► Monte-Carlo sediment tracing applied to catchments draining to the Great Barrier Reef. ► Between 77% and 89% of fine river sediment is derived from sub-surface soil. ► Removing grazing appears to ...reduce rates of both surface and subsurface erosion. ► In large catchments geochemical source tracing is best applied at river confluences. ► Tracing results verify data limitations in sediment budget spatial modelling.
Identifying how agricultural practices can be changed to reduce sediment loss requires knowledge of the erosion processes and spatial areas contributing to end of catchment sediment loads. The Burdekin River basin in northeast Australia is a priority for such knowledge because of its large size (130,000km2), ongoing public investment in changing agricultural practices, and because sediment exports are known to affect the health of a significant aquatic ecosystem, the Great Barrier Reef (GBR). This study applied sediment tracing techniques within the Burdekin River basin to identify the contributions of surface versus subsurface soil, and spatial areas to fine sediment export. Tracer properties included fallout radionuclides and geochemistry. The contributions of each sediment source to river sediment were identified with 95% confidence intervals using a Monte-Carlo numerical mixing model. Between 77% and 89% of fine sediment loss in the study area was derived from subsurface soil sources. High-resolution monitoring of river suspended sediment concentrations indicated that sediment sources were in close proximity to the drainage network, since concentrations were higher on the rising limb than the falling limb of large hydrographs. Gully erosion is likely to be the dominant subsurface soil erosion process, although channel bank erosion and hillslope rilling cannot be discounted. The results contrast with previous sediment budget spatial modelling, which predicted that hillslope erosion was the dominant sediment source in the area, thus demonstrating the need to independently verify modelling predictions where input datasets are poor. The contribution of surface soil to river sediment was generally similar between catchments which were currently grazed and two catchments where livestock grazing ceased 7 years ago. Concurrent increases in vegetation cover in the non-grazed catchments indicate that surface erosion rates had declined, suggesting that subsurface soil erosion rates had also declined by a similar amount. The estimated contributions of spatial source areas within the large study catchments had narrower confidence intervals when source areas were defined using sediment from geologically distinct river tributaries, rather than using soil sampled from geological units in the catchment, since tributary sediment had less-variable geochemistry than catchment soil. Programs to reduce fine sediment losses from the Burdekin River basin should primarily focus on reducing sub-surface soil erosion proximal to the basin's drainage network. Understanding the biophysical processes of pollutant generation is important to help guide on-ground activities to improve water quality.
► This study reports current and pre-European river pollutant loads to the GBR lagoon. ► The mean-annual load of total suspended solids has increased by 5.5 times. ► Mean-annual loads of nitrogen and ...phosphorus have increased by 5.7 and 8.9 times. ► The current mean-annual load of PSII herbicides is 30,000 kg/yr. ► These estimates can facilitate target setting and enable management prioritisation.
Degradation of coastal ecosystems in the Great Barrier Reef (GBR) lagoon, Australia, has been linked with increased land-based runoff of suspended solids, nutrients and pesticides since European settlement. This study estimated the increase in river loads for all 35 GBR basins, using the best available estimates of pre-European and current loads derived from catchment modelling and monitoring. The mean-annual load to the GBR lagoon for (i) total suspended solids has increased by 5.5 times to 17,000ktonnes/year, (ii) total nitrogen by 5.7 times to 80,000tonnes/year, (iii) total phosphorus by 8.9 times to 16,000tonnes/year, and (iv) PSII herbicides is 30,000kg/year. The increases in river loads differ across the 10 pollutants and 35 basins examined, reflecting differences in surface runoff, urbanisation, deforestation, agricultural practices, mining and retention by reservoirs. These estimates will facilitate target setting for water quality and desired ecosystem states, and enable prioritisation of critical sources for management.
Modification of terrestrial sediment fluxes can result in increased sedimentation and turbidity in receiving waters, with detrimental impacts on coral reef ecosystems. Preventing anthropogenic ...sediment reaching coral reefs requires a better understanding of the specific characteristics, sources and processes generating the anthropogenic sediment, so that effective watershed management strategies can be implemented. Here, we review and synthesise research on measured runoff, sediment erosion and sediment delivery from watersheds to near-shore marine areas, with a strong focus on the Burdekin watershed in the Great Barrier Reef region, Australia. We first investigate the characteristics of sediment that pose the greatest risk to coral reef ecosystems. Next we track this sediment back from the marine system into the watershed to determine the storage zones, source areas and processes responsible for sediment generation and run-off.
