Preferential Flow Effects on Infiltration and Runoff in Grassland and Forest Soils

Due to the heterogeneity of soil hydraulic properties, there are many ways in which natural soils respond to rainfall, making upscaling of flow processes from plot to catchment scale difficult. The objectives of this study were to qualitatively characterize the flow pathways on forest and grassland hillslopes and to quantitatively define the relevant parameters controlling surface runoff generation by using infiltration and dye tracer experiments supplemented with measurements of the saturated hydraulic conductivity K-sat, the structural porosity n(SP), and the pore-size distribution PSD. While infiltration excess overland flow dominates in grassland, forest soil structure characterized by relatively high values of K-sat and n(SP) enhances the infiltrability of the soil and consequently prevents or at least reduces surface runoff. The dye patterns suggest that macropores are more efficient in forest than in grassland soil. The low efficiency of grassland soil macropores in transporting all water vertically downward can be explained by (i) the fine and dense few topsoil layers caused by the land use that limit water flux into the underlying macropores and (ii) their restricted number, their tortuosity, and the restricted interaction between macropores and the matrix below the topsoil layer. The larger root water uptake of forest soil as compared to grassland soil can be viewed as an additional factor enhancing its storage capacity and, consequently, may reduce the generation of surface runoff. It remains unclear, however, what effect the low interaction between macropores and soil matrix in the upper part of the subsoil has on surface runoff in grassland soil; this should be investigated in future studies.