A Closer Look at a Key Wildfire Indicator

As wildfires grow in frequency and intensity across the western U.S., researchers are delving deeper into the climate factors that fuel the flames. A recent study funded in part by the Southwest Climate Adaptation Science Center sheds new light on the methods behind calculating a critical indicator of wildfire potential. 

Vapor pressure deficit (VPD) is a metric for measuring atmospheric aridity that takes into account temperature and humidity. For our forests, this means that at higher temperatures (where there’s a higher VPD), trees lose more moisture to the air quicker as water diffuses from a highly concentrated environment within plant cells to the thirsty atmosphere. This creates ample fuel for the elevated spread and intensity of wildfires. Understanding VPD, therefore, is key to understanding climate-wildfire dynamics. 

However, since temperature and humidity can’t be assessed on a second-by-second basis, the standard calculations of VPD are often imperfect estimates. The researchers wanted to understand how these imperfections might influence our understanding of the relationship between VPD and wildfire. Using different temporal resolutions for VPD calculations (such as humidity and temperature data), they found that VPD is more accurate when calculated from daily data versus monthly data. However, this difference has only a minimal effect when studying the linkages between annually averaged VPD and yearly or seasonal wildfire patterns in the western U.S., suggesting that monthly data work well enough for this purpose. 

By refining how we understand and calculate wildfire indicators like VPD, researchers are helping improve our understanding of the atmosphere’s role in shaping fire-prone landscapes. Overall, this research suggests that choices between different calculation methods can influence the accuracy of the data. While historical relationships between VPD and fire do not seem to be sensitive to these differences, future changes in temperature and humidity could mean these different methods will be important for future projections of wildfire activity and should be researched further. 

Read the full publication online at AGU Journals.