The flow of water over a hillslope surface. When water infiltrates a soil and all the pore spaces are full, the soil is saturated and the water table is at the surface. Overland flow will then occur. This is saturation-excess overland flow, which can occur at much lower rainfall intensities than those required to generate infiltration-excess overland flow (also known as Hortonian overland flow). Saturation-excess overland flow can occur at the foot of a hillslope even when it is not raining. Overland flow may be laminar, turbulent, and transitional, or consist of patches of any of these flow states (A. J. Parsons and A. D. Abrahams 1992). Horton (1945) GSA Bull. 56 suggests that overland flow is likely to be laminar near watersheds, but with distance downslope overland flow becomes more turbulent (Smith and Bretherton (1972) Water Resources Res. 8). Horton (1945) explains that these changes result from the microtopography of the soil surface: run-off gathers into depressions, which increase in size downslope. As the depressions deepen they capture more flow through the cross-grading of the hillslope surface (Dunne et al. in J. Costa and P. J. Fleisher, eds 1995). Surface run-off deepens as a result of this ‘micro piracy’; see Croke et al. (2005) Geomorph. 68, 3–4.

The Darcy–Weisbach, Chézy, and Manning equations are used to predict overland flow velocity (Nunnally (1985) Env. Manage. 9, 5). Smith et al. (2007) PPG 31, 4 review the problems of measuring overland flow, and evaluate some solutions.