# Fallout models

VOLCANBOX includes two fallout models, HAZMAP and a simplified version of Fall3D.

HAZMAP (Macedonio et al 2005) is a code, originally written in FORTRAN, for the solution of the equations of diffusion, transport and sedimentation of small particles, in order to model the dispersion of ash generated by a convective column. The model was developed simplifying the advection–diffusion–sedimentation equation for volcanic ash transport in the atmosphere from three to two dimensions and using a semi-analytical computational method to greatly reduce the required computer time and memory. HAZMAP can be used either to simulate the mass distribution of the deposit if a wind profile is given (‘’deposit mode’’), or to define the probability of a given accumulation of mass on the ground if a statistical set of wind profiles is provided (‘‘probability mode’’). The model needs the definition of the physical system (spatial distribution of the point sources, total erupted mass, settling velocity distribution, wind velocity profile, atmospheric turbulent diffusion coefficients), and of a computational grid. The output values of mass accumulation on the ground or the probability of mass accumulation are computed on a regular grid, suitable for further analysis.

FALL3D (Folch et al., 2009) FALL3D is a 3-D time-dependent Eulerian model for the transport and deposition of volcanic ashes and lapilli. The model solves the advection–diffusion–sedimentation (ADS) equation on a structured terrain-following grid using a second-order finite differences(FD) explicit scheme. Different parameterizations for the eddy diffusivity tensor and for the particle terminal settling velocities can be used. Although the model has been designed to forecast volcanic ash concentration in the atmosphere and ash loading at ground, it can also be used to model the transport of any kind of airborne solid particles. The model inputs are meteorological data, topography, grain-size distribution, shape and density of particles, and  mass rate of particle injected into the atmosphere.Optionally, FALL3D can be coupled with the output of the meteorological processor CALMET, a diagnostic model which generates 3-D time-dependent zero-divergence wind fields from mesoscale forecasts incorporating local terrain effects. The FALL3D model can be a tool for short- term ash deposition forecasting and for volcanic fallout hazard assessment.