The review determined that only a small proportion of the sediment that has been eroded from the watershed makes it to the mid and outer reefs. The sediment transported >1km offshore is generally the clay to fine silt (<4–16μm) fraction, yet there is considerable potential for other terrestrially derived sediment fractions (<63μm) to be stored in the near-shore zone and remobilised during wind and tide driven re-suspension. The specific source of the fine clay sediments is still under investigation; however, the Bowen, Upper Burdekin and Lower Burdekin sub-watersheds appear to be the dominant source of the clay and fine silt fractions. Sub-surface erosion is the dominant process responsible for the fine sediment exported from these watersheds in recent times, although further work on the particle size of this material is required. Maintaining average minimum ground cover >75% will likely be required to reduce runoff and prevent sub-soil erosion; however, it is not known whether ground cover management alone will reduce sediment supply to ecologically acceptable levels.
•This paper reviews the impact of sediment delivery to coral reefs.•The sources, processes and management options of excess sediment are discussed.•The synthesis is based primarily on measured data sets.•The approaches and outcomes are relevant to coral reefs around the world.
Structure from Motion with Multi-View Stereo photogrammetry (SfM-MVS) is increasingly used in geoscience investigations, but has not been thoroughly tested in gullied savanna systems. The aim of this ...study was to test the accuracy of topographic models derived from aerial (via Unmanned Aerial Vehicle, ‘UAV’) and ground-based (via handheld digital camera, ‘ground’) SfM-MVS in modelling hillslope gully systems in a dry-tropical savanna, and to assess the strengths and limitations of the approach at a hillslope scale and an individual gully scale. UAV surveys covered three separate hillslope gully systems (with areas of 0.412–0.715 km2), while ground surveys assessed individual gullies within the broader systems (with areas of 350–750 m2). SfM-MVS topographic models, including Digital Surface Models (DSM) and dense point clouds, were compared against RTK-GPS point data and a pre-existing airborne LiDAR Digital Elevation Model (DEM). Results indicate that UAV SfM-MVS can deliver topographic models with a resolution and accuracy suitable to define gully systems at a hillslope scale (e.g., approximately 0.1 m resolution with 0.4–1.2 m elevation error), while ground-based SfM-MVS is more capable of quantifying gully morphology (e.g., approximately 0.01 m resolution with 0.04–0.1 m elevation error). Despite difficulties in reconstructing vegetated surfaces, uncertainty as to optimal survey and processing designs, and high computational demands, this study has demonstrated great potential for SfM-MVS to be used as a cost-effective tool to aid in the mapping, modelling and management of hillslope gully systems at different scales, in savanna landscapes and elsewhere.
ABSTRACT
The Canada–France Imaging Survey (CFIS) will consist of deep, high-resolution r-band imaging over ∼5000 deg2 of the sky, representing a first-rate opportunity to identify recently merged ...galaxies. Because of the large number of galaxies in CFIS, we investigate the use of a convolutional neural network (CNN) for automated merger classification. Training samples of post-merger and isolated galaxy images are generated from the IllustrisTNG simulation processed with the observational realism code RealSim. The CNN’s overall classification accuracy is 88 per cent, remaining stable over a wide range of intrinsic and environmental parameters. We generate a mock galaxy survey from IllustrisTNG in order to explore the expected purity of post-merger samples identified by the CNN. Despite the CNN’s good performance in training, the intrinsic rarity of post-mergers leads to a sample that is only ∼6 per cent pure when the default decision threshold is used. We investigate trade-offs in purity and completeness with a variable decision threshold and find that we recover the statistical distribution of merger-induced star formation rate enhancements. Finally, the performance of the CNN is compared with both traditional automated methods and human classifiers. The CNN is shown to outperform Gini–M20 and asymmetry methods by an order of magnitude in post-merger sample purity on the mock survey data. Although the CNN outperforms the human classifiers on sample completeness, the purity of the post-merger sample identified by humans is frequently higher, indicating that a hybrid approach to classifications may be an effective solution to merger classifications in large surveys.
Achieving change through gully erosion research Wilkinson, Scott N.; Rutherfurd, Ian D.; Brooks, Andrew P. ...
Earth surface processes and landforms,
January 2024, 2024-01-00, 20240101, Letnik:
49, Številka:
1
Journal Article
Recenzirano
Odprti dostop
This is an introduction to a special issue arising from the 8th International Conference on Gully Erosion, held in Townsville, Australia in 2019. Research has improved understanding of gully erosion ...processes and increasingly emphasizes sophisticated near‐and‐remotely sensed methods to characterize and measure gullies. These data can then be analysed using equally sophisticated models and scenarios can be simulated. These advances improve the capacity to predict gully initiation and development over time and space. It is often assumed that better prediction translates into greater impact and uptake of research to solve real world gully erosion problems. Examples of impressive associations between research programmes and major gully management programmes are evident. However, there has been little assessment of the impact of gully research. We argue for a greater focus on achieving impact including interventions that better manage and prevent gully erosion. Opportunities to deliver research impact are assessed using three indicators of progression towards impact; the practical usability of research, whether it is in use by non‐researchers, and whether it is useful in guiding improvements in management. Like other natural hazards, gully erosion is a phenomenon of the social, economic and environmental context in which it occurs. Defining the contexts and consequences of gully erosion and using these to frame further research is therefore a means to increase research impact. Enhancing collaboration between research disciplines and with practitioners who act on the research, and a greater focus on the translation of results into practice, is another avenue. Expanding the monitoring and evaluation of gully management can better demonstrate the impact of past research and enable further useful investigations. We urge gully erosion researchers to consider the potential impact of their research, including how it can more effectively inform better and more cost‐effective management and political decisions.
