Modernizing the US National Fire Danger Rating System (version 4): Simplified fuel models and improved live and dead fuel moisture calculations

The US National Fire Danger Rating System (USNFDRS) supports wildfire management decisions nationwide, but it has not been updated since 1988. Here we implement new fuel moisture models, and we simplify the fuel models while maintaining the overall USNFDRS structure. Modeled and measured live fuel moisture content values were highly correlated ( r 2 = 0 . 629 with defaults and r 2 = 0 . 693 when species and location optimized). We also consolidated fuel models to five fuel types that eliminated significant index cross-correlation. Index seasonality compared between old (V2) and new USNFDRS models (v4) across six US National Forests was very similar ( ρ = 0.97). V4 was as good or better than V2 at predicting fire days in 92% of the cases tested and V4 effectively predicted wildfire days and large fire ignition days (AUCs 0.647 to 0.915). USNFDRS V4 can adequately depict spatial and temporal wildland fire potential and it can be adapted for worldwide use. • The US National Fire Danger Rating System (USNFDRS) has not been updated in 37 years. • We update USNFDRS with new live and dead fuel moisture models and simpler fuel types. • The new USNFDRS is as good or better at predicting fire activity than the old system. • This new model primes future innovation, and it can be adapted for worldwide use.