| The red soil hilly region of south China is rich in hydrothermal resources and widely distributed with slope farmland,which is an important production base of grain,subtropical and tropical economic crops in China.However,affected by natural and human factors,the red soil hilly region is suffering from the most widely distributed and severe soil erosion in China,and the severity of soil erosion is second only to the Loess Plateau.Rill erosion caused by slope runoff is a common pattern of soil erosion in the red soil hilly region of south China,while the red soil slope rill erosion mechanisms and the main impacting factors still need further research.The rill morphology on red soil slopes,as well as the sediment sorting and transport mechanisms,and flow hydrodynamic mechanisms during rill erosion processes,are not clear enough.Therefore,this paper seleted two types of red soils which are derived from typical parent materials in the red soil hilly region（UDS:Ultisols derived from shale;UDQ:Ultisols derived from quaternary red clay）to conduct a comparative investigation of the rill erosion processes on the two types of red soil slopes through a series of indoor simulated runoff scouring experiments.And based on the observation data of red soil slope runoff plots in Xianning City,a typical red soil area in Hubei Province,the influence of soil and water conservation measures on red soil erosion was discussed.The main results are as follows.（1）Runoff and sediment yield characteristics on slopeThe average runoff rates of each treatment are not significantly different between the two red soils.The everage infiltration rates of UDQ are 1.09-1.54 times that of UDS,while the average sediment yield rates and sediment concentrations of UDS are 2.26-4.71 and1.94-3.96 times that of UDQ(except for the treatment of the 8.5 L min-1 m-1 inflow rate).When rills generated on slopes,the soil loss amounts and flow sediment concentrations of UDS are much higher than that of UDQ,indicating its more severe soil erosion degree,while the soil infiltration capacity of UDQ is stronger.The average runoff and sediment yield rates of the two red soils are both linearly positively correlated with the inflow rates（p<0.05）.Moreover,the linear growth coefficient of sediment yield rate with inflow rate is much higher for UDS than UDQ.（2）Particle size characteristics of eroded sedimentsThe eroded sediments of the two red soils are both enriched in coarse particles（sand:0.05–2 mm）（UDS:62.65%-67.22%;UDQ:72.91%-75.12%）,which is related to the high fine particles（<0.05 mm）content（UDS:80.30%;UDQ:95.59%）in the two red soils and the rill flow with adequate energy.With the increase of inflow rate,the mean weight diameter（MWD）of the eroded sediments decreases while the fractal dimension（D）increases,that is,the grain size of rill eroded sediments gradually refines and more aggregates break down.Compared with UDS,the eroded sediments of UDQ have lower silt content,higher sand content,higher MWD and lower D,indicating its coarser eroded sediments.In the rill erosion processes on red soil slopes,the sediment transport mechanism of suspension–saltation takes a major proportion（UDS:66.33-91.94%;UDQ:72.18-93.14%）,and with the development of rills on hillslope,bed load transport by rolling becomes more active.（3）Rill morphologyThe average width,depth and width-depth ratio of rills developed on UDS slope are3.95 cm,5.63 cm,0.94,respectively.The average width,depth and width-depth ratio of rills developed on UDQ slope are 3.63 cm,1.76 cm,2.92,respectively.Under the same inflow rate,the average rill widths are not significantly different between the two red soil,while the average rill depths of UDS are much higher than that of UDQ,and its average rill width-depth ratios are significantly lower than that of UDQ.Therefore,rills deveoped on UDQ slope tend to be“wide-shallow type”,and rills developed on UDS slope tend to be“narrow-deep type”with more severe downcutting erosion.The rill erosion amount of UDS is much larger than that of UDQ(except for the treatment of the 8.5 L min-1 m-1inflow rate),indicating its more severe rill erosion degree.（4）Slope flow hydrodynamicsMost flow hydrodynamic paramters increase with inflow rate.The rill flow Darcy-Weisbach resistance coefficient and Manning roughness coefficient of UDQ are higher than that of UDS,while other hydrodynamic parameters（flow velocity,runoff depth,Reynolds number,Froude number,flow shear stress,stream power,unit stream power）are basically lower than that of UDS.This indicated the higher resistance,weaker erosive power of the rill flow on UDQ slope.Under relatively large inflow rates(11.5-14.5 L min-1 m-1),the flow sediment transport capacity of UDS is 2.35-5.07 times that of UDQ.The occurrence and development of rills on red soil slopes significantly enhanced the turbulence,resistance,erosivity,energy and sediment transport capacity of slope flow.（5）Soil erodibility and soil and water conservation measuresThe occurrence of rills significantly decreased the anti-scouribility of red soils.According to EPIC model,the soil erodibility factor（K）of UDS and UDQ are 0.0288 and0.0278 t·hm2·h MJ-1·mm-1·hm-2,respectively,and the former is 1.03 times of the latter.The linear regression equation between rill erosion amount（RER）and flow shear stress（τ）indicated the rill erodibility parameter of 3.5×10-4 s m–1 and 1.9×10-4 s m–1 for UDS and UDQ,respectively,and the former is 1.84 times of the latter.Under the inflow rate of 10-14.5 L min-1 m-1,the anti-scouribility of UDQ is 2.26-4.71 times that of UDS.The different soil texture and organic matter content in red soils derived from different parent materials can lead to the difference in soil erodibility,which in turn,has significant impacts on rill erosion susceptibility and anti-scouribility.Based on the comprehensive analysis of the soil and water conservation practice factor（P）and the observed sample numbers,the measure of planting tea along slope has relatively high soil and water conservation benefits（P value:0.420;sample numbers:6）for red soil slopes. |
PDF Full Text Request