Changes in Preferential Flow Path Distribution and Its Affecting Factors in Southwest China

Preferential flow is important in solute transport in the soil, and it indirectly affects the quality of the groundwater. The objectives of the present study were to investigate the distribution characteristics of preferential flow paths under different land-use types (forestland, shrubland, and farmland) and to determine the factors that affect the formation of these paths. Dye tracer experiments were conducted at six sites covering the three land-use types. The relationship of the preferential flow paths to the properties of the soil, root length per soil volume, root holes, and root surface area was analyzed using a partial correlation method. The results show dye coverage of greater than 50% at a soil depth of 0 to 10 cm and dye coverage of less than 30% below a depth of 20 cm in all of the land-use types. The vertical distribution differed widely in the various land-use types. Shrubland and farmland had more preferential flow paths in the topsoil compared with forestland, in contrast to the conditions in the deeper soil layers. The soil chemical properties and the saturated hydraulic conductivity in the preferential flow paths were quite different from those in the soil matrix. The soil organic matter content was approximately 5% to 20% higher in the preferential flow paths than in the soil matrix. The average value of the saturated hydraulic conductivity was 41.3 mm min(-1) in the preferential flow paths compared with 21.4 mm min(-1) in the soil matrix. The percentage of preferential flow paths in the soil was significantly affected by roots with a diameter of less than 5 mm, root holes, and the root surface area. These findings suggest that land use is significant in the distribution of preferential flow paths, and root characteristics are important in the formation of preferential flow paths.