Following the 8th International Symposium on Gully Erosion we propose an increased focus on research impact on gully management and prevention, by defining the consequences of gully erosion, enhancing collaboration, focusing on use by practitioners, and expanding monitoring and evaluation.
Increased erosion related to climate and/or land cover change has adverse impacts on terrestrial and aquatic ecosystems. Mapping of erosion hotspots improves our ability to identify and potentially ...remediate the most active erosion sources. The topical savannas of the Great Barrier Reef catchments, in northeast Australia, are generating excessive sediment yields, primarily from gully erosion. To reduce the adverse impacts on marine ecosystems, there is an urgent need to identify priority erosion hotspots and implement mitigation measures. While repeat airborne LiDAR surveys allow subtle topographic change detection, they are cost-intensive for catchment-scale applications and their applicability is constrained by data avalability and detection treshold of ground level change. In contrast, satellite-based radar imagery can allow large scale tracking of geomorphic change at high temporal resolution. Here we apply a new method based on Sentinel-1 C-band radar images and Coherence Change Detection (CCD), where large stacks of interferometric coherence images are subdivided with rain gauge time-series for separation of erosion-rich and erosion-free coherence information. After correcting the former with the latter, the resulting corrected coherence maps are compared with differential elevation models derived from multitemporal LiDAR, regional scale gully delineation maps, maps of gullying potential and in-situ field verification. Our results demonstrate the promising potential of this technique in detecting gully erosion hotspots. The coherence loss indicating erosion/deposition is well detected in wide gully morphologies, however, the line-of-sight angle does not allow penetration into narrow linear gullies. Further, CCD detects sheetwash or rill erosion occurring in areas identified as at risk of gully expansion, which is commonly below detection threshold for multitemporal LiDAR datasets. When used with LiDAR-derived geomorphic change mapping and gullying potential maps, CCD allows identification of gully erosion dynamics and forecasting gully evolution and creation, which is critical for supporting mitigation measures.
•Sentinel-1 radar coherence stacks map erosion hotspots in northeast Australia.•Erosion-free coherence is used to correct erosion-affected coherence images.•Comparison with DoD, gully maps, gullying potential maps and in situ observations.•Capability is indicated to detect early development and deepening of gullies.•A cost-effective, semi-quantitative mapping of large-scale erosion patterns.
Detailed understanding of gully erosion processes is essential for monitoring gully remediation and requires fine-scale monitoring. Hand-held laser scanning systems (HLS) enable rapid ground-based ...data acquisition at centimeter precision and ranges of 10–100 m. This study quantified errors in measuring gully morphology and erosion over a four year period using two models of HLS. Reference datasets were provided by Real-Time-Kinematic (RTK) GPS and a RIEGL Terrestrial Laser Scanner (TLS). The study site was representative of linear gullies that occur extensively on hillslopes throughout Great Barrier Reef catchments, where gully erosion is the dominant source of fine sediment. The RMSE error against RTK survey points varied 0.058–0.097 m over five annual scans. HLS was found to measure annual gully headcut extension within 0.035 m of RTK. HLS was, on average, within 6% of TLS for morphological metrics of depth, area and volume. Volumetric change over a 60 m length of the gully and four years was estimated to within 23% of TLS. Errors could potentially be improved by scanning at times of year with lower ground vegetation cover. HLS provided similar levels of error and was relatively more rapid than TLS and RTK for monitoring gully morphology and change.
Land use in the catchments draining to the Great Barrier Reef lagoon has changed considerably since the introduction of livestock grazing, various crops, mining and urban development. Together these ...changes have resulted in increased pollutant loads and impaired coastal water quality. This study compiled records to produce annual time-series since 1860 of human population, livestock numbers and agricultural areas at the scale of surface drainage river basins, natural resource management regions and the whole Great Barrier Reef catchment area. Cattle and several crops have experienced progressive expansion interspersed by declines associated with droughts and diseases. Land uses which have experienced all time maxima since the year 2000 include cattle numbers and the areas of sugar cane, bananas and cotton. A Burdekin Basin case study shows that sediment loads initially increased with the introduction of livestock and mining, remained elevated with agricultural development, and declined slightly with the Burdekin Falls Dam construction.
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