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Universiteit van Amsterdam (2016)

Soil erosion and sediment deposition on semiarid agricultural fields since abandonment

Scholten R.

Titre : Soil erosion and sediment deposition on semiarid agricultural fields since abandonment

Auteur : Scholten R.

Université de soutenance : Universiteit van Amsterdam Institute for Biodiversity and Ecosystem Dynamics (IBED

Grade : Master programme : Environmental Management 2016

In this thesis the sediment budget of abandoned terraces in a semi-arid environment was studied. Erosion, sedimentation and transport processes have been quantified, the spatial distribution of erosion and sedimentation features has been recorded and the physical and chemical soil properties have been determined. The research area is located in the Guadalentin basin, southeast Spain, and was studied before by De Boer (2015) and Van der Meulen et al. (2006). The strong pressure of human activities, the climatic conditions, the presence of steep slopes and the occurrence of tectonic activity made the region extremely vulnerable to erosion processes and land degradation. Many terraces in the area are deeply incised by gullies and pipes. Erosion processes are well studied in the area, however, this research is distinctive due to the unique research area were plastic evaporation sheets are still present, since they have not been removed by the farmers at abandonment. Through the years sediment has accumulated on top of this layer, which makes it possible to quantify sedimentation processes and get insight in the spatial distribution. A DGPS device together with manual measurements have been applied to create detailed DEM’s that gave insight in the current (2015) conditions and the condition at time of abandonment. These DEM’s were subtracted from each other to calculate erosion and sedimentation volumes. Bulk density samples have been taken to turn the volumes into rates. Severe erosion rates were found, varying between 38.31 and 93.27 t ha-1 y-1, since abondonment. However, sedimentation rates were generally higher (33.79 – 117.44 t ha-1 y-1), which resulted in a positive sediment budget of 470.16 m3. The most important sediment sources were upslope located terraces, connected areas and catchments outside the terraces, while the most important sinks were alluvial fans around the mouth of gullies and pipes. Connectivity was found to play an important role in the redistribution and sinks of sediment. Erosion features generally follow the downslope gradient on the terraces and are mainly concentrated along the eastern part of the terraces as a consequence of a connected catchment on the eastern side of terrace one. It is for this reason that relative recently rills have been formed between different gullies in the eastern part of the terraces, reflecting increasing connectivity within and between the different terraces. Observations have been conducted on the terraces and soil samples have been taken from the terraces to get insight into chemical and physical soil properties, and erodibility of the soil on the terraces. It was found that the soils are highly erodible as a consequence of the fine soil texture and the presence of smectite clays, which is reflected in a high average K-factor of 0.067. The pH, EC, bulk density, silt, gypsum and soil moisture were found to increase with depth. On the other hand, the total carbon content, sand, clay and SOM content decrease with depth, while the CaCO3 remains stable. As a consequence of sedimentation processes and internal soil processes, like shrinking and expansion processes, the plastic layer has been buried and integrated into the soil profile. However, parts of the plastic layer have also been disintegrating into smaller pieces at many places as a consequence of (mainly) shrinking and expansion processes and consequently been relocated by overland flow, erosion and sedimentation processes, nevertheless the main part of the plastic layer is still unchanged since abandonment. In conlusion it can be said that soil erosion is significant on abandoned terraces in the Guadalentin basin, however sedimentation rates are significantly higher, which is reflected in a positive sediment budget. Chemical and physical soil properties indicate calcaric silty soils with a high erodibility in which the plastic layer has been buried by sedimentation processes, and been moved and disintegrated by shrinking and expansion processes of the soil.

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Page publiée le 8 novembre 2016, mise à jour le 18 octobre 2018