Executive Summary
Executive Summary
Wildfires in southern California threaten millions of homes and suppression costs have risen to more than $1 billion annually (Wells 2007, Stephens et al. 2009). The most costly wildfires are those which are wind-driven (Keeley and Zedler 2009, Radtke et al. 1982, Westerling et al. 2004). The main elements determining fire hazard (or the likelihood that an area will burn) are vegetation, weather (including wind) and topography. SAMO is dominated by chaparral and coastal sage scrub vegetation types, which provide an abundance of highly ignitable fuel and thus contribute to extreme fire danger (Witter et al. 2007). The area’s steep terrain, with major canyons running north and south, is conducive to rapid fire spread (Radtke et al. 1982). Additionally, local patterns of warm temperatures and relatively low humidity similarly increase fire hazard. However, this conceptual model of hazard does not account for extreme wind conditions, such as Santa Ana wind events. Climate change effects add complexity to SAMO’s future land management planning, as the current fire regime (e.g. fire frequency, seasonality and intensity) may be altered.
The primary goal of this project is to take Santa Ana winds into consideration in fire policy and management in Santa Monica Mountains National Recreation Area (SAMO). With a more accurate model of the spatial pattern of fire spread under strong wind conditions, managers could improve 1) fire management, including prevention, suppression or modification of fire behavior through various landscape treatment practices, and 2) education, which may improve community and private landowner fire management. Wind intensity in SAMO has never been formally documented on a fine scale. Combined with other factors, such as topography, vegetation and weather, information on wind intensity could be used to identify the areas facing higher fire hazard and facilitate a more effective allocation of fire management and education resources. Due to future land development potential, it is important to know which areas will be indefensible during Santa Ana wind events, and where development should be avoided.
Santa Ana winds occur seasonally when a cool, dry air mass from the interior western U.S. flows towards the Pacific Coast. The air mass sinks, compresses, strengthens and warms, desiccating vegetation and increasing fire hazard (Westerling et al. 2004). Multi-day Santa Ana wind events, occurring mostly between late September and December, are the primary drivers of fire danger in southern California (Witter et al. 2007, Dennison et al. 2008). In some portions of SAMO, canyons with high fuel loads run parallel with the prevailing north to northeasterly directions of the Santa Ana winds. In such areas, it will be particularly important to identify the spatial distribution of high intensity surface winds from the prevailing Santa Ana winds in order to plan for defensibility.
To model Santa Ana wind events in SAMO, WindWizard, a gridded wind-modeling program, will be used. Information from the National Weather Service (NWS) currently serves as the baseline for wind data. WindWizard provides finer scale results than the NWS without requiring high intensity computing power, and its validity can be checked against historical wind data. The WindWizard simulations will be used to create a Santa Ana wind intensity map for SAMO. The output can also be used to improve the accuracy of other models, such as Fire Area Simulator (FARSITE), a fire behavior and growth simulator that will be used to update fire hazard maps for SAMO. Hazard will be determined based on how frequently a given location burns in simulations, and how quickly a fire would reach a given location when started from various ignition points. The fire hazard map will be used to identify defensible and indefensible areas during Santa Ana wind events. Based on the mapping results, recommendations will be made for SAMO’s fire management planning and education programs for hazard mitigation and prevention.
Because SAMO is concerned about long-term management, the effects of climate change should be taken into account. There is consensus in the scientific community that fire hazard could increase as a result of changes in fuel loads and weather patterns (IPCC 2007, Moritz and Stephens 2008). However, the precise effects on a local scale are uncertain, with 20-year predictions relying on hypothetical scenarios of population growth, regulatory decisions and technological advancements (IPCC 2007). Despite the uncertainty, managers can consider the effects of climate changes in the landscape with some certainty, and modify current policy, zoning and mitigation plans to address future fire hazard (Lenihan et al. 2003). Regional climate change scenario models could be used by fire and land managers to project a potential range of future fuel moisture levels. The FARSITE model can incorporate both the projected fuel moisture values and the previously calculated wind data to create SAMO-specific future fire regime models. Based on the analysis of these model-based scenarios, long-term recommendations for fire management and outreach program modifications for SAMO will be provided.
SAMO has long recognized the importance of education in preventing both the ignition and spread of fire. Given the potential for additional population growth in the WUI, a fire hazard map will help to articulate areas of highest hazard from uncontrollable wildfires. More precise information on fire hazard could be used to better inform land development decisions by residents and planners. Although education efforts are important and should continue, it may be more cost-effective in the long-term to advocate changes to zoning in areas of high fire hazard and limit population growth where practicable.