So for me the question is, what is the FDI for?
Short answer is to have some sort of quantifying system so as to be able to describe
an expected level of fire behaviour simply, without having to detail the science and
data used to calculate it. GFDI & FFDI numbers mean something to people in the
business of managing & fighting fires.
As Mr.Teddy also quite rightly points out, as fuel loads vary wildly across a district,
the actual FDI in specific locations also varies wildly. An over-grazed paddock will
have a far lower FDI than the standing wheat crop or the scrub either side of it. FDI
numbers are meaningful at specific locations, not so meaningful across whole districts
unless they are all very big or very small numbers.
So different organisations use FDIs to nominate risk levels & trigger points relevant
to them.
For example, ETSA set 3 levels of risk regarding their infrastructure - see
http://sacfs.org/publications/Operations_Management_Guidelines_Third_Edition_Nov2004.pdf page 77
Most fire services use 5 descriptive levels of fire behaviour & risk for general public
consumption. Loosely describe the likelihood of an ignition escaping out of control.
Low (0-20), moderate(20-30), high(30-40), v.high(40-50) & extreme(>50). The system is
stating that fire fighters are very unlikely to be able to control any fire when the
FDI is greater than 50. Sometimes we get lucky. Sometimes we don't.
As with any gross simplification, it has short-comings. The cut-offs between each level
are arbitrary. As Mr Teddy quite rightly points out, there is sod-all difference in
fire behaviour between FDI of 49 & 50, but one is oficially "Very High", the other
is "Extreme".
But we need to have a quantifiable trigger-point, above which certain behaviours are
demanded of the public, and penalties can be enforced for non-compliance.