Effect of spatial scale on runoff coefficient: Evidence from the Ethiopian highlands

The runoff coefficient (RC) is the ratio between the runoff and rainfall amounts and is scale dependent, which is due in part to the heterogeneity of watershed characteristics. This study quantified the spatial scale effects on runoff using long-term rainfall-runoff data on runoff plots and small watersheds. Effect of spatial scale on RC was studied for 12 runoff plots (2 m by 15 m) and three small watersheds (113-477 ha) in the highlands of Ethiopia using a total of 4397 and 13,925 15-day cumulative pairs of rainfall and runoff data at watershed and runoff plot scales, respectively. The observed average RC of runoff plots was extrapolated based on the extent of representation of a particular watershed in terms of slope, land use, cover and soil type. The weighted RC of plots was then compared with the observed RC of the watershed to determine a scale factor for extrapolation. A decrease in RC from plot to the watershed was observed in Anjeni and Andit Tid watersheds, while an increase in RC in Maybar watershed illustrates the role of specific watershed conditions in determining the scale effect. This, in turn, suggests that the variation in scale factor is not well explained by the difference in the area alone. The scale effect of runoff generation was better explained by extrapolating the RC based on the representation of different watershed characteristics. Thus, extrapolation exercises in runoff modeling and scaling efforts of soil and water conservation practices should consider the scale effect cautiously.