| Aerial Imagery and GIS Data Combine for Effective Prescribed Burning Programs
By Tim Walsh GIS for Pre-Suppression Planning
The importance and use of geospatial data for fire control and prevention has increased dramatically in recent years. The summer of 2000 brought about one of the worst years on record for fires throughout the West. Almost seven million acres were burned, with losses in the hundreds of millions of dollars.
Using GIS, GPS and aerial imagery data, firefighters were provided powerful tools to aid in firefighting efforts out in the field. Geospatial technologies were valuable tools used for fire-perimeter mapping during the blaze and for damage assessment after the fire was extinguished.
An additional area that has seen increased benefit from geospatial tools is in the Pre-Suppression Planning phase. At this phase, GIS analysis is used to focus on high risk/high hazard areas. Once areas are identified, projects are implemented to reduce the chance of a large, damaging fire. Projects include prescribed burning over large areas, cutting vegetation around structures and pile burning, fuel break construction, and public education. Wildland Fire Hazard/Risk Assessment
Four factors are ranked to determine the overall fire severity of a geographical area: presence of hazardous fuel (or vegetation), assessment of property at risk, severity of fire weather, and service levels. All are analyzed to determine the fire problem.
Hazardous fuels are mapped based on vegetation types. Vegetation types are determined by using several different GIS layers of information. In Marin County, several layers of vegetation data were collected from various agencies. However, there are inherent limitations in the collection of this data. For instance, data from the Department of Fish and Game had excellent information for riparian areas, but not for brush-covered slopes. Each layer was queried and compared to 30-meter-resolution Infrared and Spot imagery. A draft vegetation map was created with a need for serious field validation.
Fortunately, the County purchased high-resolution aerial image sets in pixel resolutions of one-foot for rural areas and six-inch for urban and residential areas. Unfortunately, these sets are not field-friendly for firefighters on location at the burn site-at this level of resolution, each image was almost 100MB in file size. To cover the entire county, 65 CDs containing four to six images each were needed. Working off a CD is clumsy and slow, not to mention the additional weight load and packing space needed for storing the CDs. To solve this problem, MrSID Geospatial Edition from LizardTech, located in Seattle, Wash., was used to reduce each image at a 20:1 reduction ratio. The MrSID images remain visually indistinguishable from the original TIFF files, a requirement necessary for zooming in on vegetation details while in the field. With this reduction ratio, the entire 65-CD collection now fits onto three CDs, making field validations on laptops fast and easy.
Another important factor affecting burn rate is the steepness of the slope. The steeper the hillside, the faster the fire spreads. With the vegetation converted to fuel models, the next step is to create a slope map. Ten-meter digital elevation models (DEM) were downloaded from the U.S. Geological Survey Bay Area Regional Database (BARD). Using ESRI's Spatial Analyst product, the DEMs were converted into slope models with six values used in ranking. A slope between zero and 10 percent received a rank of zero. A slope between 11 and 20 received a rank of one, and so on.
Using high-resolution images works well for asset validation. Assets at risk are defined as anything someone wants to protect from fire. Obviously homes come to mind, but there are other properties and resources that can suffer from a wildland fire. These include power lines and other public utilities, watersheds and water quality, soil erosion, damage to recreational areas and campsites, and much more. Assets are mapped and then compared to surrounding fuel data.
Most large, damaging fires occur under serious north-northeast wind events. Modeling this phenomenon would be very difficult due to the dynamic process of wind. To focus on weather potential, a more static assessment was used. The four components that make up the weather assessment include fire weather severity index, areas above the inversion layer, areas under tree canopies, and aspect analysis.
Fire weather severity indexes the number of moderate, high, and very high fire-danger days cataloged from the department's five fire-weather stations. Marin County's proximity to the ocean produces a marine influence on a high number of days. The entire county is often draped in fog, and it would appear that nothing would burn. However, above the inversion layer are areas that are still very dry.
The tree canopy improves the severity of several weather influences. It shades the ground producing higher fuel moisture, moderates wind speed, and limits the growth of flammable vegetation. Lastly, the direction a slope faces determines the amount of solar heating throughout the day. South and southwest slopes are much drier and burn more readily. All four of the components were mapped, and a rank was established based on overall fire-weather severity.
Level of service is an assessment aimed at finding areas at the highest risk from a large, damaging fire. The department chose four service factors that contribute to large, damaging fires. These include response time, past ignitions, history of fire resistance, and structure density. Response times were established based on distance from fire stations. Each fire station was mapped, and response zones were modeled based on five-, eight-, and 10-minute responses. Mapping every wildland fire dispatch that occurred over an eight-year period identified areas with a history of past ignitions. Areas with a resistance to fire control are defined as an area with slopes steeper than what department vehicles can climb, fuels that burn very hot or very fast, and areas with no service roads. Potential structure-loss areas were mapped based on structure density from the assessor's parcel map. When the four-part assessment was completed, it was used to plan and prioritize pre-suppression projects that will reduce the hazard to a community or asset at risk.
One of the ways to decrease the threat is by reducing the fuel through a program of prescribed suppression burning. Suppression burning is an intentional fire ignited by authorized personnel to meet specific objectives. To perform suppression burning, a written, prescribed fire plan must exist, and specific requirements must be met prior to ignition. Given the strict parameters, suppression burning is a very important part of proper forest management and planned accordingly, can effectively reduce the threat of fire and the potential loss of assets. GIS Provides The Tools For Prevention and Management
Today's advanced geospatial technologies have contributed to the accuracy and effectiveness of advanced pre-suppression burn programs. GIS provides the capabilities to properly analyze the factors determining the fire severity of a geographical area. Integrated into ESRI's ArcView GIS, vector-theme data combined with high-resolution MrSID imagery provides firefighting professionals with the ability to assess the probability of fires beforehand, and in the field. Back |
