Abstract
Topography controls surface flows, and thereby exerts a significant action on soil formation. At the hillslope scale, infiltrability of the surface horizon varies gradually along the slope. In semi‐arid zones, and especially in the Sahel, runoff is Hortonian and depends mainly on the hydraulic properties of the soil surface horizon (saturated hydraulic conductivity K~s~ and hydraulic roughness of the soil surface n). Using the fully distributed hydrologic model r.water.fea as an experimentation tool, this paper investigates the effects of various spatial distributions of K~s~ (deterministic, stochastic or a combination of both, all with an invariant global mean) and related n (taken as fully correlated to K~s~) on the outflow of a small catchment representative of Sahelian conditions. In addition to a uniform distribution used as reference, deterministic distributions here consist of linear variations of K~s~ with elevation. A stochastic component is then added by drawing from a log‐normal distribution with different variation coefficients C~v~. Both hypothetical and real rainstorm events are tested.
All K~s~ distributions studied produce hydrographs that are very close to the uniform K~s~ case when rainfall is long and intense. For most other rain events, runoff increases with K~s~ variability. Whatever the rainfall event and C~v~, outflow is greater when the less infiltrative surfaces are located downhill. The ratio of deterministic to purely stochastic variation is a good indication of the relative importance of the two K~s~ variation sources for catchment runoff. Given the high local‐scale (stochastic) soil variability typically encountered, only strong catchment‐scale contrasts really deserve to be included in the K~s~ distribution for runoff modelling of all but insignificant events. Spatial K~s~ representation can be further simplified down to a uniform distribution, when only a seasonal water yield is the required result. Copyright © 2002 John Wiley & Sons, Ltd.