School of Agriculture, Food and Ecosystem Sciences - Theses
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ItemAridity as a predictor of the hydrogeomorphic response of burnt landscapes( 2016)Wildfire is an important disturbance in natural systems which can lead to changes in runoff and erosion processes. Increased runoff and erosion, including extreme erosion events such as debris flows, pose a hazard to soil and water resources, habitats, infrastructure, and lives. Natural resource managers require information about potential post-fire response in order to plan prevention, mitigation, or recovery activities which deal with the impacts of runoff and erosion events. Changes in hydrogeomorphic processes due to fire may also have important implications for long-term sediment budgets and landscape development. However, post-fire responses vary widely due to the landscape, fire, and post-fire rainfall properties, making it important to identify and understand mechanisms which control response variation. Previous studies, anecdotal evidence, and soil development theory suggest that moisture availability may influence post-fire runoff and erosion response. Therefore, this study aimed to investigate and quantify the relationship between moisture availability (characterised by an aridity index) and hydrogeomorphic response following high severity wildfire in the upper reaches of forested water catchments in central Victoria, Australia. This aim was addressed by examining the relationships between aridity index (AI) and soil infiltration capacity, runoff generation, and debris flow occurrence. In the first year following fire, infiltration rates and patterns, soil moisture, and soil water repellency were measured in the field. Saturated hydraulic conductivity (Ksat), soil porosity, texture, and density as well as water repellency of air-dried soils and the change in repellency with soil moisture were measured in the laboratory. To quantify the relationship between AI and surface runoff, total (all events) and real-time (within events) surface runoff and rainfall were monitored over 10 months. Aerial photography and spatial datasets were used to model (logistic regression) the relationship between the AI and the probability of post-fire debris flow occurrence. Overall the results suggest aridity exerts control over both long-term (decades to centuries) and short-term (daily to annual) system properties which result in a strong, quantifiable relationship between AI and post-fire soil infiltration capacity, runoff generation, and debris flow occurrence in the forested catchments of Victorian uplands. Increased AI resulted in reduced saturated hydraulic conductivity, suggesting a long-term control of aridity on soil structure. This, coupled with long- and short-term control of aridity on soil water repellency, led to field infiltration rates three times higher and over twice the proportion of the soil actively contributing to infiltration at the lowest AI site (AI 1.1) compared with the highest AI site (AI 2.4). Higher AI sites were consistently drier (had lower soil moisture content) leading to increased actual water repellency (measured in situ), as well as having an increased level of potential (air dry) water repellency. The reduction in infiltration capacity resulted in higher AI sites producing a greater percentage of runoff and higher peak discharge, for longer timeframes than lower AI sites. Average runoff ratio of the highest AI site (33.6%) was an order of magnitude higher than the lowest AI site (0.3%). Peak discharge during rainfall events also increased with increasing AI, with up to a thousand fold difference observed during one event. Undergrowth on the lower AI sites (AI 1.2 and 1.4) recovered more quickly (> 30% projected foliage cover within the first 6 months) than higher AI sites (AI 1.9 and 2.4) (< 5%), suggesting increased AI increases the window of disturbance. At the single headwater catchment scale, increased AI was empirically related to increased probability of post-fire debris flows. Post-fire debris flow producing catchments had significantly higher minimum, mean, and maximum AI values than non-debris flow producing catchments on average. Results indicated debris flows only occurred in catchments which contained pixels with an AI of 2.2 or higher. Logistic regression modelling results showed the probability of debris flows was highly sensitive to changes in maximum AI (sensitivity 0.95 and elasticity 2.05). This study supports the theory that aridity is a dominant first order control of hydrogeomorphic sensitivity to wildfire in this environment. Over geomorphic timescales, aridity-driven variation in runoff and erosion could have significant implications for landscape and ecosystem development. The effect of aridity is likely to become even more pronounced as climate change alters current rainfall regimes, and as the frequency and intensity of fires subsequently increases. This is the first study to quantify the relationship between AI and runoff and erosion processes following wildfire in this environment. The results of the study are a first step in using AI to predict and map soil hydrologic properties, runoff potential, and debris flow risk across burnt landscapes. As these properties and post-fire processes are resource intensive to determine in the field, AI provides an alternative spatial predictor variable which can be used to estimate these factors. Results of the study suggest AI could be a useful environmental indicator for management, capable of identifying areas of post-fire runoff and erosion risk in S.E. Australia.
