# McArthur

The McArthur Forest Fire Wildfire model rate of spread model was developed for predicting fire spread rates in Eucalyptus Forest.

Eucalypt Forest

### Fuel inputs

temp
rel_hum
D
• Temperature (degrees C)
• Relative humidity (%)
• Drought factor (range from 1 to 10)

### Code

```// Dry Eucalypt model - McArthur Mk V

// -------------------------------------------
// Model parameters
// These must be defined below, or included as a user-defined layer
//
// 1. Temperature, 'temp' (input)
// 2. Relative humidity, 'rel_hum' (input)
// 3. Drought factor, 'D'
// -------------------------------------------

// Backing and flanking coefficients compared to head fire ROS
REAL wind_speed = length(wind_vector);

REAL LBR = 1.0;
if (wind_speed < 5){
LBR = 1.0;
} else if (wind_speed < 25){
LBR = 0.9286 * exp(0.0505 * wind_speed);
} else {
LBR = 0.1143 * wind_speed + 0.4143;
}

// Determine coefficient for flank rank of spread, Rf = cf * Rh, using Andrew's LBR equations
REAL cc = sqrt(1.0-pow(LBR, -2.0));
REAL cb = (1.0-cc)/(1.0+cc);
REAL a_LBR = 0.5*(cb+1.0);
REAL cf = a_LBR/LBR;

// Determine shape parameters
REAL f = 0.5*(1.0+cb);
REAL g = 0.5*(1.0-cb);
REAL h = cf;

// Now calculate a speed coefficient using normal flow formula
REAL speed_fraction = (g*wdot+sqrt(h*h+(f*f-h*h)*wdot*wdot));

// Calculating FFDI using McArthur and Noble et al. (1980)
REAL FFDI = 2 * exp(-0.450 + 0.987*log(D) - 0.0345*rel_hum + 0.0338*temp + 0.0234*wind_speed);

// Calculate spread rate in km/h using McArthur's Mk 5 and Noble et al. (1980)