Soil hydrophobicity variations with depth and particle size fraction in burned and unburned Eucalyptus globulus and Pinus pinaster forest terrain in the Agueda Basin, Portugal

A laboratory programme of water drop penetration tests is used to investigate the nature of hydrophobicity of soils in burnt and unburnt Eucalyptus globulus and Pinus pinaster forest areas of northern Portugal. Variations in hydrophobicity of air-dried soil with soil depth, soil particle size fraction, land use and burn history are assessed. Results differ from those found by many other studies in several respects: (1) fire was not found to enhance hydrophobicity, as unburnt and old-burn soils are as hydrophobic as newly-burnt ones; (2) hydrophobicity was found to be characteristic of soils from the surface down to the weathered (Cw) horizon rather than confined to a near-surface layer, (3) it is also associated with the finer rather than the coarser size fractions of the soils. Soils under E. globulus are distinctly more hydrophobic than those under P. pinaster. Implications for the generation of overland flow are briefly explored.