Modelling rainfall erosivity for dynamic hillslope erosion estimation in events of wildland fires, snowmelt, and extreme rainfall

University of Technology Sydney. Faculty of Science.Rainfall erosivity and soil erosion have being significantly affected by more frequent extreme weather events and ongoing climate change. Projected warmer and drier climate in Australia will change the erosion rates through more intensive storm events, more severe and frequent wildfire and less snowmelt. To estimate the near real-time rainfall erosivity and erosion change, it is essential to link the extreme weather events and hillslope erosion model in response to provide effective ecosystem and environment management. In this study, I selected two case study areas in southeast Australia to assess the effect of extreme weather events on hillslope erosion modelling (e.g. Warrumbungle National Park (WNP) and NSW and ACT Alpine region). Radar rainfall data (1km, 10-min), calibrated by rain gauges rainfall were applied to estimate the near real-time rainfall erosivity on a daily basis. There was a positive correlation between radar-based and gauged rainfall. The highest rainfall erosivity was estimated as 826.76 MJ mm ha-1 hr-1 for a single storm event. The modelled average annual rate of hillslope erosion appears to be declining due to the vegetation recovery after the wildfire. Six extreme rainfall indices (ERIs) were selected to assess the extreme rainfall impact on rainfall erosivity over 60 years. In comparison with the result from Australia Bureau of Meteorology, it is possible to estimate the approximately erosivity value from ERIs especially to where without radar or gauged rainfall data. Snow and temperature projections for the 60 years derived from NARCliM were applied to adjust the snowmelt runoff and rainfall erosivity model during the melting season. Weekly measurements of snow depth and snow water equivalent at three filed sites in the Snowy Mountains were obtained to assess the snowmelt-adjusted rainfall erosivity model. Snowmelt in spring is estimated to increase the rainfall erosivity by 12.95% for baseline. However, the snow impact is projected to be 24.84% for the near future and then less (1.63%) for the fat future due to the projected higher temperature and less snow depth into NARCliM. The erosion amount and change is comprehensively derived from various factors, includes rainfall erosivity, groundcover, slope length and steepness and soil erodibility. These factors always combine and interact to influence and accelerate the mechanism of the erosion process under more frequent and more extreme weather events. The current outcomes would effectively enhance the capability of government, and provide adaptation and mitigation strategies in responding to a changing climate

Assessment of post-wildfire erosion risk and effects on water quality in south-western Australia

Investigations of wildfire impact on water resources have escalated globally over the last decade owing to an awareness of climate-related vulnerabilities. Within Australia, research into post-wildfire erosion has focused on water supply catchments in the south-eastern region. Here, we examine post-wildfire erosion risk and its potential for water quality impacts in a catchment in south-western Australia. The catchment of the Harvey River, which drains from forested escarpments onto an agricultural coastal plain and into valuable coastal wetlands, was burnt by wildfire in 2016. The aims of this study were to determine erosion risk across contrasting landforms and variable fire severity, using the Revised Universal Soil Loss Equation (RUSLE), and to determine whether post-fire water quality impacts could be detected at permanent river monitoring stations located on the coastal plain. RUSLE outputs showed erosion hot-spots at intersections of steep terrain and high fire severity and that these areas were confined to forested headwaters and coastal dunes. Monthly water quality data showed conspicuous seasonal patterns, but that sampling frequency was temporally too coarse to pick up predicted event-related effects, particularly given that the pre-existing monitoring sites were distal to the predicted zone of contamination. © IAWF 2020 Open Access

Soil erosion modelling as a tool for future land management and conservation planning

