Study On The Development Processes Of Rill Erosion In Purple Soil Region

Rill erosion is one of the most serious erosion problems in cultivated slope land of purple soil.The development of rill erosion can aggravate soil erosion in slope farmland,and the change of rill network and rill morphyology will also affect surface runoff energy and surface microtopography.Conversely,the change of surface runoff energy and surface microtopography also affect the development of rill erosion.Therefore,this paper took the purple soil as the research object,which was collected from Hilly Area of Central Sichuan Basin.Methods involving artificial scouring,GIS technology and multifractal method were adapted to investigate the rill erosion processes and charateristics of rill morphology,flow hydraulics and surface microtopography under different runoff rates（4 L/min,6 L/min,8 L/min）,slope gradients（10°,15°,20°）and upslope filling areas（0.15 m²,0.3 m²,0.45 m²）.Revelead the relationship among rill morphyology,flow hydraulics parameters and surface microtopography during rill erosion processes,and construction the empirical prediction model of surface microtopography.This research provided a theoretical basis for the dynamic mechanism of rill erosion and rill erosion prediction model of sloping farmland in purple soil region.The main results are presented as follows:（1）Rill erosion process of purple soil can be divided into:drop-pits development-intermittent rill erosion-headward erosion（downward erosion）-rill stabilize stage.With the increased of runoff rate,the number of rills and the proportion of high-level rills increased.At the same time,the rill network gradually changed from dendritic to single.The development of rill network slowed down when the runoff rate less than 6 L/min,the rill cross section shape gradually changed from“U”to“V”.The proportion of high level accounted for 38.9%under 20°slope gradient,and rill network shape shown as single drainage.Rill depth obviously increased with the slope gradients increased,and the maximum value was 0.074 m under 20°slope gradient,rill cross section shape shown as“V”.Rill curvature increased with the upslope filling areas increased,rill network shown as dendritic.The rill complexity gradually decreased when the upslope filling areas less than 0.3 m²,and rill cross section shape gradually changed from“V”to“box”or“U”.（2）With the increased of runoff rates,the flow patteren of runoff gradually changed from subcritical laminar flow to supericritical transition flow,Darch-Weisbach resistance coefficient garudlly decreased,and the difference of Darch-Weisbach resistance coefficient was small under intermitten rill and headward erosion stages.The value shear stress,stream power and unit stream power reached the maximum under 8L/min condition,and the difference shown more less as runoff rate greater than 6 L/min conditions.The value of flow velocity,stream power and unit stream power under 20°slope much bigger than other slope gradients,the regime of surface runoff belong to supercritical transition flow,and the flow regime belong to supericritical laminar flow when the slope gradient less than 15°.The value of Darch-Weisbach sharply decreased at drop-pits development and intermittent rill erosion stage under 10°slope,but the value bigger than other slope gradients.When the upslope filling areas less than0.3 m²,the regime of runoff belongs to subcritical laminar flow,and the flow regime changed to supericritical transition flow as the runoff rate increased.The flow velocity,shear stress,stream power and unit stream power of 0.45 m² upslope filling area less than other conditions,but the difference of Darch-Weisbach resistance coefficient shown less.（3）The range of surface elevation between-120 mm to 20 mm under 8 L/min condition.The value of simple fractal dimension f（α）_max,singular exponential span△αand△f（α）reached maximum at 8 L/min runoff rate condition.When the runoff rate greater than 6 L/min,the change of surface microtopography increased,and the surface microtography tended to sharp.The change value of surface elevation reached maximun at 20°condition,and rill erosion tended to sharp.Meantime,The value of simple fractal dimension f（α）_max,singular exponential span△αand△f（α）increased with the slope gradient increased,and the change of surface elevation sharply under intermittent rill erosion and rill stabilize satges.The range of surface elevation variation reduced from-120 mm to-110 mm.The value of simple fractal dimension f（α）_max,singular exponential span△αand△f（α）reached minimum at 0.45 m² upslope filling area,and the change of surface microtopography decreased during rill erosion processes.（4）The runoff rates have a great effect on the surface microtopography.The△α,△f（α）are significantly or extremely siginificantly related to rill network,rill morphyology,and hydraulics parameters.The prediction effect of Froude number to△αwas better,and rill length,rill complexity and Darch-Weisbach resisitance coefficient has the best prediction effect for△f（α）.The rill width,shear stress and Darch-Weisbach resistance coefficient have the best prediction effect for△a,and the best predictive factor for△f（α）was flow stream power.The relationship between upslope filling areas and△f（α）was significantly,and the prediction of rill complexity to△f（α）was better than other parameters.The rill density,rill weidth and Darch-Weisbach resisitance coefficient was better to predict△f（α）.