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University of Melbourne (2012)

Effects of prescribed burning on surface runoff and erosion

Cawson, Jane Greenslade

Titre : Effects of prescribed burning on surface runoff and erosion

Auteur : Cawson, Jane Greenslade

Université de soutenance : University of Melbourne

Grade : Doctor of Philosophy (PhD) 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

Mots Clés : fire ; prescribed burning ; runoff ; erosion ; water quality ; forest ; hydrologic connectivity ; fire severity ; burn patchiness ; fire frequency ; fire season


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Page publiée le 11 septembre 2016, mise à jour le 31 mai 2017