Table 1 A description of the FlowDIR inputs, along with a guide for their selection, suggested ranges and the default values. Default values are provided as a general guidance, however we suggest that the user becomes fully acquainted with the tools inputs before simulating

DEM file^b

Path to a.tif file projected to UTM coordinates.

Starting coordinate^b

A single point in UTM coordinates (X,Y).

This should be informed by past events. For volcanoes with a summit crater, a reasonable assumption is that magma pathways will be the same as in the past, and a central location within the summit crater may be chosen, while for a volcano without a summit crater the highest point is reasonable. If unrest is observed and monitoring data suggest differently (i.e., shallow seismicity away from a past crater) then this location should be chosen.

Swath length (m)^a

The initial swath length prior to clipping.

500 – 1000 m (default 800 m)

This needs to be long enough to extend from the starting coordinate to outside of the summit region in all directions. The exact length is not important since this is used to generate the first swaths which are later clipped to define the summit region. If FlowDIR is being run for a single volcano, this can be measured using Google Earth, whereas if FlowDIR is being run for multiple volcanoes with different summit morphologies a larger value should be used to ensure that swaths reach outside of the summit region for all volcanoes.

Buffer (m)^b

Swaths are clipped to the maximum elevation plus the buffer. This is used to extract the summit region.

50 – 150 m (depends on the topography, no default)

For breached craters, or convex summit regions, a buffer towards the higher end of the range may be required to manually enlarge the area included in the calculation. For a clearly defined crater rim with no irregularities or minor breaches a smaller value should be used.

Elevation threshold (m)^a

The elevation change values calculated as part of the AED procedure for each azimuth bin are compared to this value. If the value is below this threshold the crater wall section is likely to be overtopped and if the value is above this threshold the crater wall section is unlikely to be overtopped.

20 – 50 m (no default)

Should be based on knowledge of the current crater morphology along with past erupted volumes, flow rheologies, and the heights of past lavas. If this information is not available, an intermediate value that lies between the maximum and minimum elevation difference between the starting coordinate elevation and the crater rim is suitable.

Capture uncertainty in start? (0/1)^b

Input 1 to run FlowDIR with uncertainty in the starting coordinate. When uncertainty = 0, the initialisation polygon edges are 1 DEM cell width from the starting coordinate, resulting in nine initialisation points.

Default 1

Uncertainty can be epistemic or aleatory. There is also some uncertainty in representing a flow’s initialisation location as a single point when a flow usually initiates over an area (especially for high resolution DEMs). Use of start uncertainty is encouraged.

Start uncertainty (m)^b

When uncertainty = 1, the user can set the size of the polygon width in meters to increase the number of initialisation points (see Fig. 1a).

No default

For a concave summit, the initialisation polygon should fill the crater while ensuring that all points are within the crater walls, whereas for a convex summit, the initialisation polygon should cover the summit region. Note that this should be set in the context of the DEM’s resolution, setting a start uncertainty of 10 m for a DEM with resolution 30 m will have no effect.