Maintaining future agricultural productivity and ensuring soil security is of global concern and requires evidence-based management practices. Moreover, understanding where and when land is at risk of erosion is a fundamental step to combatting future soil loss and reach Land Degradation Neutrality (LDN). However, this is a difficult task because of the high spatial and temporal variability of the controlling factors involved. Therefore, tools investigating the impact and frequency of extreme erosive events are crucial for land managers and policymakers to apply corrective measures for better erosion management in the future. While the utility of using wind and water erosion models for management is well established, there is a paucity of work on the impact of climate change and extreme environmental conditions (e.g. wildfires) on soil erosion by wind and water simultaneously. Both erosion types are controlled by different environmental variable that vary highly in space and time. Therefore, the overarching aim of this study was to develop a joint wind-water erosion modelling method and demonstrate the utility of this approach to identify (1) the spatio-temporal variability of extreme erosion events in the South Australian agricultural zone (Australia) and (2) assess the likely increase of this variability in the face of climate change and the recurrence of wildfires. To fulfil the aim of the research project, we adapted two state-of-the-art wind and water (hillslope) erosion models to integrate modern high-resolution datasets for spatial and temporal analysis of erosion. The adaptation of these models to local conditions and the use of high-resolution datasets was essential to ensure reliable erosion assessment. First, we applied these models separately in the Eyre Peninsula and Mid-North agricultural regions. We evaluated the spatio-temporal variability of extreme erosion events between 2001 and 2017 and described the complex interactions between each erosional process and their influencing factors (e.g. soil types, climate conditions, and vegetation cover). Hillslope erosion was very low for most of the Eyre Peninsula; however, a large proportion of the central Mid-North region frequently recorded severe erosion (> 0.022 t ha-1) two to three months per year, for most of the years in the time-series. The most severe erosion events were primarily driven by topography, low ground cover ( 500 MJ mm ha-1 h-1). Average annual wind erosion was very low and comparable in the two regions. Nonetheless, most of the west coast of the Eyre Peninsula frequently registered severe erosion (> 0.000945 t ha-1 or 0.945 kg ha-1) two to three months per year, for most of the years. The most severe erosion events were largely driven by the soil type (sandy soils), recurring low ground cover ( 68 km h-1). We identified that erosion severity was low for the vast majority of the study area, while 4% and 9% of the total area suffered severe erosion by water and wind respectively, demonstrating an extreme spatial and temporal skewness of soil erosion processes. Then we combined the modelling outputs from the wind and water erosion models and tested the models’ response to major wildfire events. This research demonstrated how erosion modelling could be used to predict the impact of severe wildfire events on soil erosion. The two models satisfactorily captured the spatial and temporal variability of post-fire erosion. However, a very small fraction of the region (0.7%) was severely impacted by both wind and water erosion. We observed that soil erosion increased immediately after the wildfires or within the first six months for the ten fire-affected regions. For three of the wildfire events, the models showed an increase in wind and water erosion in consecutive months or at the same time. These results highlighted the importance to consider wind and water erosion simultaneously for post-fire erosion assessment in dryland agricultural regions. Finally, we had the rare opportunity to assess the impact of a catastrophic wildfire event on wind erosion in an agricultural landscape by examining the influence of unburnt stubble patches on adjacent burnt or bare plots using a spatio-temporal sampling design. The field study allowed a quantitative assessment of spatial and temporal patterns of wind erosion and sediment transport after a catastrophic wildfire event. It showed very high levels of spatial variability of erosion processes between burnt and bare patches and demonstrated how measuring field-scale sediment transport could complement fine-scale experimental studies to assess environmental processes at the field scale. This research highlights the utility of erosion models to inform corrective measures for future land management. We have implemented tools that allow a realistic assessment of the influence of climate change and extreme environmental conditions scenarios on soil erosion for a wide range of land cover over large regions. Here, the models enabled the identification of the relative post-fire wind or water erosion risk in dryland agricultural landscapes, making them particularly useful for land management under future uncertainty. Spatial patterns compared well with previous modelling approaches and underpinned the benefit of erosion models to assess spatial differences in erosion risk and evaluate corrective measures at the regional scale. However, modelled soil erosion magnitudes strongly depend on how the influence of soils is implemented in the models, making it difficult to set absolute quantitative soil loss targets for land management. The thesis has provided a proof of concept of the approach for South Australia. However, all input data can be freely sourced Australia-wide and similar dataset are available globally.Thesis (Ph.D.) -- University of Adelaide, School of Biological Sciences, 202

Runoff production in blanket peat covered catchments

Although blanket peat covers many major headwater areas in Britain, runoff production within these upland catchments is poorly understood. This thesis examines runoff production mechanisms within the blanket peat catchments of the Moor House National Nature Reserve, North Pennines, UK. Catchments ranging from 11.4 km^ down to the hillslope and plot-scale are examined. Runoff from the monitored catchments was flashy. Lag times are short and rainwater is efficiently transported via quickflow- generating mechanisms such that flood peaks are high and low flows poorly maintained. Hillslope and plot-scale runoff measurements show that the flashy catchment response is the result of the dominance of overland flow. Typically 80 % of runoff is produced as overland flow. This occurs both on bare and vegetated surfaces. Most of the remaining runoff is generated from the upper 10 cm of the peat, except where well-connected macropore and pipe networks transfer flow through the lower layers. Below 10 cm depth the blanket peat matrix fails to contribute any significant runoff Thus most groundwater-based models of peat hydrological process are not readily applicable to these catchments.Suggestions that infiltration-excess overland flow may be largely responsible for the flashy regime of these upland catchments are not substantiated by the blanket peat infiltration data presented in this thesis. Monitoring of hillslope runoff mechanisms combined with rainfall simulation (at realistic intensities of 3-12 mm hr(^-1)) and tension- infiltrometer experiments shows that saturation-excess mechanisms dominate the response. Infiltration is relatively rapid in the upper peat layers until they become saturated and overland flow begins. High water tables result in rapid saturation of the peat mass such that even at low rainfall intensity runoff production is just as efficient as during high intensity events. While macropores have largely been ignored in blanket peat, results presented suggest that up to 30 % of runoff may be generated through them. Occasionally these macropore networks develop through the deeper peat such that runoff bypasses the matrix and runs off at depth from small outlets and larger pipe networks. Seasonal variations in runoff- generating processes can be exacerbated by drought which causes structural changes to the near-surface of the peat. This was found to result in enhanced infiltration and macropore flow which may encourage pipe network development. Flow has been monitored simultaneously from several natural pipes in a 0.4 km(^2) headwater catchment of the Tees. This catchment provides one of the few examples of pipeflow monitoring outside the shallow peaty-podzols of mid-Wales. Natural pipes are found throughout the soil profile and can be at depths of up to three metres. Ground penetrating radar was useful in identifying deep subsurface piping and suggestions are made for improvements to the application. The pipe networks were found to be complex and results demonstrate that outlet location and size may bear little relation to the form and depth of the pipe a short distance upslope. Pipes generally contribute less than 10 % to catchment runoff but on the rising and falling hydrograph limbs can contribute over 30 % to streamflow. Pipeflow lag times are short suggesting that both the shallow and deep pipes may be well connected to the surface. Thus while matrix runoff contributions at depth within the peat may be low, macropore flow mechanisms can be significant

