Patrick N.J. Lane

Streamflow response of mixed-species eucalypt forests to patch cutting and thinning treatments

Abstract An 11-year paired-catchment study was implemented to assess the effect of differing logging practices on streamflow in mixed-species eucalypt forest at Tantawangalo Creek, southeastern Australia. Following a 3-year calibration period, Ceb catchment was retained as a control, Wicksend catchment was patch-cut to remove 22% of basal area, and Willbob catchment was thinned to remove 12% of basal area. The treatments resulted in an initial increase in monthly total streamflow and baseflow at both treated catchments. In annual terms, total streamflow increased by 10% in the first 3 years after logging at Wicksend, and by 31% for the first 4 years at Willbob, despite the difference in basal area removed. Streamflow then returned to pre-treatment levels for 1 year and subsequently declined by 20% below pre-treatment levels over the next 4 years at Wicksend. Willbob displayed a return to, but not below, pre-treatment levels over the same period. Although there were some large post-treatment stormflows, changes in baseflows were largely responsible for the streamflow increases, and at Wicksend, explained the streamflow decreases in the latter observation period. Differences in regeneration densities between the treated catchments indicate the greater magnitude and persistence of flow increases at Willbob were due to poor regeneration following the thinning treatment, while dense and vigorous under- and overstorey regeneration dampened initial flow increases and facilitated significant decreases at Wicksend. Significant streamflow decreases after disturbance have not been widely reported for mixed-species eucalypt forests, and the magnitude of streamflow increases after only 12% basal area removal at Willbob is unusual.

Carbon loads, forms and sequestration potential within ash deposits produced by wildfire: new insights from the 2009 ‘Black Saturday’ fires, Australia

Forest fires release substantial amounts of carbon (C). Much of it is emitted to the atmosphere, but some is deposited within ash on the ground. Little is known about amount and types of C deposited in ash. Here, we quantify total C, and total inorganic, water-soluble and particulate organic fractions deposited in ash during the catastrophic 2009 ‘Black Saturday’ wildfires in Australia. These fires coincided with the highest air temperatures and lowest humidity ever recorded in the local area, which, combined with high fuel loads of mostly long unburnt eucalypt forests, generated extreme burning conditions. In three mixed-species eucalypt forest sites sampled, the canopy, understorey and litter fuels were almost completely consumed, resulting in substantial ash deposition (mean, 81.9 t ha−1), with 5.9 t ha−1 of C being transferred from vegetation to the forest floor. In five temperate rainforest sites sampled, the canopy was not burnt and ash deposition was lower (mean, 48.3 t ha−1) than in the mixed-species eucalypt forest, but overall their higher C content resulted in higher C deposition (8.1 t ha−1). In all cases, most C contained in ash was organic and its pyrogenic nature infers increased resistance to degradation. Pyrogenic C is viewed by many as an important C sink, which could contribute to long-term C sequestration when incorporated into soils or sediments. Our results highlight the potential importance of the pyrogenic C pool in freshly deposited ash and, therefore, the necessity of a systematic and detailed analysis of ash deposition and C forms in ash to improve our understanding of C fluxes by forest fires.

Modelling the long term water yield impact of wildfire and other forest disturbance in Eucalypt forests

Disturbance of forested catchments by fire, logging, or other natural or human induced events that alter the evapotranspiration regime may be a substantial threat to domestic, environmental and industrial water supplies. This paper describes the physically-based modelling of the long term changes in water yield from two wildfire affected catchments in north-eastern Victoria, Australia, and of fire and climate change scenarios in Melbournes principal water supply catchment. The effect of scale, data availability and quality, and of forest species parameterisation are explored. The modelling demonstrates the importance of precipitation inputs, with Nash and Sutcliffe Coefficients of Efficiency of predicted versus observed monthly flows increasing from 0.5 to 0.8 with a higher density of rainfall stations, and where forest types are well parameterised. Total predicted flow volumes for the calibrations were within 1% of the observed for the Mitta Mitta River catchment and <4% for the Thomson River, but almost -10% for the less well parameterised Tambo River. Despite the issues of data availability simulations demonstrated the potential for significant impacts to water supply in SE Australia from wildfire and climate change. For example, for the catchments modelled the moderate climate change impact on water yield was more pronounced than the worst fire scenario. Both modelled cases resulted in long term water yield declines exceeding 20%, with the climate change impact nearing 30%. A simulation using observed data for the first four post-fire years at the Mitta Mitta River catchment showed Macaque was able to accurately predict total flow.

