Petter Nyman

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.

Relations between soil hydraulic properties and burn severity

Wildfire can affect soil hydraulic properties, often resulting in reduced infiltration. The magnitude of change in infiltration varies depending on the burn severity. Quantitative approaches to link burn severity with changes in infiltration are lacking. This study uses controlled laboratory measurements to determine relations between a remotely sensed burn severity metric (dNBR, change in normalised burn ratio) and soil hydraulic properties (SHPs). SHPs were measured on soil cores collected from an area burned by the 2013 Black Forest fire in Colorado, USA. Six sites with the same soil type were selected across a range of burn severities, and 10 random soil cores were collected from each site within a 30-m diameter circle. Cumulative infiltration measurements were made in the laboratory using a tension infiltrometer to determine field-saturated hydraulic conductivity, Kfs, and sorptivity, S. These measurements were correlated with dNBR for values ranging from 124 (low severity) to 886 (high severity). SHPs were related to dNBR by inverse functions for specific conditions of water repellency (at the time of sampling) and soil texture. Both functions had a threshold value for dNBR between 124 and 420, where Kfs and S were unchanged and equal to values for soil unaffected by fire. For dNBRs >~420, the Kfs was an exponentially decreasing function of dNBR and S was a linearly decreasing function of dNBR. These initial quantitative empirical relations provide a first step to link SHPs to burn severity, and can be used in quantitative infiltration models to predict post-wildfire infiltration and resulting runoff.

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.

Effects of aridity in controlling the magnitude of runoff and erosion after wildfire

This study represents a uniquely high-resolution observation of postwildfire runoff and erosion from dry forested uplands of SE Australia. We monitored runoff and sediment load, and temporal changes in soil surface properties from two (0.2–0.3 ha) dry forested catchments burned during the 2009 Black Saturday wildfire. Event-based surface runoff to rainfall ratios approached 0.45 during the first year postwildfire, compared to reported values <0.01 for less arid hillslopes. Extremely high runoff ratios in these dry forests were attributed to wildfire-induced soil water repellency and inherently low hydraulic conductivity. Mean ponded hydraulic conductivity ranged from 3 to 29 mm h−1, much lower than values commonly reported for wetter forest. Annual sediment yields peaked at 10 t ha−1 during the first year before declining dramatically to background levels, suggesting high-magnitude erosion processes may become limited by sediment availability on hillslopes. Small differences in aridity between equatorial and polar-facing catchments produced substantial differences in surface runoff and erosion, most likely due to higher infiltration and surface roughness on polar-facing slopes. In summary, the results show that postwildfire erosion processes in Eucalypt forests in south-east Australia are highly variable and that distinctive response domains within the region exist between different forest types, therefore regional generalizations are problematic. The large differences in erosion processes with relatively small changes in aridity have large implications for predicting hydrologic-driven geomorphic changes, land degradation, and water contamination through erosion after wildfire across the landscape.

ENSO controls interannual fire activity in southeast Australia

El Nino Southern Oscillation (ENSO) is the main mode controlling the variability in the ocean-atmosphere system in the South Pacific. While the ENSO influence on rainfall regimes in the South Pacific is well documented, its role in driving spatiotemporal trends in fire activity in this region has not been rigorously investigated. This is particularly the case for the highly flammable and densely populated southeast Australian sector, where ENSO is a major control over climatic variability. Here we conduct the first region-wide analysis of how ENSO controls fire activity in southeast Australia. We identify a significant relationship between ENSO and both fire frequency and area burnt. Critically, wavelet analyses reveal that, despite substantial temporal variability in the ENSO system, ENSO exerts a persistent and significant influence on southeast Australian fire activity. Our analysis has direct application for developing robust predictive capacity for the increasingly important efforts at fire management.

Quantifying the effects of topographic aspect on water content and temperature in fine surface fuel

This study quantifies the effects of topographic aspect on surface fine fuel moisture content (FFMC) in order to better represent landscape-scale variability in fire risk. Surface FFMC in a eucalypt forest was measured from December to May (180 days) on different aspects using a novel method for in situ monitoring of moisture content (GWClit) and temperature (Tlit) in litter. Daily mean GWClit varied systematically with aspect. North (0.07 ≤ GWClit ≤ 1.30 kg kg–1) and south (0.11 ≤ GWClit ≤ 1.83 kg kg–1) aspects were driest and wettest respectively, whereas east and west were somewhere in between. On the warmest day (38.9°C), the maximum Tlit on north (43.7°C) and south (29.8°C) aspects differed by 13.9°C. Aspect-driven variation in Tlit and GWClit is exacerbated by vegetation, which increases markedly in density with decreasing solar exposure. GWClit was below fibre saturation point (<0.35 kg kg–1) on 49 and 128 days on south and north aspects, respectively, demonstrating that fuels beds are often in different stages of drying and therefore subject to different hydrological processes depending on landscape position. This terrain-related variability in moisture dynamics strongly affects the spatial connectivity of fuels, and may be more important for predicting landscape-scale burn outcomes than sub-daily fluctuations at a point.

