¹Climate Research Division, Scripps Institution of Oceanography, University of California, La Jolla, California
²Desert Research Institute, University of Nevada, Reno, Nevada
³US Geological Survey, La Jolla, California

Please click here for full article (pdf file); summary below

• Abstract
• Introduction
• Data
• Seasonality of Wildfire

This information is from an article to be published in the
Bulletin of the American Meteorological Society (accepted Nov 2002)

ABSTRACT

A twenty-one year gridded monthly fire starts and acres burned dataset from US Forest Service, Bureau of Land Management, National Park Service and Bureau of Indian Affairs fire reports recreates the seasonality and interannual variability of wildfire in the western US. Despite pervasive human influence in western fire regimes, it is striking how strongly these data reveal a fire season responding to variations in climate. Correlating anomalous wildfire frequency and extent with the Palmer Drought Severity Index illustrates the importance of prior and accumulated precipitation anomalies for future wildfire season severity. This link to antecedent seasons' moisture conditions varies widely with differences in predominant fuel type. Furthermore, these data demonstrate that the relationship between wildfire season severity and observed moisture anomalies from antecedent seasons is strong enough to forecast fire season severity at lead times of one season to a year in advance.

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1. Introduction

The number and extent of wildfires in the western United States each season are driven by natural factors such as fuel availability, temperature, precipitation, wind, humidity, and the location of lightning strikes, as well as anthropogenic factors. It is well known that climate fluctuations significantly affect these natural factors, and thus the severity of the western wildfire season, at a variety of temporal and spatial scales.

Until now, the lack of a comprehensive fire climatology has hampered detailed studies of climate-fire relationships. Our study is the first to consider the larger picture of wildfire for the entire western U.S., including fire histories from several federal agencies whose territories span a diverse array of ecosystems. The quality of the location data for some of these fire histories constrains a comprehensive, regional-scale analysis to a one-degree grid resolution. This level of aggregation obscures some of the rich detail of wildfire variability related to the enormous variability in vegetation within the western U.S. However, at the same time it provides a common fire history with comprehensive coverage over a large region to study patterns of spatial variability in wildfire. As will be shown, this scale is commensurate with that of some important climate variations and still allows us to make comparisons across diverse ecosystems.

Currently, twenty to thirty years of digitized data on wildfire activity are available from the U.S. Deprtment of Agriculture's Forest Service (USFS) and from the U.S. Department of the Interior's Bureau of Land Management (BLM), National Park Service (NPS), and Bureau of Indian Affairs (BIA). Longer historical records for specific sites, especially some USFS and NPS managed areas, are available but limited in spatial representation. Results shown here indicate that, despite their foibles, an amalgamations of these data sets yields a spatial and temporal history that is of sufficient quality, spatial resolution and duration to resolve regional characteristics of the wildfire season, and that these characteristics are related to modes of interannual climate variability. We report here results of our analysis of wildfire frequency and acres burned for the contiguous western U.S. based on these data. The next sections review the data, describe the seasonality of wildfire, and describe relationships between the Palmer Drought Severity Index (PDSI) and anomalous fire season severity. Finally, these relationships are used to predict fire season severity at lead times of one season to one year in advance of the fire season in regions with diverse fuels and climes.

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2. Data

Over 410,000 fire reports from the USFS, BLM, NPS and BIA were compiled and combined in a data set of monthly fire start counts and acres burned on a 1 x 1 degree grid. A 1 x 1 degree grid was the finest resolution practical given the quality of the location data in many fire reports. Digitized fire reports are available from USFS beginning in 1970, from NPS before 1970 in some places, from BIA starting in 1972, and from BLM in 1964, but because of data-quality concerns pre-1980 BLM fire reports are not included in this analysis. The longest common data set encompasses approximately 300,000 fires for the region from 31^o N to 49^oN latitude and 101^oW to 125^o W longitude for the period 1980-2000. In many locations, however, the coverage could be extended 8 to 10 years using an additional 110,000 USFS, NPS and BIA fire reports.

While these data do not record every wildland fire ignition for the past 21 years, they provide a comprehensive representation of wildfire activity in the region. The four source agencies own roughly half the land in the western U.S., and the area under protection accounts for an even larger share of western wildlands. Fires recorded by these four agencies on federal, state, and private lands regularly account for over 90 percent of the wildland fire area burned reported by state and federal sources for the western U.S.

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3. Seasonality of Wildfire

Even though roughly half the fires included in this analysis are human-caused rather than 'natural' in origin, Figures 1-3 clearly show western wildfire to be a process largely governed by climate.

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