INTRODUCTION

The nature of the soil surface is the key factor in deciding how rainfall will infiltrate and move through the soil, i.e. whether water will move downwards or sideways. Surface soil hydraulic properties control the rate of entry (i.e. infiltration) but, if unimpeded vertically, incoming water will move through the regolith as percolation to reach the water table. More commonly, however, there is a reduction in the permeability in the upper soil horizons at various points because of the presence of more impervious soil layers. These deflect water laterally, either at the surface (as infiltration excess (Hortonian) overland flow, HOF (Horton, 1933; 1945)) or subsurface (as subsurface stormflow, SSF, or interflow) (Chorley, 1978). This SSF can emerge at the surface as return flow and combine with precipitation falling on saturated soils to produce saturation (or saturation-excess) overland flow, SOF. This is also known as the Dunne mechanism (Dunne and Black, 1970a, b).

As highlighted by Bonell and Williams (1989), the soil hydraulic properties of ‘undisturbed’ tropical landscapes tend to be in equilibrium with the prevailing rainfall characteristics (notably short-term rain intensities). Thus in closed tropical forest, HOF is not generally favoured (exceptions will be outlined later) because the dense root mat and the incorporation of soil organic matter in the topmost soil layers encourage very high infiltration rates. Annual erosion rates from closed tropical forests at the headwater catchment scale are thus small in comparison with disturbed landscapes (see Douglas and Guyot; Chappell, Tych et al., this volume).