Modelling the effects of fire and rainfall regimes on extreme erosion events in forested landscapes

Existing models of post-fire erosion have focused primarily on using empirical or deterministic approaches to predict the magnitude of response from catchments given some initial rainfall and burn conditions. These models are concerned with reducing uncertainties associated with hydro-geomorphic transfer processes and typically operate at event timescales. There have been relatively few attempts at modelling the stochastic interplay between fire disturbance and rainfall as factors which determine the frequency and severity with which catchments are conditioned (or primed) for a hazardous event. This process is sensitive to non-stationarity in fire and rainfall regime parameters and therefore suitable for evaluating the effects of climate change and strategic fire management on hydro-geomorphic hazards from burnt areas. In this paper we ask the question, “What is the first-order effect of climate change on the interaction between fire disturbance and storms?” The aim is to isolate the effects of fire and rainfall regimes on the frequency of extreme erosion events. Fire disturbance and storms are represented as independent stochastic processes with properties of spatial extent, temporal duration, and frequency of occurrence, and used in a germ–grain model to quantify the annual area affected by extreme erosion events due to the intersection of fire disturbance and storms. The model indicates that the frequency of extreme erosion events will increase as a result of climate change, although regions with frequent storms were most sensitive.

Hillslope-scale prediction of terrain and forest canopy effects on temperature and near-surface soil moisture deficit

Soil moisture has important effects on fuel availability, but is often assessed using drought indices at coarse spatial resolution, without accounting for the fine-scale spatial effects of terrain and canopy variation on forest floor moisture. In this study, we examined the spatial variability of air temperature, litter temperature and near-surface soil moisture (θ, 0–100 mm) using data from field experiments at 17 sites in south-east Australia, covering a range of topographic aspects and vegetation types, within climates from semiarid to wet montane. Temperatures and θ in mountainous environments were found to vary at much finer spatial scales than typical drought index grid dimensions (several kilometres). Using terrain elevation, local insolation ratio and plant area index, we developed semi-empirical microclimate models for air and litter temperatures, then used modelled temperatures as input into calculations of the Keetch–Byram Drought Index, a widely used index of soil moisture deficit. Drought index results based on predicted litter temperature were found to explain 91% of the spatial variation in near-surface soil moisture at our experimental sites. These results suggest the potential for routine hillslope-scale predictions of forest floor moisture status, which may be useful in the management of fire, particularly prescribed burning, in complex terrain.

Quantifying relations between surface runoff and aridity after wildfire: Relations between surface runoff and aridity after wildfire

Post-wildfire runoff and erosion are major concerns in fire-prone landscapes around the world, but these hydrogeomorphic responses have been found to be highly variable and difficult to predict. Some variations have been observed to be associated with landscape aridity, which in turn can influence soil hydraulic properties. However, to date there has been no attempt to systematically evaluate the apparent relations between aridity and post-wildfire runoff. In this study, five sites in a wildfire burnt area were instrumented with rainfall-runoff plots across an aridity index (AI) gradient. Surface runoff and effective rainfall were measured over 10months to allow investigation of short(peak runoff) and longer-term (runoff ratio) runoff characteristics over the recovery period. The results show a systematic and strong relation between aridity and post-wildfire runoff. The average runoff ratio at the driest AI site (33.6%) was two orders of magnitude higher than at the wettest AI site (0.3%). Peak runoff also increased with AI, with up to a thousand-fold difference observed during one event between the driest and wettest sites. The relation between AI, peak 15-min runoff (Q15) and peak 15-min rainfall intensity (I15) (both in mm h ) could be quantified by the equation:Q15 = 0.1086I15 ×AI 2.691 (0.65<AI<1.80, 0<I15<45) (adjusted r 2 = 0.84). The runoff ratios remained higher at drier AI sites (AI 1.24 and 1.80) throughout the monitoring period, suggesting higher AI also lengthens the window of disturbance after wildfire. The strong quantifiable link which this study has determined between AI and post-wildfire surface runoff could greatly improve our capacity to predict the magnitude and location of hydro-geomorphic processes such as flash floods and debris flows following wildfire, and may help explain aridity-related patterns of soil properties in complex upland landscapes. Copyright