Abstract
Obtaining good quality soil loss data from plots requires knowledge of the factors that affect natural and measurement data variability and of the erosion processes that occur on plots of different sizes. Data variability was investigated in southern Italy by collecting runoff and soil loss from four universal soilβloss equation (USLE) plots of 176 m^2^, 20 βlargeβ microplots (0Β·16 m^2^) and 40 βsmallβ microplots (0Β·04 m^2^). For the four most erosive events (event erosivity index, R~e~ β₯ 139 MJ mm ha^β1^ h^β1^), mean soil loss from the USLE plots was significantly correlated with R~e~. Variability of soil loss measurements from microplots was five to ten times greater than that of runoff measurements. Doubling the linear size of the microplots reduced mean runoff and soil loss measurements by a factor of 2Β·6β2Β·8 and increased data variability. Using sieved soil instead of natural soil increased runoff and soil loss by a factor of 1Β·3β1Β·5. Interrill erosion was a minor part (0Β·1β7Β·1%) of rill plus interrill erosion. The developed analysis showed that the USLE scheme was usable to predict mean soil loss at plot scale in Mediterranean areas. A microplot of 0Β·04 m^2^ could be used in practice to obtain field measurements of interrill soil erodibility in areas having steep slopes. Copyright Β© 2003 John Wiley & Sons, Ltd.