Distribution Of Rock Fragments And Their Effects On Hillslope Soil Erosion In Purple Soil

Purple soil is widely distributed in Sichuan Basin and Three Gorges Reservoir Area. Purple soil region is abundant in soil fertility and hydrothermal resources, playing an important role in the agricultural development of China. Soil erosion has long been recognized as a major environmental problem in the purple soil region where the population is large and slope farming is commonly practiced, and rainstorm is numerous. The existence of rock fragments is one of the most important characteristics of purple soil. Rock fragments at the soil surface or in the soil layer affect soil erosion processes by water in various direct and indirect ways, thus the erosion processes of soil containing rock fragments have unique features. Against the severe soil degradation by erosion of purple soil slope, carrying out the research about the characteristics of purple soil containing rock fragments and understanding the influence of rock fragments on soil erosion processes have important significance, which would promote the rational utilization of purple soil slope land resources and accurate prediction of purple soil loss. Therefore, the aims of this study were to investigate the distribution of rock fragments in purple soil slope and the impact of rock fragment content on soil physical properties and soil erosion. First, field sampling methods were used to survey the spatial variability of rock fragments in soil profiles and along slope and the physical properties of soils containing rock fragments. Secondly, indoor simulated rainfall experiments were used to exam the effect of rock fragments in the soil layer on soil erosion processes and the relationships between rainfall infiltration, change of surface flow velocity, surface runoff volume and sediment on one hand, and rock fragment content (Rv,0%～30%, which was determined according the results of field investigation for rock fragment distribution) on the other were investigated. Thirdly, systematic analysis about the influence of rock fragment cover on purple soil slope erosion process were carried on, under different conditions with two kind of rock fragment positions (resting on soil surface and embedded into top soil layer), varied rock fragment coverage (Rc,0%～40%), two kind of soils with textural porosity or structural porosity, and three kind of rainfall intensities (I,1mm/min,1.5mm/min and2mm/min). Simulated rainfall experiments in situ plots in the field, combined with simulated rainfall experiments in soil pans indoor, were used. The main conclusions of this dissertation are as following:1. The spatial distribution characteristics of rock fragments in purple soil slope and its effects on the soil physical properties were clarified basically:(1) Nearly all of the purple soils and each layer of it contained rock fragments. Rock fragment content of purple soil concentrated in0%-40%. The small rock fragments (5-20mm) and medium rock fragments (20～76mm) were the major component of rock fragments in the soil. The mean ratios of content of rock fragments with diameter<76mm (the content of rock fragments (<76mm)/the total content of rock fragments (5～250mm),%) in different soil layers were larger than80%.(2) Soil depth had significant influence on rock fragment content and ratio of rock fragments with varied diameters because of the soil formation processes. Medium rock fragment content, big rock fragment (76～250mm) content and total rock fragment content increased from top soil layer to bottom soil layer. However, small rock fragment content hadn't significant differences in soil layers at different depth. Meanwhile, with increasing soil depth, ratio of small rock fragment content decreased, ratios of medium rock fragment content and big rock fragment content were increased, thus equivalent diameter increased.(3) The changes of rock fragment content along slopes with different gradients were different. Because the main controlling factors which influence the distribution of rock fragment were determined by slope gradient to a large extent. On the steep hillslope with gradient of28°, the processes of rock fragments rolling or creeping to the lower slope by gravity were dominant for the distribution of rock fragment. The total rock fragment content and equivalent diameters in different soil layers decreased from bottom (19.5%～54.9%,50～84mm) to top (3.6%～6.6%,18～46mm) along the slope and rock fragments with various sizes (5～20,20～76, and76～250mm) showed a similar increasing trend. On the gentle hillslope with gradient of20°, the total rock fragment content in different soil layers increased from bottom (0.1%～2.0%) to top (18.2%～28.3%) and rock fragments with various sizes (5～20,20～76, and76～250mm) showed a similar increasing trend. However, the equivalent diameters of rock fragments except in bottom soil layer hadn't significant difference along the slope.