Following the disastrous 1980 fire season, Senator Ayala introduced
legislation which required the Director of CDF to zone all
State Responsibility Area (SRA) lands in accordance with the degree
of severity of fire hazard. The purpose of the zoning was for identifying
measures to be taken to retard the rate of spreading and to reduce
the potential intensity of uncontrolled fires that threaten to destroy
resources, life, or property. Each zone was "to embrace relatively
homogeneous lands and shall be based on
fuel loading, slope, fire weather, and other relevant factors present"
(PRC 4201-4204). The process used by CDF in developing and rating
the zones is described in a publication titled "Instructions
for Zoning Fire Hazard Severity in State Responsibility Area in
California",
by Clinton B. Phillips, dated December 1983. This process was conducted
between 1981 and 1985, and the final maps were adopted into regulation
as Section 1280 of CCR Title 14 on March 12, 1985. The end result
was that each zone received one of three ratings: Moderate, High,
or Very High. Copies of the maps were distributed to county planning
departments in early 1988, with written receipt of acknowledgment.
Mapping Procedures
The following is a summarization of the main steps used for mapping Fire Hazard Severity Zones (FHSZs).
1) Selection of long-term planning period: Ranger units were directed
to confer with local government planners and building inspectors
to
determine the time span over which the regulations should remain
stable.
2) Zone delineation: Zones were to be based on areas of similar
vegetative type, slope, and weather. Zone boundaries were either
identifiable on the ground or well-established, legally surveyed
lines. Some ranger units defined zones on non-SRA as well as SRA
lands, while others zoned SRA lands only. Zone delineation typically
involved drawing in boundaries on USGS 7-1/2 minute quadrangle maps.
3) Fuel model selection: A representative National Fire Danger
Rating System fuel model was assigned to each zone. The long-term
planning period is a critical factor, since fuel models can be assigned
based on current conditions as determined by recent disturbances
(fires, timber harvest) or based on the longer-term average fuel
conditions.
4) Slope class selection: A representative National Fire Danger Rating System slope class was assigned to each zone.
5) Weather station selection: The CDF weather monitoring station
most representative of each zone was selected.
6) Zone rating assignment: The FIRDAT computer program was used
to estimate the number of days in an average year that each zone
will experience a Burn Index of 61 or higher, based on the assigned
fuel model, slope class, and weather data. Burn Index
is a measure of both the rate of spread and energy release based
on fuels, slope, and weather. The FIRDAT results are
used to assign each zone a rating of Moderate, High, or Very High.
The first five steps were performed by staff at ranger units. The last step was performed at Sacramento Headquarters.
Data Capture Procedures
The quads sheets were digitized at Deuel Vocational Institution
(DVI) by inmate labor using Atlas GIS software. There was no effort
made to match FHSZ boundaries with digital data sources such as
county boundaries, streams, roads, or public land survey section
lines.
The original quad sheets (over 2200) are currently stored at DVI.
CDF-FRAP has worked to merge ranger unit data into a statewide
data set in a format accessible to the GIS community. Since zoning
for non-SRA lands was incomplete, it has been removed from the data
set.
Data Limitations
FHSZ mapping was basically a CDF field exercise to carry out the
above set of procedures. Examination of the data suggests that little
effort was made to standardize zoning procedures between ranger
units. There are cases where ranger units that might be expected
to have similar hazard characteristics appear to be much different
in terms of FHSZs. For example, there are ranger units zoned exclusively
as Very High while an adjacent unit has a variety of zones of different
ratings. There are also cases where ratings change abruptly at ranger
unit boundaries. These inconsistencies are a direct result of a
lack of procedural standardization, and could be due to different
long-term
planning periods, different interpretation of fuel models, or the
level of detail used to map the various zones.
Data Usage
The gross inconsistencies in the data are obvious from even a cursory
examination of the statewide FHSZ map. The fact that there was no
effort to standardize the mapping effort or review the data for
accuracy suggest that the data be used with extreme caution.
Finally, the zones are designed to give an average hazard rating
for the area. This does not necessarily define the exact conditions
for
all areas within the zone. Variations in fuels, slope, weather,
as well as factors not considered in this exercise such as aspect,
elevation, and air stability will influence hazard conditions at
actual locations within each zone. For an individual structure,
the
risk of damage from fire also depends on site-specific factors such
as access, water supply, clearance, and characteristics of the
structure. Since statewide hazard zoning cannot capture these factors,
it should not be used as a measure of the risk faced
by individual structures. "