Impact of plot length on the effectiveness of different soil-surface covers in reducing runoff and soil loss by water

Covering the soil surface by rock fragments, organic mulches or vegetation is often done to reduce runoff and soil loss by water erosion compared to a bare soil treatment. The runoff or erosion-reducing effectiveness of these soil surface covers has been investigated for a range of plot lengths under different environmental conditions. Recent research indicates that the effectiveness of soil surface covers in reducing runoff and soil loss by water erosion may be affected by the size of the laboratory or field plots (ie, spatial scale). Therefore, the main objective of this review is to explore to what extent the impact of plot length on the effectiveness of different surface covers (ie, rock fragments, organic mulch and vegetation) in reducing runoff and soil loss by water erosion emerges from a worldwide data set. Furthermore, it is investigated whether there is a significant difference in runoff or erosion-reducing effectiveness between rock fragments, organic mulches and vegetation. Data from 65 experimental studies, investigating the impact of surface cover by rock fragments, organic mulch or vegetation on runoff or soil loss, are collected and analysed in this review. The results indicate that for plot lengths < 11 m there is a large variation in the runoff and erosion-reducing effectiveness of a soil cover, depending on various factors and on the larger number of studies conducted on these plots compared to longer field plots. However, with an increasing plot length (up to 50 m) this variation is reduced and surface covers by rock fragments, organic mulches and vegetation become on average more effective in reducing runoff or soil loss by water erosion. A vegetation cover is significantly more effective in reducing runoff rate compared to a rock fragment cover. No other significant differences in runoff or in erosion-reducing effectiveness between the studied soil surface covers are observed. Finally, two equations are proposed describing the possible effect of plot length and cover by rock fragments, organic mulches and vegetation on relative runoff and soil loss by water erosion. These findings have important consequences for the design of runoff and erosion plots, for modelling runoff and soil erosion rates and for scaling up plot data.