Effects Of Bedrock Fragmentation Induced By Hoeing Into Purple Mudstone On Hydrological Processes Of Hillslopes

The purple soil area of Sichuan Basin is a typical rock-soil dual structure,the bedrock is covered with shallow soil layer.Farmers would actively use tillage tools to break the soft bedrock,and rock fragments from the bedrock can rapidly weather to offset soil loss by water and tillage erosion,so as to maintain soil productivity of upper slope positions in hilly agricultural regions with mudstone and shale,southwest China.Bedrock fragmentation by tillage will cause the changes of soil-debris mixture thickness,content and spatial distribution of rock fragments,which can change the hydraulic properties of soil and affect the processes of water erosion.The effects of purple mudstone fragmentation induced by tillage on hydrological processes of hillslope were explored by simulated tillage and rainfall experiments.Simulated tillage experiments were used to determine the fragmentation and movement of rock fragments induced by tillage based on different tillage depth,soil-covered thickness,moisture content and slope gradient.Then,the indoor simulated rainfall experiments were carried out to find out the effects of soil-debris mixture thickness,content and spatial distribution of rock fragments on hydrological and erosion characteristics under different slope gradients.Finally,the relationship between the bedrock fragmentation by tillage and the hydrological processes of hillslope was established.The main results and conclusions are as follows:1.The occurrence and movement of rock fragments induced by tillageThe bedrock fragmentation and the movement of rock fragments caused by tillage are affected by both anthropogenic factor(e.g.,tillage depth)and natural factors(e.g.,rock moisture content,overlying soil thickness,slope gradient).The fragmentation degree of bedrock has a significant negative correlation with tillage depth,but has no significant relationship with slope gradient.With the increase of tillage depth,the proportion of coarse rock fragments(>20 mm)increases,while the proportion of fine rock fragments(<10 mm)decreases,and the proportion of rock fragments limited in 10-20 mm size changes little.More intense movement of rock fragments induced by tillage occurred in bedrock slopes with higher moisture content.For bedrock slopes with different moisture contents,the responses of mean displacement distance(MDD)of rock fragments to slope gradient and tillage depth are different.On the bedrock slopes with higher water content,the MDD of rock fragments has a significant negative correlation with tillage depth,but no significant correlation with slope gradient.Conversely,on the bedrock slopes with lower water content,the MDD of rock fragments has no significant relationship with tillage depth,but has a significant positive correlation with slope gradient.The movement of rock fragments induced by tillage varies greatly in different depth-layers,and the smaller MDD of rock fragments occurred in the deeper layer.Soil-covered thickness played an important role in diminishing the movement of rock fragments in the process of hoeing,and there was no significant difference in the movement of rock fragments at different depth-layers under the overlying soil layer.In addition,slope gradient has no significant effect on the fragmentation degree and movement of rock fragments induced by hoeing into soil-covered bedrock.2.Effects of soil-debris mixture properties on hydro-erosional processesThe changes of soil-debris mixture thickness(Tsr),rock fragment content(W)and vertical gradient of rock fragment contents(Gv)caused by tillage of broken bedrock directly affect the hydrological properties and erosion durability of soil,and then affect the hydrological and erosional processes.1)With the increase of Tsr,the straight distance of water inflow infiltration path lengthens,and the preferential flow path decreases,which leads to the decrease of soil infiltration.Under 60 mm h-1 rain intensity,the stable infiltration rate decreased by 77.88%when the Tsr increased from 24 cm to 44 cm.Conversely,the soil-debris mixture with greater thickness shows worse erosion durability,and the sediment yield of 44 cm is 59.28%higher than that of 24 cm.2)The surface roughness and the number of soil non-capillary porosity increase with the increase of rock fragment content which can increase soil infiltration and reduce runoff volume.Compared with W=0%slope,the stable infiltration rate of W=20%and W=40%slope increased by 32.18%and 86.31%,respectively.The increase of rock fragment