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Universiteit van Amsterdam (IBED) 2017

Comparing soil losses in heterogeneous vegetated semi-arid hillslopes by remote sensing of topography and vegetation patterns

Timmerman J.

Titre : Comparing soil losses in heterogeneous vegetated semi-arid hillslopes by remote sensing of topography and vegetation patterns

Auteur : Timmerman J.

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

Grade : Master programme : Geo-ecological Dynamics 2017

[[ Résumé
Mediterranean semi-arid landscapes are often vulnerable to erosion and are under pressure of a multitude of processes like land use change, agricultural land abandonment, increasing grazing intensity and a changing climate. The spatial distribution of vegetated patches on slopes can be used as an indicator for the amount of sediment production and erosion in these areas. A variety of studies used elaborate field measurements on vegetation patterns and the topography to explain hillslope runoff rates and sediment yields by bare patch connectivity with successful results (p <0.05 ; r >0.9). However, little is known about the transferability of the plot scale results to similar other areas. This study explores the possibilities of upscaling previous finding by using an Unmanned Arial Vehicle for large scale multitemporal high resolution data acquisition. The study focusses on a semi-arid catchment in SE Spain near the city of Lorca. The area contains several Gerlach troughs with sediment yield measurements starting from 1998. The slopes are mostly covered by patchy vegetation of grass tussocks. The area is covered by an UAV flight in 2013 and 2016, resulting in detailed multispectral images and a Digital Surface model from both flights. This data package offers a unique opportunity to compare the long term sediment yield measurements with the UAV derived dataset. The UAV data is used to calculate the hydrologic catchments corresponding with the Gerlach troughs and to look for an earlier proven correlation between bare patch connectivity and sediment production. The bare patch connectivity is quantified by using flow path calculations as the hydrology takes both the microtopography as the vegetation pattern into account. The vegetation pattern in the study area proved to be well able to retain most of the sediment during rainfall events up to 15mm/h, only during more intense rainfall events sediment yield increased substantially. Therefore only the sediment yield data corresponding to intense rainfall events appeared to have stronger correlations between sediment yield and bare patch connectivity. Three connectivity indexes appeared to show a positive and significant correlation with sediment yield (p < 0.05 ; r ≥ 0.6) : The Mean Flow Path of a catchment, the Maximum Flow Path of a catchment and the Direct Bare Area draining directly in a Gerlach trough. For locating sediment producing areas in the larger catchment surrounding the long term measurement plots a DSM subtraction between 2016 and 2013 is used to indicate bare patches with a clear reduction in elevation over the three year period. For the corresponding catchment of these sediment producing areas the three connectivity indexes are calculated and compared to stable catchments in the surroundings. Especially the Mean Flow Path appeared to be significantly larger on sediment producing areas (>0.44 m) if compared to the surroundings. The DSM subtraction indicated clearly areas with elevation change, but is vulnerable to disturbances of larger vegetation types like trees. The potential of UAV flights for upscaling plot scale knowledge and study results has been proven by the results of this research. However, the approach is limited by the need for high quality data and is therefore dependant of the application of future UAV flights.

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Page publiée le 7 novembre 2018