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Solar radiation driven differences in fine fuel moisture content on north and south slopes |
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Solar radiation driven differences in fine fuel moisture content on north and south slopes |
Project ResearchersRationaleDecision support tools such as the Canadian Forest Fire Danger Rating System (CFFDRS) exist to aid managers in assessing and integrating the numerous factors influencing the potential for dangerous wildfire behaviour. One component of CFFDRS, the Fire Weather Index (FWI), uses common weather inputs to create numerical rankings of fire danger related to wind conditions and the moisture content of surface fuels. Outputs of the FWI system represent standard fuel conditions on flat terrain and in an open closed canopy jack or lodgepole pine stand. Realistically, fire managers must constantly adapt these baseline measurements to suit their local surroundings. Managers in regions with complicated topographical features often find large discrepancies between predicted and actual fuel moisture conditions on slopes. Although simple physical relationships can be used to estimate temperature and relative humidity changes related to elevation, it is difficult to accurately quantify the length of exposure and amount of solar radiation received at the forest floor. In complex topography, slope angle and orientation coupled with time of year and latitude create a complicated mosaic of solar energy conditions. These differences are further complicated by the fact that the underlying models within the FWI System do not explicitly take solar radiation (and variations therein) into account. Project Objectives
Materials and MethodsIn early spring, a study site will be located based on local slope and stand characteristics, on the vicinity to nearest research facilities and on general ease of access. As a minimum, four weather stations will be installed at the site, each one representing a different mountain microclimate: north-facing slope (in-stand), south-facing slope (in-stand), level terrain (in-stand) and level terrain (open area: standard fire weather station location). The weather stations will collect in-stand values for temperature, relative humidity and solar radiation throughout the fire season, supplemented by the installation of rain gauges and additional solar radiometers throughout the stand. The weather stations must meet provincial fire weather collection guidelines and be calibrated to match standard FTS station outputs for temperature and relative humidity measurements. Destructive sampling of litter and duff layers will occur once in the early morning and again during peak burning conditions at three locations at each weather station site. Sampling will occur throughout the fire season and attempt to collect a range of values representing both fuel wetting and drying cycles. Applications and Future Research DirectionsAdjusting the FFMC for aspect and slope conditions allows fire managers to improve assessment of forest floor ignition potential as well as the accuracy of predicted of rates of spread in areas of prescribed burn units. Using a calibrated FFMC, managers will be better equipped to choose burn days that will result in fire behaviour appropriate to their prescription. The fine fuel moisture content ultimately determines the ability of the fire to sustain itself and spread which is crucial to the effective execution of prescribed burns. An adjustment factor will allow managers to quickly assess the flammability of different areas of their burn unit and plan ignition procedures accordingly. In this way, managers can predict when the wettest slopes will be ready to ignite, minimizing repeat efforts to sample areas and install weather stations. The private forestry industry may also find calibration codes useful, especially concerning harvesting operation restrictions during high fire hazard periods. Based on aspect and slope, companies may be able to find areas within their harvest management zone that have danger ratings less than those predicted by standard fire weather stations. A simple drying and wetting model that considers the influence of solar radiation on the moisture content of fine fuels can also be applied to operational activities such as thinning to predict changes in local fire potential. Adjustment factors can also be used to justify changes in suppression resource allocation, often in accordance with sustained action policies employed by certain agencies. A simple drying and wetting model that considers the influence of solar radiation on the moisture content of fine fuels can also be applied to operational activities such as thinning to predict changes in local fire potential. The 2008 season's research is to act as a pilot study to guide additional future research. Once the scientific method has been declared as sound, more sites comparing fine fuel moisture conditions on various slopes may be installed for the 2009 fire season, based on interest from fire agencies. Although creating a slope and aspect adjusted model for FFMC is the priority of this project, destructive duff samples will be collected and analyzed to begin building an adjusted model for DMC. The ability to adjust all the FWI System outputs will allow managers to better predict ease of ignition, fuel consumption, holdover potential, expected rates of spread, fuel availability for combustion and overall fire intensity in forest management areas with mountainous terrain. This project strives to be a catalyst for future improvements to CFFDRS and FWI systems related to topography, as well as to provide future directions for private (FPInnovations) and academic research activities. |
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