Influence of Time-Averaging of Climate Data on Estimates of Atmospheric Vapor Pressure Deficit and Inferred Relationships With Wildfire Area in the Western United States

Abstract

Vapor pressure deficit (VPD) is a driver of evaporative demand and correlates strongly with wildfire extent in the western United States (WUS). Vapor pressure deficit is the difference between saturation vapor pressure (e_s) and actual vapor pressure (e_a). Because e_s increases nonlinearly with temperature, calculations of time-averaged VPD vary depending on the frequency of temperature measurements and how e_a is calculated, potentially limiting our understanding of fire-climate relationships. We calculate eight versions of monthly VPD across the WUS and assess their differences. Monthly VPDs calculated from daily data are 2%–6% higher, and more accurate, than when calculated from monthly data. Using daily maximum and minimum temperature, instead of mean, increases VPD by ∼20%, but can overestimate true values depending on how e_a is calculated. These differences do not meaningfully impact correlations with annual wildfire area, however, suggesting our understanding of historical fire-VPD relations is not very sensitive to how VPD is calculated.