NASA scientists have found that a wet winter corresponds to more small wildfires in the following fire season, not fewer, as is commonly assumed.
Large fires behave more "logically," with fewer large fires after a wet winter and more after a dry one.
"This is the most surprising result from our study, because we would expect small fires to follow suit with larger fires," said Daniel Jensen of University of California, Los Angeles (UCLA) who worked on the project with J.T. Reager of NASA's Jet Propulsion Laboratory in Pasadena, California.
The researchers explained that a wet winter causes grasses and other small plants to grow profusely.
These plants dry out and die at the end of the growing season, leaving abundant fuel for a wildfire.
Trees and larger shrubs, however, retain more moisture after a wet winter. That might hamper the ability of small fires to grow into large ones in landscapes containing trees, said the study published online in the journal Environmental Research Letters.
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When there is ample moisture for plant growth, "it seems that the buildup of fuel content alone causes there to be more fires -- but not necessarily more devastating fires", Jensen pointed out.
The research is a step toward understanding the role of fuel moisture in wildfires, which could help in determining how severe a fire season may be several months before it arrives.
Knowing the moisture of a fuel supply can improve predictions of how fast a wildfire may spread, but measuring it from samples collected in the field is time-consuming and labor-intensive.
Remote sensing offers a possible alternative, and earlier studies have shown that soil moisture (the water contained in the soil) correlates well with fuel moisture.
The researchers correlated records of wildfire occurrences across the US from 2003 through 2012 with soil moisture measurements from the U.S./German Gravity Recovery and Climate Experiment (GRACE) satellite mission and US Geological Survey data on vegetation and landscape types.
They found that although each landscape type varied in average soil moisture and average number of fires, in every landscape type, the number of small fires increased after a wet pre-season.
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