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A new method for classifying soils on the basis of aggregate stability is presented. The method, based on the concept of an instability index β is simple, quick, and gives reliable results comparable to those obtained with the laborious more commonly used methods. The optimum time of oscillation for the determination of the instability index, β was determined by statistically analyzing data for nine soils. According to this analysis, β is a reliable index of the relative aggregate water‐stability when oscillation times of at least 4 min are used.

For interrill erosion, raindrop-induced detachment and transport of sediment by rainfalldisturbed sheet flow are the predominant processes, while detachment by sheet flow and transport by raindrop impact are negligible. In general, interrill subprocesses are interactively affected by rainfall, soil and surface properties. The objective of this work was to study the relationships among interrill runoff and sediment loss and some selected parameters, for cultivated soils in central Greece, and also the development of a formula for predicting single storm sediment delivery. Runoff and soil loss measurement field experiments have been conducted for a 3·5-year period, under natural storms. The soils studied were developed on Tertiary calcareous materials and Quaternary alluvial deposits and were textured from sandy loam to clay. The second group of soils showed greater susceptibility to sealing and erosion than the first group. Single storm sediment loss was mainly affected by rain and runoff erosivity, being significantly correlated with rain kinetic energy (r = 0·64***), its maximum 30-minute intensity (r = 0·64***) and runoff amount (r = 0·56***). Runoff had the greatest correlation with rain kinetic energy (r = 0·64***). A complementary effect on soil loss was detected between rain kinetic energy and its maximum 30-minute intensity. The same was true for rain kinetic energy and topsoil aggregate instability, on surface seal formation and thus on infiltration characteristics and overland flow rate. Empirical analysis showed that the following formula can be used for the successful prediction of sediment delivery (D i ): D i = 0·638β β β β βEI 30 tan(θ θ θ θ θ) (R 2 = 0·893***), where β β β β β is a topsoil aggregate instability index, E the rain kinetic energy, I 30 the maximum 30-minute rain intensity and θ θ θ θ θ the slope angle. It describes soil erodibility using a topsoil aggregate instability index, which can be determined easily by a simple laboratory technique, and runoff through the product of this index and rain kinetic energy.

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