Surface runoff and erosion after prescribed burning and the effect of different fire regimes in forests and shrublands: a review

This paper examines the state of knowledge about the effects of prescribed burning on surface runoff and erosion at point to catchment scales in forests and shrublands. Fires can increase surface runoff and erosion by removing vegetation, changing soil hydrologic properties and providing a readily erodible layer of sediment and ash. Catchment-scale studies in prescribed-burnt areas usually report minimal impacts from the burn. However, measurements at smaller spatial scales suggest that large changes to hydrologic properties and processes do occur, and a debris-flow example from Australia demonstrates that large catchment-scale impacts are possible. It appears that existing catchment-scale studies in prescribed burns do not capture these large events as the sample size (i.e. number of studies) is too small relative to the infrequency of such events. Furthermore, numerous knowledge gaps across all spatial scales limit understanding of the processes contributing to post-prescribed burn runoff and erosion. Understanding the influence of fire regime characteristics on post-fire runoff and erosion is particularly important in the context of prescribed burning, as fire regimes can be manipulated to reduce erosion and water-quality impacts. Therefore, two directions for future research are recommended: (1) process-based studies to understand the factors controlling surface runoff and erosion, particularly in relation to aspects of the fire regime; and (2) landscape-scale surveys to quantify large erosion events.

Downscaling regional climate data to calculate the radiative index of dryness in complex terrain

The radiative index of dryness (or aridity index) is a non-dimensional measure of the long-term balance between rainfall and net radiation. Quantifying aridity requires spatially distributed information on net radiation and rainfall. The variability in net radiation in complex terrain can be modelled at high spatial resolution by combining point data with equations that incorporate the effects of elevation, surface geometry and atmospheric attenuation of incoming radiation. At large spatial scales and over long time periods, however, the combination of seasonality, year to year variations and spatial variability in climate result in complex spatial-temporal patterns of incoming radiation, which are more effectively captured in satellite-based measurements. This study uses a high resolution model of shortwave radiation as a tool for downscaling satellite-derived data on incoming radiation. The aim was to incorporate topographic effects on net radiation in complex terrain while retaining information on regional and seasonal trends captured in satellite data. The method relies on satellite-based measures of incoming radiation from the Australian Bureau of Meteorology (BoM) to provide the spatial coverage and long-term data that represent the average incoming radiation across the state of Victoria in southeast Australia. These long-term data were coupled with a topographic downscaling algorithm to produce estimates of net radiation and aridity at the resolution of a 20 m digital elevation model. Results show that annual precipitation (and cloud fraction) gradients drive the variability in aridity at large scales (10–100 km) while topography (e.g. slope aspect and slope angle) are the main drivers at small scales (e.g. 1 km). The aridity index varied between 0.24 and 10.95 across the state of Victoria. The effect of aridity on vegetation was apparent at local scales through systematic variations in tree-height along rainfall gradients and across aspects with different levels of exposure to solar radiation.

Hydro-geomorphic response models for burned areas and their applications in land management

Erosion, flash floods and debris flows are hydro-geomorphic processes that intensify due to catchment disturbance by wildland fire. Predictive models of these processes are used by land managers to quantify rehabilitation effectiveness, prioritize resources and evaluate trade-offs between different management strategies. Predictions can be difficult to make, however, because of heterogeneous landscapes, stochastic rainfall, and the transient and variable fire effects. This paper reviews hydro-geomorphic response models for burned areas and explores how modelling approaches and sources of uncertainty change depending on the focus question (or purpose) and the associated spatial-temporal scale of the model domain. The review shows that current models focus primarily on predicting catchment responses during a recovery period (within-burn timescales), a relatively short temporal window during which rainfall is an important source of uncertainty. At longer (between-burn) timescales, the fire regime itself, and not just fire severity, becomes a variable component of the model. At this temporal scale, the catchment processes respond to variations in the frequency and severity with which a landscape is conditioned (or ‘primed’) by fire and rain storms. Conditioning is a stochastic process that is determined by the spatial-temporal overlap of fire disturbance and rain storms. The translation of overlaps to hydro-geomorphic responses is a function of intrinsic catchment attributes (e.g. permeability, slope and catchment area). Capturing the stochastic interplay between fire and rain storms is important when land-management questions shift towards the issues of climate change and landscape-scale interventions such as prescribed burning. The review therefore includes a discussion on fire and rainfall regimes as variables which drive decadal and regional variability in hydro-geomorphic processes.