(4) Rock fragment content had significant influence on physical properties of soil such as total bulk density, fine earth bulk density and porosity distribution. With increasing rock fragment content, total bulk density increased while fine earth bulk density decreased, total porosity and capillary porosity decreased while non-capillary porosity increased.2. The mechanism of influence of rock fragments within top soil layer on soil erosion processes was understood and a threshold of rock fragment content on the infiltration was figured out:(1) The influences of rock fragments within top soil layer on infiltration were ambivalent. On one hand, the existence of rock fragments within top soil layer restricted the movement of water by reducing the cross-sectional area available for flow. On the other hand, rock fragments create new non-porosity, thereby increasing infiltration rates. When the rock fragment content (Rv) was less than20%-30%, infiltration capacity of soil was strengthened by rock fragments. With increasing rock fragment content, infiltration rate and accumulative infiltration volume increased from0.14mm/min and14.25mm for soil with0%rock fragment content to0.18mm/min and19.19mm for soil with20%rock fragment content. However,30%volume content of rock fragments would restrict the infiltration processes, the infiltration rate and accumulative infiltration volume were decreased to0.12mm/min and11.59mm, respectively.(2) Rock fragments within top soil layer influenced the changes of interrill flow velocity and rill flow velocity by different mechanisms. In the mixture of rock fragments and fine earth, the rock fragments were embedded into the soil surface, the interrill flow was concentrated on the soil surface between rock fragments. With increasing rock fragment content, mean interrill flow velocity increased from4.0cm/s for soil with0%rock fragment content to5.7cm/s for soil with30%rock fragment content. However, when interrill area evolved into rills, more rock fragments outcropped in rills. With increasing rock fragment content, hydraulic roughness increased, thus the mean rill flow velocity decreased from16.0cm/s to11.2cm/s.(3) The existence of rock fragments in top layer had changed the composition and structure of soil, and enhanced the anti-erodibility and anti-scourability of soil. Sediment concentration decreased significantly by increased rock fragment content. Thus, with increasing rock fragment content, soil loss decreased and mean soil erosion rate decreased from155g/(m2·min) for soil with0%rock fragment content to29g/(m2·min) for soil with30%rock fragment content. Rock fragment content (Rv) was related to relative soil loss (the ratio of soil loss rates between soil with rock fragments and soil without rock fragments, Rsl) and by a negatively exponential function with a high degree of reliability:Rsl=a·exp(-b·Rv).3. The relationships between surface rock fragment cover and hillslope soil erosion in purple soil under different conditions with varied rock fragment positions, soil structures and rainfall intensities were obtained and the soil and water conservation function of surface rock fragment cover on reducing soil loss was affirmed:(1) The relationships between rock fragment coverage (Rc) and parameters related to soil erosion processes influenced by rock fragment position and soil structure and rock fragment cover couldn't always reduce surface runoff volume and sediment yield. In soil with texture porosity, when rock fragments rested on soil surface, part or whole rock flow (runoff generated by the rock surface) would be absorbed by soil beneath the rock fragments and the infiltration of rainwater increased. Thus, surface flow volume and sediment yield decreased with increasing rock fragment coverage. When rock fragments embedded into soil surface, all of the rock flow converged into surface flow between rock fragments and the infiltration of rainwater decreased. Thus, surface flow volume and sediment yield increased with increasing rock fragment coverage.(2) In soil with structure porosity, rock fragments resting on soil surface had significant influence on parameters related to erosion processes under different rainfall intensities. First, rock fragment cover had a function to adjust the distribution of rainwater. Rock fragment cover increased surface roughness and surface depression. Meanwhile, rock fragment cover protected surface soil from splash of raindrop and maintained soil infiltration capacity. The steady-state infiltration rates increased from0.28,0.33,0.40mm/min (Rc=0%) to0.71,0.85,0.94mm/min (Rc=40%) under three rainfall intensities, respectively. With increasing rock fragment coverage, the times to start surface flow were delayed and the times to start subsurface flow were shortened, surface flow coefficients were deduced, subsurface flow coefficients and deep infiltration coefficients were increased under different rainfall intensities. Secondly, rock fragment cover protected soil beneath rock fragments from splash by raindrops and flush by surface flow. With increasing rock fragment coverage, sediment concentrations decreased from39～51g/L (Rc=0%) to10～15g/L (Rc=40%). Rock fragment cover inhibited soil loss by reducing surface flow and surface soil area which could be eroded. Rock fragment coverage (Rc) was related to relative soil loss (the ratio of soil loss rates between soil with rock fragments and soil without rock fragments, Rsl) and by a negatively exponential function with a high degree of reliability under different rainfall intensities:Rsl=a·exp(-b·Rc). Meanwhile, rock fragment coverages of20%-30%were the optimal because of the marginal effect of rock fragment cover on parameters related to soil erosion.(3) The effectiveness (b-value) of rock fragment cover in reducing surface flow volume and sediment yield depended on soil structure. The surface flow rate, sediment cocencration and erosion rate was generally lower for soil with structural porosity than for soil with textural porosity. Rock fragment cover was far less efficient in reducing soil loss for soil with textural porosity (b=0.036) than for soil with structural porosity (b=0.004).