Study On Characteristics Of Preferential Flow On Four Land Use Types In Simian Mountain Of Chongqing

Preferential flow is a common type of water infiltration in soil that can lead to several problems, such as soil nutrient loss, groundwater pollution, and slope stability failure. The characteristics of soil preferential flow must be investigated to reveal their effects on soil water infiltration processes and to understand the influence of soil hydrological processes in the regional water cycle. The findings can provide scientific support for the assessment of regional water resources, agricultural water-saving irrigation, groundwater pollution risk evaluation, and geological disaster prevention.This study was conducted in the conifer-broadleaf forestland (Pinus massoniana, Schima superba, and Lithocarpus glaber), Phyllostachys pubescens forestland, grassland(Arachniodes exilis, Dichondra repen, and Aster ageratoides), and farmland (Glycine max) of Simian Mountain in Chongqing, which is located in the Three Gorges Reservoir Area of China. Combined with experimental methods, such as the brilliant blue dyeing test, and simulating the soil column patterns of soil water flow infiltration, this study applies image analytics and mathematical statistics analysis, stand spatial structure analysis, spatial point pattern analysis, and grey relational analysis to investigate the preferential flow dyed morphological characteristics, the preferential paths spatial structure characteristics, the preferential flow degree, and the major factors that influence the preferential flow of the four typical land use types, respectively. The characteristics of preferential flow were systematically revealed on four land use types in Simian Mountain. The main conclusions are summarized as follows:(1) The preferential flow accompanies the matrix flow in the water infiltration processes of the four land use types. The dyed pattern areas of preferential flow were 0.7 to 10.7 times greater than those of the matrix flow. At the same water supply condition, the occurrence of preferential flow was the fastest in the conifer-broadleaf forestland. The preferential flow dyed pattern in soil profiles with vertical extension and differentiation degree was most obvious in all land use types. Nevertheless, the differentiation degrees of the preferential flow dyed pattern were decreased in the Phyllostachys pubescens forestland and grassland, which indicated that the morphological characteristics were laterally deviated and block dyed to a certain extent. The occurrence of preferential flow was the slowest in farmland, and the matrix flow accounted for the majority of the soil water movement in farmland. Therefore, the occurrence of preferential flow was the slowest in farmland. The soil dyed coverage ratio reduced along with the depths of soil vertical direction. The matrix flow occurred 0 cm to 10 cm from the soil surface, and the preferential flow occurred 10 cm to 40 cm from the soil surface. The morphological dyed distribution patterns in the soil horizontal profile could be divided into "multimodal pattern," "bimodal pattern," "unimodal pattern," and "uniform pattern," while those in conifer-broadleaf forestland, Phyllostachys pubescens forestland, and grassland were identified as "multimodal pattern," "bimodal pattern," and "from bimodal to unimodal to uniform pattern," respectively. However, the dyed pattern of farmland was mainly uniform, which suggested the lowest degree of dyed pattern differentiation. The occurrence and change degrees of preferential flow in conifer-broadleaf forestland and Phyllostachys pubescens forestland changed from active, second active, active, and to fast-changing, while the change degrees of preferential flow in grassland and farmland changed from second active, active, fast-changing, and to relatively stable.(2) The number and spatial connectivity of preferential paths were determined as follows: conifer-broadleaf forestland> Phyllostachys pubescens forestland> grassland> farmland. The soil preferential paths tended to demonstrate a clumped distribution pattern along with increasing soil depths. The four land use types were arranged from highest to lowest as follows in terms of degrees of clumped distribution:conifer-broadleaf forestland> Phyllostachys pubescens forestland> grassland> farmland. The spatial structures of the preferential paths of these four land use types exhibited a high mixture pattern or complexity in the surface soil and community structures (i.e., all paths are of uniform pore size) in deeper soil. The development degrees of the different preferential paths showed a decreasing tendency from the surface to the bottom layers of all experimental plots. The formation and distribution of preferential paths were influenced by other preferential paths and by the biological activities in the conifer-broadleaf forestland, Phyllostachys pubescens forestland, and grassland. The preferential paths were more significantly associated with the biological activities in farmland than those in the other land use types.(3) A contrastive experiment of disturbed and undisturbed soil columns showed that the water and solute flowed through undisturbed soil columns much faster than through disturbed soil columns, and that higher preferential flow dyed pattern differentiation degrees were observed in undisturbed than in disturbed soil columns. The characteristics of water and solute transport significantly differed across various structured undisturbed soil columns. The four land use types could be ranked from shortest to longest as follows in terms of the time for the water to flow through undisturbed soil columns: conifer-broadleaf forestland, Phyllostachys pubescens forestland, grassland, and farmland. Conifer-broadleaf forestland had the largest saturated hydraulic conductivity. By contrast, the characteristics of water and solute transport in farmland were not significant as compared with those of the other land use types. The variation coefficient of the soil dyed coverage ratio in the preferential flow area of soil columns reflected the occurrence degrees of preferential flow. Therefore, the four land use types could be ranked as follows in terms of the occurrence and change degrees of the preferential flow of undisturbed soil columns:conifer-broadleaf forestland> Phyllostachys pubescens forestland> grassland> farmland. As revealed in the grey correlation analysis, the four land use types were ranked from highest to lowest as follows in terms of degrees of preferential flow:conifer-broadleaf forestland (91.62%)> Phyllostachys pubescens forestland (90.77%)> grassland (87.70%)> farmland (85.51%). The average degree of preferential flow was 88.90% in all land use types.(4) The occurrence and development of preferential flow were determined by external and internal factors. Following Spearman’s correlation analysis, we used soil density, sand grain content, slit and clay ratio, capillary porosity, non-capillary porosity, total porosity, soil organic matter content, soil saturated water conductivity, soil volumetric moisture content, diameters of root length density, number of soil root holes, and density weight of root to analyze further the major factors that influenced the occurrence and development of preferential flow via principal component analysis. Soil pore structure (48.92% contribution rate), soil moisture condition (23.46% contribution rate), and soil type (18.33% contribution rate) were the main three factors that influenced the occurrence and development of preferential flow in Simian Mountain, with soil pore structure being the most dominant.