content helps to enhance the soil anti-erodibility,and the sediment yield of W=20%and W=40%slope is 54.82%and 62.05%less than that of W=0%slope,respectively.3)For slopes with same rock fragment content,the vertical gradient of rock fragment content(Gv,positive when the cuttings content in the lower layer is high)has an effect on slope hydro-erosional processes by changing the soil properties of the top layer.Firstly,the existence of Gv will lead to differences in soil infiltration in different depth-layers,and this difference will gradually increase with the increase of Gv.The top layer controls the total amount of water infiltration into the soil,and the infiltration amount of Gv=10%and Gv=20%slope is 19.92%and 23.42%less than that of Gv=0%,respectively Secondly,soil property of the upper layer affect the surface characteristic,and then affect the hydrodynamic characteristics of slope flow and the degree of soil erosion,and greaterrunoff volume,runoff coefficient,sediment content of runoff,Darcy-Weisbach friction factor and sediment yield occurred at slope with greater Gv In addition,the topsoil condition of Gv=20%slope is consistent with that of pure soil slope(CK),and there was no significant difference in stable infiltration rate,hydrodynamic characteristics,sediment yield between the two groups3.Effects of slope gradient on hydro-erosional processes for soil-debris mixture slopesSlope gradient is an important factor affecting hydrological process,but its action mechanism is different under different rainfall intensity.With the decrease of slope gradient,the infiltration rate and runoff amount increased gradually,and greater change amplitude of them occurred at greater rainfall intensity.There is a critical rainfall intensity(about 60 mm h-1)in the relationship of slope gradient to flow velocity and Reynolds number.When the rainfall intensity is less than the critical value,the flow velocity decreases gradually with the increase of slope gradient,and the Reynolds number has no obvious change.When the rainfall intensity approaches the critical value,the velocity has no obvious change with the increase of slope gradient,but Reynolds number decreased linearly.However,when the rainfall intensity is greater than the critical value,with the increase of slope gradient,flow velocity increases significantly and the Reynolds number decreases as a power function.Under great rainfall(120 mm h-1),the Froude number increases linearly with the increase of slope gradient.Meanwhile,slope gradient has no significant effect on Froude number under smaller rainfall intensity(26 and 60 mm h-1).The Darcy-Weisbach friction factor increases with the increase of slope gradient,but decreases significantly with the increase of rainfall intensity.Moreover,sediment yield increases linearly with the increase of slope gradient,and greater increase amplitude occurred at greater rainfall intensity4.The mechanism of bedrock fragmentation by tillage on hydro-erosional processesIn the short term,bedrock fragmentation by tillage can increase the thickness of soil layer and the content of rock fragments,but the movement of rock fragments varies greatly in different deep layers,and the overlying soil layer of bedrock can significantly restrain the movement of rock fragments,so there is a vertical difference in the distribution of rock fragments induced by hoeing into bedrock From a long-term view,bedrock fragmentation by tillage can lower the bedrock surface and flatten hillslopes gradient.The increase of soil-debris mixture thickness and the vertical gradient of rock fragment content,which caused by bedrock fragmentation induced by tillage,could lead to the decrease of infiltration,the increase of runoff volume,and the enhancement of sediment carrying capacity of runoff,which aggravate water erosion.Meanwhile,the increase of rock fragment content caused by hoeing into soil-covered bedrock and the slowness of topography caused by long-term tillage are helpful to increase infiltration,reduce runoff volume and sediment-carrying capacity of overland flow,which helps to reduce slope erosion.Therefore,it is suggested that when crushing bare bedrock to replenish soil,smaller depth should be used to reduce the average particle size of rock fragments,thus helping to improve the effect of water and soil conservation of rock fragments coverage.Moreover,when crushing soil-covered bedrock,it is recommended to using a tillage depth slightly greater than the thickness of the overlying soil,and to turn up or mix the rock fragments with the soil,which will help to reduce soil erosion on the slope.