Measurement of topographic controls on the moisture content of surface fuels in south east Australian forests

Abstract

Prediction of fuel moisture content (FMC) is important for estimating the rate of spread of wildfires, the ignition probability of firebrands, and for the efficient scheduling of prescribed fire. The moisture content of fine surface fuels varies dramatically at a range of spatial scales; at large scales (10's to 100's km) due to variation in meteorological variables (eg. temperature, relative humidity, precipitation), and at smaller scales (100's of metres) in steep topography due to factors that include differences in radiation due to aspect and slope, differences in precipitation, temperature and relative humidity due to elevation, and differences in soil moisture due to hillslope drainage position. Forest structure and canopy shading responses to these topographic influences adds further to the spatial variability in surface fuel moisture. Finally, it is likely that the interactions between these topographic influences, vegetation response and fuel moisture content will vary across climatic gradients, potentially creating a high level of complexity in the relationship between topography and fuel moisture. As a result of this complexity there have been few attempts to model FMC at smaller spatial scales that could assist fire managers in prediction and planning. In this study we aim to “untangle” these factors, and in particular answer the following questions i) How does fuel moisture vary with aspect? ii) How does fuel moisture vary with hillslope drainage position? iii) How do these topographic variables interact with vegetation structure to result in net FMC effects, and iv) How do these topographic and vegetation interactions change along a climatic gradient? To achieve the project aims, a new method was developed and validated to enable the monitoring of FMC over seasonal timescales. Microclimate stations were established in southeast Australian forests to monitor surface fine fuel moisture at 15-minute intervals using these newly developed instrumented litter packs, in addition to temperature and relative humidity measurements inside the litter pack, and measurement of precipitation and energy inputs above and below the forest canopy. Stations were established to monitor FMC and microclimate throughout a fire season across a gradient of aspect, drainage position, forest structure, and climate in order to address the research objectives. Preliminary conclusions from three months of data collection are that; 1) aspect effects on FMC are mostly due to secondary effects on canopy cover and shading, rather than the direct effect of aspect on incoming above-canopy radiation, 2) drainage position influences FMC due to the secondary effects on canopy cover and shading as well as the direct effect of drainage area on soil moisture, and, 3) as a result, both aspect and drainage position effects on FMC are strong when they result in significant change in canopy cover, and weak when they don't, resulting in highly variable topographic effects across a climatic gradient. These results based on an unprecedented field measurement campaign provide a major step forward towards the larger goal of constructing high spatial resolution models of FMC for implementation in complex landscapes.