Multicriteria Evaluation of Wildfire Hazard in California

Data Acquisition Data Preparation and Analysis Multi-Criteria Evaluation and Results Discussion References

Introduction

Fire is an incredibly important component of ecosystems all over the world. It is a form of disturbance that helps maintain diversity and therefore stability in an ecosystem. Too short a time between fires can prevent plant species from reestablishing themselves resulting in extinctions and lack of diversity (Krebs 2001). Too long between fires can lead to a build up of fuel and a lack of diversity due to competitive species taking over (among other problems). The build up in fuel can result in catastrophic high intensity wildfires (Wright & Bailey 1982). Fire frequency therefore needs to be controlled to encourage diversity and prevent large wildfires destroying vegetation stands and causing damage to people and property. Knowledge of areas most at risk of having a wildfire can be useful in implementing preventative measures. This project has performed an evaluation on how high a risk there is of a serious, high intensity wildfire occurring in any area of California. This knowledge can help determine the allocation of funding for wildfire prevention and fire services.

For this project several data sets were required.

The heat yield of a fire is affected by how flammable the vegetation is. Naturally volatile substances such as oils will increase the heat yield substantially as well as increasing the chance of catching fire (Granger & Schelling 2003). Pine trees, chaparral, and eucalypts are examples of plants that contain these substances Granger & Schelling 2003, Omi 2005). Certain plants, such as succulents, will hold more water than others. This will decrease the ability of fire to catch hold in this vegetation.

Temperature is another important aspect of fire hazard. High temperatures can cause the plants to release those volatile substances into the air as a gas that can very easily catch on fire (Omi, 2005). High temperatures may also mean that the plants have had less water and are therefore drier and will burn more fiercely. Precipitation will increase the available moisture in the plant and make it much harder for fire to take hold

The structure and size of the vegetation is important. A middle layer of shrubs, saplings or seedlings in a forest will act as ladder fuels, facilitating the spread of fire from the ground to the canopy and increasing its intensity as it finds new materials to burn. The thin branches and twigs of shrubs and young trees catch fire much more easily than larger more mature trees (Shoenberg et al 2003).

Density of vegetation also plays an important role in the spread of fire. If the vegetation is too dense the fire will not be able to get enough oxygen because it cannot move freely among the vegetation. If it is not dense enough flames in one bush will not be able to reach another and will be stopped when it burns out on that one bush (Omi, 2005).

The amount of fuel is important as if there is a lot the intensity of the fire will be much greater. The longer it has been since the last fire the more fuel that will build up. To determine this datasets with information about the time since a wildfire was last in the area and the time since a prescribed burn was last carried out in the area is needed (Granger & Schelling 2003).

Slope and Aspect play an important role in the intensity and speed of a fire. Fires burn much more quickly when moving up slope than down slope and the steeper then slope the faster it will move. In the Northern hemisphere slopes facing south receive more energy from the sun. This extra energy means fires burning on south facing slopes burn much more faster than those on north facing slopes (Granger & Schelling 2003, Omi 2005, ).