| Soil and landscape formation is a dynamic process in terrestrial ecosystems.Soil development on a specific landscape leads to soil-horizon interface differentiation and changes in soil architecture,thereby affecting more complex interactions between different hydrological processes and water storage.In the Three Gorges Reservoir area with undulating terrain,the soil characterized by"overlying soil and underlying parent material"is widely distributed in the middle and upper parts of the mountainous area due to the loose vegetation cover and serious soil erosion.Hydrological processes such as seepage,evaporation and runoff are significantly affected by soil and the soil/parent material interface.These soils had special runoff processes,which have an important impact on the transformation process of precipitation and soil water on hillslopes and the regional water cycle.In view of the particularity of parent material and the importance of the dual medium of soil and parent rock,the study of the typical soil profiles developed upon different parent rocks and their soil hydrological process in the Three Gorges Reservoir area is helpful to reveal the inherent mechanism of runoff generation in mountainous area and provide a theory for the simulation of hydrological process.Moreover,it can also provide a scientific basis for the rational utilization of water and soil resources in mountainous areas of China.This study takes the typical soil profile architecture distributed in the Three Gorges Reservoir area as the research object,and takes the influence mechanism of soil architecture on hydrological process as the main research content.In this study,two typical soil profiles(A-C profile and A-B-C profile)developed on three parent materials including granite,shale and purple sandstone in the Three Gorges Reservoir area were studied.Firstly,the bulk density,pore characteristics,rock fragment content,particle size distribution,water retention and water conduction properties of soil layers and parent material layers of typical soil profile configurations were investigated through a large number of sampling,and the differences in soil physicochemical and hydraulic properties between different profile layers were clarified.Through CT scanning technology and field in-situ infiltration experiments,the characteristics of soil macropore structure in typical profiles were quantified,and the infiltration characteristics of water in soil horizons,soil horizon interfaces,and parent material horizons of soils with different profiles were studied.The internal influence mechanism of macropore structure and soil horizon property difference on soil infiltration was studied.Finally,the in-situ plot simulation rainfall experiment was used to study the soil architecture of different profiles under three rainfall intensities(60,90,120 mm/h).The process and characteristics of surface runoff,subsurface runoff in soil,and water redistribution,the differences of hydrological processes under different soil interfaces and soil/parent material interfaces and their responses to rainfall intensity and profile structure were compared and analyzed,and several typical profiles were summarized.Intrinsic links of runoff generation mechanisms between soil properties in a soil profile and hydrological processes were generalized.The main findings of the study include:(1)The A-C profiles developed by the three parent materials were all characterized by shallow soil thickness with an average soil depth of 17 cm and high rock fragment contents.These soils are typical lithological soils and the soil particles are mainly sand and silt,similar to parent materials.Among the three parent materials,the soil in the A-C profiles developed by granite had the highest sand content,the lowest clay content,the lowest rock fragment content and smaller particle sizes,and low saturated hydraulic conductivity.The rock fragments were dominant by the size fractions of 5-10 mm and 10-20 mm;soil developed by shales had the highest clay content,the highest rock fragment content with large size,and high saturated hydraulic conductivity,and the rock fragments were dominant by the size fractions of 10-20 mm and>20 mm;the soil developed by purple sandstone had the middle contents of sand and clay,the largest saturated hydraulic conductivity,and the high rock fragment content with the dominant particle size of>20mm.Compared with the A-C profile,the soil in the A-B-C profile developed by the three parent materials had the characteristics of relatively thick soil thickness,low gravel content,high clay content and low saturated hydraulic conductivity.(2)There were significant differences in soil properties between different soil horizons.In the soil of A-C profile,the bulk density,rock fragment and sand content of C horizon were significantly greater than those of A horizon,saturated hydraulic conductivity,total porosity,non-capillary porosity,clay and organic matter were significantly lower than those of A horizon.In the soil of A-B-C profile,the bulk density and clay content of B horizon were significantly higher than those of A horizon,the content of organic matter is significantly lower than that of A horizon,and other soil properties had no significant differences;The differences in soil properties between A-C profile and A-B-C profile were different.The differences in soil rock fragment content,mechanical composition,bulk density and non-capillary porosity of the A-C profiles were significantly larger than those of the A-B-C profiles.Soil properties of different horizons were affected both by profile architecture and parent material.The difference of soil properties between the horizons were mainly reflected in the saturated hydraulic conductivity,sand,silt,organic matter content and non-capillary porosity.Parent material and profile architecture explained 36.05%and 29.18%of the variances in saturated hydraulic conductivity and non-capillary porosity between different horizons.The results of redundancy analysis showed that soil bulk density,rock fragment content and mechanical composition were the key factors affecting the saturated hydraulic conductivity between the profile horizons.It is highly positively correlated with sand content,organic carbon content and saturated hydraulic conductivity,saturated water content.The first and second ranking axes of redundancy analysis could explain 94.90%of the relationships between soil physiochemical properties and hydraulic properties.