Hortonian runo was measured from plots with lengths of 1 . 25 and 12 m, and at watershed level for rainstorms during the 1996 rainy season in cental Coà te d'Ivoire, Africa. A clear reduction in runo coecients was found with increasing slope lengths, giving order of magnitude dierences between runo measurements at point level (1 m 2 : 30±50% of total rain) and watershed level (130 ha: 4% of total rain). Runo reduction from 1 . 25 and 12 m slopes was reproduced for each major runo-producing rainstorm at two dierent sets of plots, but the reduction was erratic for rainfall events which produced little runo. In addition, runo reduction varied wildly from one rainstorm to the next. In the analysis, we show that the spatial variability of runo parameters causes the erratic behaviour during rainstorms with little runo. During the more important, larger runo-producing events, which give 78% of total runo, the temporal dynamics of the rainfall±runo process determine the reduction of runo coecients from longer slopes. A simple in®ltration/runo model was used to simulate the ®eld results, thereby con®rming the importance of rainfall dynamics as an explanatory factor for measured reduction of runo coecients.