Paired Eucalyptus forest catchment study of prescribed fire effects on suspended sediment and nutrient exports in south-eastern Australia

The effect of prescribed fire on suspended sediment and nutrient exports was investigated in two small Eucalyptus forest catchments in south-eastern Australia. In 2005, a patchy, mostly low-severity prescribed fire was applied to both catchments, followed in 2006 by a second burn applied to riparian areas of one catchment, with the other catchment utilised as a control for this burn. Historic pre-fire weekly stream water sampling was combined with post-fire weekly and storm-based sampling to quantify the effect of the fires. The 2005 fire resulted in a significant difference (P = 0.000) in suspended sediment concentrations compared to pre-fire data and generated peak study period suspended sediment (11.5 kg ha–1 year–1) and total phosphorous (0.016 kg ha–1 year–1) exports under near-average rainfall. However, peak suspended sediment exports only slightly exceeded the average annual load from a nearby undisturbed catchment. Well-below-average rainfall in 2006 resulted in lower exports after this burn compared with the 2005 fire. The results highlighted the importance of hydrological conditions for suspended sediment and nutrient exports within the first 12–18 months after prescribed fires, beyond which generally rapid surface vegetation recovery is likely to mitigate any burns effects.

Is aridity a high-order control on the hydro–geomorphic response of burned landscapes?

Fire can result in hydro–geomorphic changes that are spatially variable and difficult to predict. In this research note we compile 294 infiltration measurements and 10 other soil, catchment runoff and erosion datasets from the eastern Victorian uplands in south-eastern Australia and argue that higher aridity (a function of the long-term mean precipitation and net radiation) is associated with lower post-fire infiltration capacities, increasing the chance of surface runoff and strongly increasing the chance of debris flows. Post-fire debris flows were only observed in the more arid locations within the Victorian uplands, and resulted in erosion rates more than two orders of magnitude greater than non-debris flow processes. We therefore argue that aridity is a high-order control on the magnitude of post-wildfire hydro–geomorphic processes. Aridity is a landscape-scale parameter that is mappable at a high resolution and therefore is a useful predictor of the spatial variability of the magnitude of post-fire hydro–geomorphic responses.

Field and laboratory calibration and test of TDR and capacitance techniques for indirect measurement of soil water content

Time domain reflectometry (TDR) and a frequency domain sensor, the Didcot Capacitance Probe, were tested in the field and laboratory. The results from an undisturbed large core TDR laboratory test found the Topp equation returned a close correspondence to thermogravimetrically derived water content, although there was a slight underestimation. Coefficients of determination and efficiency were >0.98 and 0.92, respectively, for individual cores, and 0.98 and 0.97 for the whole data set. The field exercise revealed the Topp equation to be superior to the laboratory derived equation and other published empirical equations, suggesting the Topp equation to be adequate. A field test of the capacitance probe found poor correspondence between measured and predicted observations of profile point soil water content. Although 81% of the variance was explained by the calibration regression, there was a poor fit to the 1:1 line (E = 0.34), and a non-significant relationship between measured and predicted soil water content for the A horizon. The instrument design proved problematic for use as a determiner of point profile soil water content, and the recommended calibration procedure was impossible in the study site soils.

An empirical, comparative model of changes in annual water yield associated with pine plantations in southern Australia

Summary Results from three Australian multiple catchment projects that examine the change in water yield on conversion from native eucalypt forest to radiata pine were combined. The projects were located in southern Australia near Myrtleford (Victoria), Daylesford (Victoria) and Bathurst (NSW). From these data a model of change in water yield relative to the eucalypt forest was derived using age and annual rainfall as independent variables. This model was extended to derive estimates of change in water yield from grassland sites converted to radiata pine by using the model of Zhang et al. (2001) to estimate the difference in water use between native forest and grassland. The results showed (and quantified) that conversion of native forest to radiata pine usually increases water yield but conversion of grassland sites to radiata pine usually decreases yields. Increased water yields are associated with young pines and high rainfalls, while decreased water yields are associated with older pines and low rainfalls. The models were tested using data from radiata pine plantations planted on grassland sites in Tumut (NSW), Kilmore (Victoria) and ‘fynbos’ in South Africa. In general the models performed reasonably well in estimating sequences of changes in flows. Estimates of total water yield were less accurate. Heavy or frequent thinning may be a source of change that may need to be accounted for separately if the details of this are known. The derived models may be useful in estimating the comparative changes of flow associated with the development of multiple blocks of radiata pine plantations on catchments. This can be programmed in a spreadsheet.