Wildfire Hazard and Risk Assessment

Wildfire risk can be perceived as the combination of wildfire hazards (often described by likelihood and intensity) with the susceptibility of people, property, or other valued resources to that hazard. Reflecting the seriousness of wildfire risk to communities around the world, substantial resources are devoted to assessing wildfire hazards and risks. Wildfire hazard and risk assessments are conducted at a wide range of scales, from localized to nationwide, and are often intended to communicate and support decision making about risks, including the prioritization of scarce resources. Improvements in the underlying science of wildfire hazard and risk assessment and in the development, communication, and application of these assessments support effective decisions made on all aspects of societal adaptations to wildfire, including decisions about the prevention, mitigation, and suppression of wildfire risks. To support such efforts, this Special Issue of the journal Fire compiles articles on the understanding, modeling, and addressing of wildfire risks to homes, water resources, firefighters, and landscapes

Rainfall Erosivity in Soil Erosion Processes

This book gathers recent international research on the association between aggressive rainfall and soil loss and landscape degradation. Different contributions explore these complex relationships and highlight the importance of the spatial patterns of precipitation intensity on land flow under erosive storms, with the support of observational and modelling data. This is a large and multifaceted area of research of growing importance that outlines the challenge of protecting land from natural hazards. The increase in the number of high temporal resolution rainfall records together with the development of new modelling capabilities has opened up new opportunities for the use of large-scale planning and risk prevention methods. These new perspectives should no longer be considered as an independent research topic, but should, above all, support comprehensive land use planning, which is at the core of environmental decision-making and operations. Textbooks such as this one demonstrate the significance of how hydrological science can enable tangible progress in understanding the complexity of water management and its current and future challenges

The Effect of Hydrology on Soil Erosion

This Special Issue includes manuscripts about soil erosion and degradation processes and the accelerated rates due to hydrological processes and climate change. The new research included in this issue focuses on measurements, modeling, and experiments in field or laboratory conditions developed at different scales (pedon, hillslope, and catchment). This Special Issue received investigations from different parts of the world such as Ethiopia, Morocco, China, Iran, Italy, Portugal, Greece, and Spain, among others. We are happy to see that all papers presented findings characterized as unconventional, provocative, innovative, and methodologically new. We hope that the readers of the journal Water can enjoy and learn about hydrology and soil erosion using the published material, and share the results with the scientific community, policymakers, and stakeholders to continue this amazing adventure, facing plenty of issues and challenges

Control by Vegetation Disturbance on Gully Rejuvenation Following Wildfire

Gully rejuvenation (GR) following wildfire influences landform evolution and generates flooding and debris that alters aquatic habitat and threatens human activities. Fire severity, defined as the degree of vegetation loss by wildfire, is a hypothesized control on this erosion response. I investigated three related aspects of the relationship between fire severity and GR: The capacity of vegetation disturbance to explain the occurrence or non-occurrence of GR; the spatial structure of burn mosaics relative to post-fire erosion; and the relationship between fire severity and threshold conditions required for channel initiation. I surveyed 269 burned catchments and mapped 111 cases of GR across sites in Montana and Idaho. I created the Vegetation Disturbance Index (VDI) derived from LANDSAT images to quantify fire severity and implemented geospatial and statistical analysis to quantify relationships between VDI and post-fire erosion response. Vegetation disturbance strongly explained GR with additional influences from upslope geometry and pre-fire shrub cover. As fire severity increased, the percent of the catchment area covered by continuous patches of high severity burn increased non-linearly. Trends in patch structure defined a threshold of fire severity after which the probability of GR was strongly correlated with the development of large, continuous severely burned patches. Fire severity systematically influenced the relationship between source area and steepness. Threshold conditions for channel initiation, specifically source area steepness and curvature, decreased as vegetation disturbance increased. These results provide inferential evidence that vegetation disturbance exerts first-order controls over post-fire erosion processes. The results of the patch-pattern analysis suggest that progressive loss of vegetation due to wildfire leads to critical thresholds of hydrologic connectivity after which runoff and erosion accelerate. The source area analysis suggests that forces of convergent flow are not fully expressed until a significant proportion of vegetation has been consumed such that flow resistance is minimized. The VDI as a continuous metric of vegetation disturbance may contribute to improved quantitative analysis of landform evolution relative to vegetation disturbance, ecological effects of fire, and ecosystem response to climate change. The assessment methodology outlined herein provides a first step towards a systematic quantification of the potential for GR following wildfire

