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## Fire Counts Estimation

The fire counts (Nf) in a grid cell are calculated using the following equation:

Nf = Ni * fb * fm * fse,o

where:
- Ni is the number of ignition sources from natural and human activities
- fb is the fuel availability factor
- fm is the fuel combustibility factor
- fse,o is the fraction of fires unsuppressed by human activities

The ignition sources (Ni) are calculated as:
Ni = (In + Ia) * Ag
where:
- In is the natural ignitions per unit area
- Ia is the anthropogenic ignitions per unit area
- Ag is the grid cell area

The natural ignitions (In) are estimated using lightning flash rate and latitude-dependent cloud-to-ground lightning fraction.
The anthropogenic ignitions (Ia) are modeled as a function of population density.

The fuel availability factor (fb) depends on the total biomass relative to lower and upper thresholds.
The fuel combustibility factor (fm) depends on relative humidity and root-zone soil moisture.

For regions with population density > 0.1 person/km2, the fraction of unsuppressed fires (fse,o) is calculated as the product of demographic (fd) and economic (fe) influence factors.
The demographic influence follows an exponential decay with population density.
The economic influence is parameterized differently for regions dominated by shrub/grass versus tree PFTs, based on the relationship with GDP.