Use of ground and air-based photogrammetry for soil erosion assessment

Abstract

Water erosion affects all types of soils around the world at different intensities. However, in tropics water erosion is the most important form of soil erosion and has received much concern in the last decades. Thus, understanding the processes involved in each type of water erosion (sheet, rill and gully erosion), as well as its quantification, is a key factor in managing and developing soil conservation and erosion mitigation strategies. In that way, this thesis aims to investigate the efficiency of ground and air-based photogrammetry for soil erosion assessment, as well as to address some gaps in our understanding of the evolution of erosive processes in its different forms of occurrence. In doing so, we evaluated the factors that influence the development of erosion in micro and macro scales, with experiments in the laboratory and in the field. In the first chapter, it was evaluated the influence of gradient change and runoff volumes on rill erosion process, using digital close-range photogrammetry in a laboratory soil flume. In addition, morphological rill parameters were estimated to allow a better understanding of the rill erosion behaviour under different treatments. The results showed that the flow velocity in rills increased with water flow and slope, showing a strong correlation with the amount of rill erosion. On steep slopes the soil erosion was dominated by the rill erosion with less rill network density while, on low slopes, there were other types of soil erosion occurring together with rill erosion, causing the reduction of soil loss due to rill erosion. The digital close-range photogrammetry technique provided millimetric precision, which is sufficient for rill erosion investigations. In the second chapter aimed to evaluate the efficiency of SfM based on UAV images in obtaining accurate measurements of soil loss in areas of sheet erosion, under natural rainfall, where channelized erosion is not the principal mechanism. The measurements acquired from SfM were compared to the sediments collected in each soil erosion plots. The results of the soil losses obtained by UAV-SfM presented a high correlation with the sediments collected in the plots. This is of great relevance in the context of the monitoring and modelling of water erosion, since the quantification of soil loss around the world is mainly done using plots, a method that presents high operational cost. In addition, the study of laminar erosion through the UAV-SfM allows not only to calculate the soil loss values but to visualize the spatial variation of the erosion process (detachment, transport and deposition) practically in real time along the area. In the third chapter it was evaluated the application of UAVSfM technique in a gully system. For the first time, a study was carried out evaluating the relative contribution of the different types of erosion (sheet, rill and gully sidewall) in the gully development. This was possible due to the millimetric level of precision of the point clouds, allowing to evaluate even the contribution of the laminar erosion, which is new in gullies studies. As a result, it was possible to quantify sediments volumes stored in the channels and lost from the gully system, as well as to determine the main sediment sources. The study suggests that the main source o f sediments in the gully was due to the mass movements, followed by rills and sheet erosion. The UAV-SfM proved to be effective in the gully monitoring. The results findings by this thesis indicate that the use of ground and air-based photogrammetry are precise tools in detecting soil surface changes and can be used to assess water erosion in its various forms of occurrence in nature. In addition, the UAV-SfM has proven to be a very useful technique for monitoring soil erosion over time, especially in hard-to-reach areas.