School of Agriculture, Food and Ecosystem Sciences - Theses
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ItemPost-fire debris flows in southeast Australia: initiation, magnitude and landscape controls( 2013)Surface runoff and sediment availability can increase after wildfire, potentially resulting in extreme erosion, flash floods and debris flows. These hydro-geomorphic events supply large amounts of sediment to streams and can represent a hazard to water supply systems, infrastructure and communities. This thesis combines observations, measurements and models to quantify and represent the post-fire processes that result in hazardous catchment responses. The processes that constitute risk to water quality and infrastructure were identified through field surveys of burnt catchments in the eastern upland of Victoria (southeast Australia) where impacts had occurred. The survey established that the majority of high-impact events after wildfire were linked to runoff-generated debris flows, a process previously undocumented in the region. The debris flows were initiated through progressive sediment bulking, and occurred in response to short duration and high intensity rainfall events, within one year after wildfire. Debris flows were confined to dry sclerophyll forests that had been subject to crown fire. Wet forest types displayed comparatively subdued responses, a pattern attributed to the relatively high infiltration capacity in these systems. Infiltration and sediment availability were isolated as the key hillslope components that were sensitive to burning and which strongly influenced catchment processes and debris flow susceptibility. The aims of subsequent work were therefore to develop models of infiltration and sediment availability as controls on hillslope response and use these to quantify changes in key parameters during recovery from wildfire. Infiltration was modelled as function of surface storage (H), matrix flow (Kmat) and macropore flow (Kmac). Macropore flow was found to be the main parameter driving the temporal trends in infiltration capacity during recovery from wildfire. Water repellency was ubiquitous in headwater recovering from wildfire, although the strength diminished during prolonged wet weather conditions, a dependency which could be modelled as a function of monthly weather patterns. Sediment availability was highly variable with soil depth, a feature which contrasts with assumptions underlying commonly used erosion models, typically developed in agricultural systems. The majority of erosion following wildfire was found to occur in a shallow layer of highly erodible material which could be represented through dnc, a parameter describing the depth of non-cohesive soil. This depth of available soil decreased exponentially during recovery. The models of sediment availability and infiltration were effective at capturing both spatial variability and recovery processes and form a basis on which to model debris flow initiation and magnitude in variable landscapes during recovery from wildfire.
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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.