Investigation on automatic extraction of the contributing rill network to plot soil loss

Recently, several plot‐scale investigations confirmed the reliability of the terrestrial photogrammetric technique for studying rill erosion and the reliability of an objective method, based on the convergence index, for extracting the rill network from DigitalElevation Models (DEMs) and measuring the eroded volume. Several experimental investigations have demonstrated that the distinction between contributing rills, which are connected to the network reaching the plot outlet and contribute to the total soil loss, and non‐contributing rills, which are interrupted within the plot and, therefore, do not contribute to soil loss, allows improving the accuracy of the eroded volume measurement. For the severe erosion event occurred in September 2017 at the Sparacia experimental area, Sicily, terrestrial photogrammetric surveys of the plots incised by rills were carried out to reconstruct the DEMs. This study was conducted on three experimental plots, two with length of 22 m and steepness of 22% and 26%, and one with length equal to 44 m and steepness of 15%. For each plot, the rill network was extracted from the DEMs and the non‐contributing network was distinguished from the contributing one. The eroded volumes, from which the soil loss was computed, were determined on the latter. The values of the above eroded volumes, converted into weight using soil bulk density, were compared with the corresponding soil loss values measured in the tanks located downstream of the plots and considered as reference. Finally, both the rill depths and some morphometric parameters of the rill networks surveyed in the three plots were compared. The analysis demonstrated that the rill depth, drainage density, and drainage frequency of the contributing rill network increase with plot steepness.