Exploring the multiple techniques available for developing an understanding of soil erosion in the UK

Accelerated soil erosion and the subsequent decline in soil depth has negative environmental, and consequently financial, impacts that have implications across all land cover classifications and scales of land management. Ironically, although attempts to quantify soil erosion nationally have illustrated that soil erosion can occur in the UK, understanding whether or not the UK has a soil erosion problem still remains a question to be answered. Accurately quantifying rates of soil erosion requires capturing both the volumetric nature of the visible, fluvial pathways and the subtle nature of the less-visible, diffuse pathways, across varying spatial and temporal scales. Accordingly, as we move towards a national-scale understanding of soil erosion in the UK, this thesis aims to explore some of the multiple techniques available for developing an understanding of soil erosion in the UK. The thesis first explored the information content of existing UK-based soil erosion studies, ascertaining the extent to which these existing data and methodological approaches can be used to develop an empirically derived understanding of soil erosion in the UK. The second research chapter then assessed which of two proximal sensing technologies, Terrestrial Laser Scanning and Structure-from-Motion Multi-view Stereo (SfM-MVS), is best suited to a cost-effective, replicable and robust assessment of soil erosion within a laboratory environment. The final research chapter built on these findings, using both Rare Earth Oxide tracers and SfM-MVS to elucidate retrospective information about sediment sources under changing soil erosion conditions, also within a laboratory environment Given the biased nature of the soil erosion story presented within the existing soil erosion research in the UK, it is impossible to ascertain if the frequency and magnitude of soil erosion events in the UK are problematic. However, this study has also identified that without ‘true’ observations of soil loss i.e. collection of sediment leaving known plot areas, proxies, such as the novel techniques presented in the experimental work herein and the methods used in the existing landscape scale assessments of soil erosion as included in the database chapter, are not capable of providing a complete assessment of soil erosion rates. However, this work has indicated that despite this limitation, each technique can present valuable information on the complex and spatially variable nature of soil erosion and associated processes, across different observational environments and scales.Defr

National-scale geodata describe widespread accelerated soil erosion

Accelerated soil erosion can result in substantial declines in soil fertility and has devastating environmental impacts. Consequently, understanding if rates of soil erosion are acceptable is of local and global importance. Herein we use empirical soil erosion observations collated into an open access geodatabase to identify the extent to which existing data and methodological approaches can be used to develop an empirically-derived understanding of soil erosion in the UK (by way of an example). The findings indicate that whilst mean erosion rates in the UK are low, relative to the rest of Europe for example, 16% of observations on arable land were greater than the supposedly tolerable rate of 1 t ha−1 yr−1 and maximum erosion rates were as high as 91.7 t ha−1 yr−1. However, the analysis highlights a skew in existing studies towards locations with a known erosion likelihood and methods that are biased towards single erosion pathways, rather than an all-inclusive study of erosion rates and processes. Accordingly, we suggest that future soil erosion research and policy must address these issues if an accurate assessment of soil erosion rates at the national-scale are to be established. The interactive geodatabase published alongside this paper offers a platform for the simultaneous development of soil erosion research, formulation of effective policy and better protection of soil resource

Structure-from-Motion Photogrammetry and Rare Earth Oxides can quantify diffuse and convergent soil loss and source apportionment

Accurately quantifying rates of soil erosion requires capturing both the volumetric nature of the visible, convergent fluvial pathways (also known as rills) and the subtle nature of the less-visible, diffuse pathways (interrill areas). The aim of this study was to use Rare Earth Oxide (REO) tracers and SfM (Structure-from-Motion) photogrammetry to elucidate retrospective information about soil erosion rates and sediment sources during different soil erosion conditions, within a controlled laboratory environment. The experimental conditions created erosion events consistent with diffuse and convergent erosion processes. REO tracers allowed the sediment transport distances of over 2 m to be described, and helped resolved the relative contribution of diffuse and convergent soil erosion; interrill areas were also identified as a significant sediment sources soil loss under convergent erosion conditions. While the potential for SfM photogrammetry to resolve sub-millimetre elevations changes was demonstrated, under some conditions non-erosional changes in surface elevation, such as compaction, exceeded volumes of soil loss via diffuse erosion. The discrepancies between SfM Photogrammetry calculations and REO tagged sediment export were beneficial, identifying that during soil erosion events sediment in both aggregate and particle form is deposited within the convergent features, even when the rill extended the full length of the soil surface. The combination of SfM photogrammetry and REO tracers has provided a novel platform for building a spatial understanding of patterns of soil loss and source apportionment between rill and interrill erosion

Using aerial photogrammetry to detect significant canopy height change resulting from beaver foraging.

This data consists of a set of images recorded during aerial drone surveys on 2017/09/07 and 2018/09/25 using a 3DR IRIS drone (unpiloted quadcopter), equipped with a Ricoh GR4 camera (16.2 sensor, 28mm focal length, maximum aperture f2.8, shutter speed faster than 1/1000th sec). For each survey, two flights were undertaken: nadir imagery from 60 m above ground level (agl) and oblique (ca. 20° from nadir) images from 55 m agl, providing mean ground sampling distances ca. 16 mm. Also contained are the outputs of Structure from Motion photogrammetry (SfM) processing, which includes point clouds and derived raster products such as Digital Surface Models, Terrain Models, Canopy Heights Models, and Digital Elevation models of difference. SfM photogrammetric processing was carried out using Agisoft Metashape (v1.5) and constrained using 40 fixed ground control points, the locations and spatially accuracies of which are also provided here. Surveys were undertaken at Clyst William Cross County Wildlife site, England which has an area of 10 ha and is located on the River Tale, a 4th order stream (lat: 50.820, long: -3.313). These data were recorded to evaluate changes in the canopy height within a complex willow (Salix sp.) dominant riparian woodland, following the reintroduction of beaver (Castor fiber) in 2016. Further, these data were used to test a range of error propagation methods for elevation change detection, using drone-based SfM photogrammetry, and consider their application in vegetated systems. The code required to reproduce the derived SfM prducts can be found here: https://doi.org/10.5281/zenodo.5500199 and https://doi.org/10.5281/zenodo.5500203