Effects of surface water storage by soil roughness on overland‐flow generation

Overland‐flow generation is affected by surface roughness. This study analysed effects of surface roughness on overland‐flow by means of a model simulating depression filling, flow pathways and runoff generation. It examined the relevant characteristics of surface roughness using numerically generated surfaces. At first, horizontal surfaces with random roughness of different correlation distances were considered. This showed that overland‐flow is sensitive to the random‐roughness correlation only if the correlation distance is similar to the surface size. Because this correlation distance is limited in natural soil surfaces, correlation properties are unlikely to modify overland‐flow triggering. This allowed simplification of the description of random roughness. Then a general slope was added and different amplitude ratios between general slope and random roughness were simulated. Slope modified the development of overland‐flow. While runoff was only sensitive to steep slopes, water transfer by overland‐flow inside the surface was affected by low slopes too. The ratio between general slope and random roughness is a major parameter that controls transfers both inside the surface and to the outlet.