Soil-Water Conservation, Erosion, and Landslide

The predicted climate change is likely to cause extreme storm events and, subsequently, catastrophic disasters, including soil erosion, debris and landslide formation, loss of life, etc. In the decade from 1976, natural disasters affected less than a billion lives. These numbers have surged in the last decade alone. It is said that natural disasters have affected over 3 billion lives, killed on average 750,000 people, and cost more than 600 billion US dollars. Of these numbers, a greater proportion are due to sediment-related disasters, and these numbers are an indication of the amount of work still to be done in the field of soil erosion, conservation, and landslides. Scientists, engineers, and planners are all under immense pressure to develop and improve existing scientific tools to model erosion and landslides and, in the process, better conserve the soil. Therefore, the purpose of this Special Issue is to improve our knowledge on the processes and mechanics of soil erosion and landslides. In turn, these will be crucial in developing the right tools and models for soil and water conservation, disaster mitigation, and early warning systems

Assessing the effects of site preparation treatments on erosion processes and sediment yield on a commercial Eucalyptus plantation: case study at Two Streams, KwaZulu-Natal.

Masters Degree. University of KwaZulu-Natal, Pietermaritzburg.Soil erosion monitoring and modelling is critical in the face of climate change, as erosion is detrimental to environmental and human health. It reduces soil productivity through degradation, compromises water quality through nutrient loading freshwater sources, and decreases reservoir capacity through sedimentation. This is a global challenge which is being amplified by increased levels of soil erosion on cultivated lands (e.g. commercial forestry), the combined effects of which hinder the success of several United Nations Sustainable Development Goals. However, commercial forestry is essential for human survival, providing a host of resources for human consumption, and expanding in its global coverage each year; although, this expanding need for commercial forestry creates a paradox, as it has the potential to damage environmental health and biodiversity (systems which humans rely on for survival), particularly through soil erosion and sedimentation of freshwater systems. Afforestation of plantations has been widely considered a land-use activity which reduces soil erosion; although, this is dependent on the management of the commercial plantations, where certain commercial forestry management techniques exacerbate soil erosion, such as the well-used site preparation technique of burning. Therefore, an investigation into the effect of commercial forestry site preparation techniques such as burning (at different severities) and mulching on soil erosion and the modelling thereof is required, as only a paucity of research has reported on this. Soil erosion measurements were conducted on a newly planted Eucalyptus dunnii stand, which consisted of three different site preparation techniques, namely a hot burn, a cold burn and a mulch treatment, in the Two Streams catchment, Kwa-Zulu Natal, South Africa. Micro-runoff and runoff plots were used to respectively measure splash and rill erosion of sediment, nitrogen, phosphorous, dissolved organic carbon, particulate organic carbon loss and runoff on each treatment at different slopes. In addition, soil erosion and runoff of this catchment and treatments were modelled using the ArcSWAT model, and the observations were used to validate the simulated outputs. The mulch treatment had the most consistent reduction in runoff and erosion, while the burn treatments of different severities generated greater respective runoff and erosion quantities through different erosional processes (splash vs rill). The ArcSWAT model over-simulated runoff on the hot burn and mulch treatment, while under-simulating on the cold burn treatment; however, the model consistently over-simulated sediment and nutrient loss on all treatments, indicating the model’s inability to simulate soil erosion on the defined land-use treatments. The reduced runoff and erosion produced by the mulch treatment is attributed to the protection that the mulch provides to the soil from splash erosion and the resistance posed to overland flow reducing rill erosion. The burn treatments generating more erosion and runoff through different erosional processes was attributed to the differing nature of debris produced by each burn severity. This research will contribute towards the data sets necessary to refine the land-use management tools of the ArcSWAT model to better model soil erosion on different land-use treatments. Furthermore, this research demonstrates the erosion processes that differing site preparation treatments are susceptible to, and what this means for future research and protecting soil and downstream water quality in the face of climate change