased by 8.32%~52.35%,38.99%~68.56% and 31.24%~37.02% as well as 51.06%~53.18% on grassland,and,-5.11%~51.75% ? 27.92%~72.16% and 22.75%~41.35% as well as 37.40%~58.29% on shrub-grass land.Jet kinetic energy and shear stress of jet flow ranged from 0.570 J to 1.005 J and from 356.88 N·m-2 to 434.61 N·m-2 on farmland,each parameter on grassland and shrub-grass land decreased by 63.30%~64.41% and 7.00%~16.00% on grassland,and,47.33%~70.57% and 6.17%~22.06% on shrub-grass land.No significant relationship between gully head-ward erosion rate and runoff parameters was found in most tests.Collapse,slump and solifluction as well as toppling along with dislodgment,defied as gravitational erosion,were found during tests.Collapse was the main type accounting for61.90%~100% in all of mass erosion events during 0~180min of test.The frequency of gravitational erosion events during 0-180 min was in ranges of 21~33 and 14~17 along with18~45 on farmland and grassland as well as shrub-grass land.The intensive mass-erosion period was found to be different under different condition,which seemingly showed that it was random in temporal distribution of gravitational erosion events to some extent.However,phasic rule with different needing time for a individual mass-erosion events was observed in temporal distribution during 0~180min of test,and the needing time changed with a cycletendency of increase and decrease during the test.52.38~100% of frequency of mass erosion events took place at upside of a gully-head,and the frequency was the highest on farmland and lowest on shrub-grass land.Collapse events were classified into three types such as gully-head collapse,gully-slope collapse and gully-cliff collapse based gravitational erosion position and generated morphology result.The frequency of gully-head collapse was in range of 7.69%~35.00% and 15.38%~27.27% as well as 16.67%~35.71% on farmland and grassland as well as shrub-grass land,respectively.Gully-head collapse frequency was highest on shrub-grass land under the same tableland gradient when gully-head height was 1.2m.Gully-cliff collapse frequency was in range of 55.00%~92.31% ? 23.08%~36.36% and38.89%~50% on three kinds of lands and the highest frequency was found on farmland.Significant inhibiting effect of vegetation on gully-head retreat was found in the test.Gully-head retreat rate varied in the range of 0.99~2.08 cm·min-1 on farmland,the rates decreased by 85.39%~88.58% on grassland and 72.47%~81.47% on shrub-grass land compared with farmland under the same condition.Gully-head retreat was reinforced with tableland gradient and gully-head height increasing,it increased by 110.67% and 41.84% on farmland and shrub-grass land in the plots with a tableland gradient of 9 compared to the plots with a gradient of 3.The rate was 1.60 and 1.37 times higher on farmland and shrub-grass land in the plots with a gully-head height of 1.5m compared to the plots with a gully-head height of 1.2m.The dominate diving force to advance gully-head was different on different used land.The main force of gully-head retreat on farmland was hydraulic scoring while gravitational erosion on grassland and shrub-grass land.Distribution of roots,increasing of soil infiltration coefficient and organic content and soil water-stable macro-aggregate content,decrease of disintegration rate of gully-head on grassland and shrub-grass land were the main cause to control gully-head retreat,additional,both density and length density of fine root,with diameter lower than 0.5mm,were higher on grassland compared to shrub-grass land,which might be the main cause for lower retreat rate found on grassland. |
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