(3)The three soils developed by lithologic parent materials all characterized by large porosity,wide distribution of macropores,large differences in pore structure,and irregular shapes.In the A-C profiles,the macropores in A horizon developed by granites and shales were mainly large pore volume of 10-100 mm~3,the macropores in A horizon developed by purple sandstone were mainly large pore volume of>100 mm~3;and the macropores in C horizon of granite soils were mainly medium and small pore volume of 0.1-10 mm~3.the macropores in C horizon of shale and purple sandstone soils were mainly medium and small pore volume of 10-100 mm~3.In the A-B-C profiles,the macropores in different soil horizons developed by the three parent materials were all mainly large pore volume of 10-100 mm~3.The variability of soil macropores in the A-C profile was significantly larger than that in the A-B-C profile.The soil macroporosity of the two profiles showed a significant decreasing trend at the horizon interface,and the pores were discontinuous.The soil macropore structure parameters could explain the variations of saturated hydraulic conductivity.The combination of macroporosity and average curvature in the pore parameters had the highest contribution to the variability of soil saturated hydraulic conductivity,and the coefficient of determination is 0.75.(4)There were significant differences in infiltration capacity among different profiles developed by three parent materials.The infiltration capacity of A horizon and C horizon in the A-C profiles developed by three parent materials were all in the following order:shale-developed soil>purple sandstone-developed soil>granite-developed soil.The initial infiltration rate and steady infiltration rate of A horizon in shale-developed soil were the largest,reaching 1640.22 mm/h and 1002.33 mm/h,respectively;in the A-B-C profile,except for the cumulative infiltration amount of B horizon,the infiltration parameters of different horizons were all in the order of shale-developed soil>granite-developed soil>purple sandstone-developed soil.In the A-C profile,the infiltration capacity of A horizon was significantly greater than that of C horizon.The average initial infiltration rate and stable infiltration rate of A horizon were 1.77 and 1.73 times that of C horizon,respectively.Granite,shale and purple sandstone are.The stable infiltration rates of B horizon in the A-B-C profile developed by granite,shale and purple sandstone were23.88%,55.82%and 29.57%lower than those of A horizon,respectively.With the increase of soil depths,the infiltration capacity of soil developed by different parent materials decreased.Under the condition of infiltration water volume in this study,the preferential flow paths of the two profiles had abrupt changes,and there was an obvious blocking phenomenon;the water in the A-C profiles developed by three parent materials rarely entered into the C horizon after reaching the AC interface.The stain area ratio was shown as purple sandstone-developed soil>shale-developed soil>granite-developed soil;in the A-B-C profiles,preferential flow path with better connectivity could still develop in the B horizon after the water flows the AB interface.The preferential flow ratio was in the order of purple sandstone-developed soil>shale-developed soil>granite-developed soil;soil pore structure and water conductivity were the main factors affecting the formation of the preferential flow of the typical lithologic parent materials developed soils in the Three Gorges Reservoir area,which can explain 46.92%of the variance of the variation of the preferential flow formation.(5)During the rainfall process,the distribution of rainfall and soil water in the A-C profiles developed by three parent materials was dominated by soil water storage and deep infiltration,while the surface runoff was less,and the phenomenon of subsurface flow occurs in all of them.The proportions of precipitation were all in the order of purple sandstone-developed soil>granite-developed soil>shale-developed soil,and the deep infiltration of the shale-developed A-C profile was the largest;the rainfall and soil moisture distribution of the A-B-C profiles developed by three parent materials were all dominant by soil water storage and the proportion of surface and subsurface runoff was low.With the increase of rainfall intensity,all plots showed that the proportion of surface runoff increased,and the proportion of subsurface flow,soil water storage and deep infiltration decreased significantly.The soil with A-C profile developed in granite and shale was dominant by saturation-excess runoff under moderate and small rainfall intensities,and was dominant by infiltration-excess runoff under heavy rainfall intensities.The subsurface runoff was dominant by preferential flow in A-C profile developed by granite under different rainfall intensities.In the A-C profile developed by shale,the subsurface runoff mechanism in the A horizon was dominated by matrix flow under the condition of medium and small rainfall intensities.The subsurface runoff mechanism in the C horizon was dominated by the preferential flow.The runoff mechanism of different runoff components in A-B-C profile soil was not affected by rainfall intensity.The mechanism of surface runoff was dominated by infiltration-excess runoff,and the mechanism of subsurface runoff in A and B horizons was dominated by matrix flow.Under the condition of small rainfall intensity,the differences of sand,rock fragment content and saturated hydraulic conductivity among the profile horizons were the key factors affecting the surface and subsurface runoff coefficient.Surface hydrological processes were the result of the interaction of rainfall intensity and soil properties. |
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