Hydrological And Erosion Characteristics Of Hillslopes With Different Purple Soil Thickness In The Three Gorges Area Of China

Focusing on how purple soil architecture (e.g., horizon interface, soil thickness, soil-rock interface, porosity, surface seal, and root architecture) exerts a first-order control on soil processes (including hydrological processes and the associated erosional responses), this study investigated the plot-scale hydrological and erosional characteristics of the sloping lands with different purple soil thickness under the theoretical framework of hydropedology. Firstly, various hydrological processes and flow generation mechanisms were identified in each purple soil plot with different soil thickness through field survey and in situ rainfall simulation experiments, and the soil erosion characteristics under the simultaneous influence of surface flow and subsurface flow were also investigated. Second, we studied the impact of boundary conditions, including soil surface seal and rainfall intensity, on soil water and subsurface flow in shallow purple soil slope. Lastly, the relationship between purple soil properties and hydrological/erosional characteristics was discussed, and the main controls on the hydrological and erosion responses in purple soils exerted by the purple soil architecture were illustrated. Collectively, the purple soil erosional regimes under the simultaneous influence of various flow generation mechanisms were studied, which altogether deepen the knowledge of both hydrological processes and soil erosion regime in the purple soil area. The main results of the study can be presented as follows.(1) In general, the soil profiles of all plots showed high rock fragment content and low clay content, while sand and silt were the dominant particle size fractions. It indicates that the purple soils of this area exhibited minimal horizon development and displayed features of Inceptisols for the23,31and45cm plots, and Alfisol for the59and76cm plots which showed a more significant trend of clay translocation from the upper to the lower soil layer. The data imply that thick soils reflect deeper soil development than thin soils. All of the plots displayed high Kfs values (ranged from34to78mm h-1) in the plow layer but decreased with increasing soil depth. Overall, thin soils showed higher decreasing values than that of thick soils.(2) For all the rainfall events, all portion parts of rainfall water showed approximately linear relationships with the soil thickness:surface flow and soil storage water were positively related to soil thickness, while subsurface flow and deep percolation were negatively related to soil thickness. Compared to thin soils, thick soils had higher surface flow and soil storage water but lower subsurface flow and deep percolation. Through adopting the hydropedological approach, hydrological processes of the typical sloping land plots along a hillslope with different soil thickness were investigated by conducting rainfall simulation experiments (designed rainfall intensities of60mm/h for2hours) using a portable rainfall simulator. The results can be summarized as follows. First, the23cm plot was in the form of saturated overland flow, while the45and76cm plots were infiltration-excess overland flow as to the surface flow generation mechanism. Second, as to the subsurface flow generation mechanism, the23cm plot mainly took the form of preferential flow, conversely, the76cm plot mainly in the form of matrix flow. However, in between the above two cases, the45cm plot displayed mainly the matrix flow in the A horizon and mainly the preferential flow in the AC horizon. Our results indicated that those markedly different soil architectures among soils with different thickness in the purple soil area were the main reason for different flow generation mechanisms among these three experimental plots.(3) Due to their more distinct hydrological processes of subsurface flow and deep percolation in thin soils than those of thick soils, the soil erosion rates were significantly lower in thin soils than in thick soils in all rainfall events, which increased from211g m-2h-1for the23cm plot with rainfall of60mm h-1to4220g m-2h-1for the76cm plot with rainfall of120mm h-1. The23and31cm plots showed a modest increase in soil erosion rate as rainfall intensity increased, which implies that they exhibited strong anti-erosion capacities. On the contrary, the45,59and76cm plots showed a dramatic increase in soil erosion rate, indicating a high erodibility characteristic.(4) By applying mathematical statistics methods, the parameters of the water-content wave and subsurface stormflow hydrograph which derived from in-situ simulated rainfall were analysised, and the effects of rainfall intensity and surface soil bulk density (0～5mm depth) on subsurface stormflow of a shallow purple soil slope was studied. Results show that the wetting front velocities are between0.28and1.63mm/s, which are much higher than the velocities expected from flows that are driven by the combined gradients of gravity and capillarity, so they represent preferential flow. Rainfall intensity and surface soil bulk density are the most important factors affect soil water movement and subsurface stormflow. Increasing the rainfall intensity facilitated the initiating and sustaining of preferential flow, while surface seal is the main reason impedes preferential flow.Our study demonstrated that purple soil architecture (e.g., soil thickness, horizon interface, soil-rock interface, porosity, surface seal, and root architecture) exerted a first-order control on hydrological processes and the associated erosional dynamics at the plot scale. More than the soil thickness itself, distinct pedogenic features (e.g., mechanical composition, rock fragments, KfS variation pattern in the soil profile, horizonation, and surface seal) and anthropogenic impacts (e.g., human intervention intensity and cultivation period) of the purple soils with different soil thickness were the main causes of differential hydrological and erosional characteristics among the experimental plots. Our study indicated that, relative to its parent disciplines of both pedology and hydrology, synergistic integration was generated by the hydropedological approach in the plot-scale hydrological and erosion processes investigations on the sloping lands with different soil thickness in the Three Gorges Area of China. The emerging interdisciplinary field of hydropedology promotes the holistic study of various ecological processes across space and time in the landscape with distinct soil heterogeneity as the purple soil area in this study.