Establishing a soil loss threshold for limiting rilling

In this paper a frequency analysis of event soil loss measurements collected in the period 1999–2012 at the microplots and plots of the Sparacia Experimental Area in Sicily, southern Italy, was developed. The analysis was carried out using the annual maximum soil loss measurements normalized by the mean soil loss measured at a given temporal and spatial scale. The empirical frequency distribution of the normalized variable was well fitted by two Gumbel’s theoretical probability distributions discriminated by a value of the normalized variable equal to 2. This last value discriminates between the relatively low and frequent values of the normalized variable and the high and rare ones. The annual maximum soil loss was demonstrated to be representative of the total annual soil erosion at the Sparacia Experimental Area. Then, a threshold soil loss value at the annual temporal scale was calculated by multiplying the frequency factor, equal to 2, by the mean annual maximum soil loss values for each given plot length. The value of the threshold soil loss decreased as the plot length increased. Finally, an analysis of occurrence of rilling was developed with regard both to the classical soil loss tolerance and the defined threshold soil loss. The conclusion that rilling can be limited using a threshold soil loss which is greater than the classical soil loss tolerance was drawn.