The influence of climate on colluvial hillslopes : insights from mapping hillslope asymmetry in the Eastern Mojave Desert and an exploration of digital terrain modelling using Structure from Motion
Hillslopes are the primary source of sediment generated in drainage basins. Along slopes, sediment is stored as colluvium and when stable, soils including Bt and Bk horizons can form. Climate influences colluvial production and storage by changing weathering, pedogenic, biologic, hydrologic, and erosional processes. In this study, we explore the influence of climate on slopes in a ~2km² basin near Nipton, CA. Aerial imagery collected with a UAV (drone) was processed with the photogrammetric technique of Structure from Motion (SfM) to create high resolution Digital Elevation Models (DEMs) and digital orthomosaics. Since even in dryland regions vegetation inhibits accurate modeling, we explore three techniques for removing vegetation from point clouds. A comparison of digital terrain models (DTMs, or bare-earth models) produced by the CANUPO plugin for CloudCompare, Agisoft PhotoScan, and Pix4D found that Pix4D is the software best suited for constructing basin-scale DTMs. To investigate the impact of climate on slope processes we characterized differences in surficial geology and vegetation on slopes of varying aspects. Analysis of DEM and DTM models indicate that south-facing slopes receive more insolation (4292 WH/m2 south-facing, 2861 WH/m2 north-facing at the equinox), have higher percentages of exposed bedrock (44% south-facing, 17% north-facing), and are steeper (25.99 degrees south-facing, 25.31 degrees north-facing) relative to north-facing slopes. Reduced soil moisture increases erosion via less vegetative cover that reduces soil cohesion. South-facing slopes with steeper slopes and lower infiltration rates enhance overland flow and erosion. Landforms including colluvial remnants and debris aprons suggest that climate changes throughout the Pleistocene-Holocene transition pushed hillslope processes over a critical threshold, initiating colluvium loss. Climate exerts significant control on hillslope processes, and thus equilibrium conditions are not present in the study area.
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