article · International Soil and Water Conservation Research
The runoff coefficient measures the proportion of rainfall that becomes runoff and varies depending on spatial scale and watershed heterogeneity. Evaluating long-term data from twelve small runoff plots and three larger watersheds in the Ethiopian highlands reveals how scale impacts these measurements. Runoff coefficients from plots were weighted and extrapolated using physical attributes such as slope, soil type, and land cover, then compared to observed watershed figures to determine extrapolation scale factors. In two watersheds, runoff coefficients decreased as scale increased, while the third watershed exhibited an increase, indicating that area alone does not dictate scale effects. Instead, specific local watershed characteristics govern runoff behaviour. Consequently, modelling runoff and scaling up soil and water conservation measures requires careful consideration of physical landscape attributes rather than relying solely on catchment size.
Accurate runoff estimates are essential for managing water resources and designing effective flood mitigation and land protection schemes. Because runoff processes do not scale uniformly with catchment size, relying on simple area-based extrapolations can produce misleading predictions. Recognising how specific terrain features influence runoff at different scales helps environmental managers plan soil and water conservation measures that fit local landscape realities.
This applied research provides empirical scaling insights relevant to hydrological modellers, watershed management planners, and conservation agencies. The findings can inform the calibration of catchment modelling tools and soil and water conservation engineering interventions. As the study centres on empirical data analysis and methodological warnings for scaling calculations, it represents early-stage scientific guidance rather than a ready-to-deploy software product or commercial technology.
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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. Keywords: Runoff coefficient, Spatial scale, Rainfall-runoff relation, Ethiopian highlands
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DOI: 10.1016/j.iswcr.2018.08.002
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