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ItemEffects of prescribed burning on surface runoff and erosion( 2012)Prescribed burning – the deliberate use of fire to achieve management objectives – is used extensively in fire-prone vegetation for reducing fuel hazards and enhancing ecological values. As governments set ambitious targets for more prescribed burning, it is important to understand and manage the potential negative impacts, such as increased erosion. While globally there are many studies that consider the effects of prescribed burning on surface runoff and erosion, there are critical knowledge gaps for particular forest types (e.g. dry eucalypt forests) and in relation to understanding the factors controlling particular post-fire hydrologic and erosion responses, the likelihood of large impacts, the effects of spatial scale on the magnitude of an impact and the long-term risks of repeated burning. Therefore, the aim of thesis was to quantify the effects of prescribed burning on soil hydrologic properties, surface runoff and erosion in dry eucalypt forests in Victoria, Australia. This aim was addressed by examining the effects of two potentially important aspects of fire regimes – fire severity and burn patchiness – on soil hydrologic properties, surface runoff and erosion. Measurements were conducted in unburnt, low fire severity (scorched understorey and intact canopy) and high fire severity (burnt understorey and scorched canopy) areas at three dry eucalypt forest sites. Soil water repellency (using the critical surface tension test) and infiltration capacity (using ponded and tension infiltrometers) were measured at the point-scale for all sites immediately post-burn and then at six-month intervals. Rainfall simulations were used to measure runoff and erosion at the plot-scale (3 m2) six-weeks and 11-months post-burn at one site. Additionally, at one site runoff samplers (116 unbounded plots, 10 cm wide and approximately 100 m from the catchment divide) were used to measure runoff and erosion downslope of six burn categories: (1) high severity, (2) low severity, (3) unburnt, and low severity above (4) 1 m, (5) 5 m, and (6) 10 m wide unburnt patches. Prescribed burning resulted in higher runoff and erosion rates. Cumulative hillslope runoff volumes (over16-months) were approximately two orders of magnitude higher on burnt hillslopes and cumulative sediment loads were approximately three orders of magnitude higher. Water repellency increased following burning at two sites, but loss of vegetation cover appeared to be the primary driver for increased runoff and erosion in burnt areas, as fire-induced water repellency did not affect point-scale infiltration capacities. Fire severity differences had relatively little effect on runoff and erosion, presumably because surface vegetation cover was similar in the high and low fire severities. Unburnt patches were highly effective at reducing the connectivity of runoff and erosion from upslope burnt areas, with reductions in overall sediment loads of 96.6% and 99.8% for the 5 m and 10 m wide patches, respectively. The effectiveness of the unburnt patches at reducing runoff and erosion connectivity varied with patch width and rainfall intensity. For example, the 1 m wide unburnt patch reduced the overall sediment load by 92% for rainfall events with average recurrence intervals of < 10 years but was ineffective during a 10-year storm. Overall, the results suggested that despite higher plot-scale runoff and erosion rates post-burn, prescribed burns are unlikely to substantially affect runoff and erosion at the catchment-scale for most rainfall events given their inherent patchiness. Only during particularly intense storms, when unburnt patches become less effective at intersecting runoff and erosion, might severe erosion occur. From a management perceptive, the results suggest that to minimise runoff and erosion connectivity and potential water quality impacts following prescribed burning, there should be a fine-grained mosaic of burnt and unburnt patches throughout a burn (e.g. > 50% unburnt and patches 5-10 m wide) and unburnt streamside buffers. Such burn patterns may be achieved by the ignition pattern, and burning under mild conditions when there are moisture differentials throughout the burn area. While fire severity was found to be a less significant factor in relation to post-burn runoff and erosion rates, it is likely that lower fire severities are associated with more patchy burns and therefore it would be reasonable to aim for low severity burn outcomes.