Use of Radar Data in Determining the
KBDI
The
moisture content of the upper soil and the duff layers
is an important variable in evaluating the potential
danger of wildfires. The Keetch-Byram drought index (KBDI)
is used to track the moisture content of the upper soil
and duff layers. To calculate the KBDI, 24 hour rainfall
totals and the maximum temperature need to be measured
on a daily basis. With this information it is a simple
matter to calculate the KBDI for a location; however,
problems arise when one wishes to determine the drought
index for a region such as the state of Florida.
Temperature
is routinely measured at a number of sights across
Florida and since temperature is continuous,
interpolation to regions that lack measurements is
straightforward. Rainfall on the other hand is not
continuous, limiting the usefulness of interpolation.
With Florida's rainfall being largely tied to local
thunderstorms, precipitation can vary dramatically over
short distances. Due to the hit or miss nature of these
storms a particular rain gauge may receive substantially
more precipitation than surrounding areas and may cause
the KBDI to underestimate the severity of the drought in
that region.
The Florida Division of Forestry
combines traditional rainfall observations with data
derived from the National Weather Service's WSR88D (NEXRAD)
radar network to provide a detailed view of rainfall
across the state for use in the KBDI. The radar data is
provided to the Division by Meteorologix
in the form of a Composite Rainfall Estimate that
combines precipitation information from all of the
NEXRAD radars. This dataset provides rainfall
information every 4 kilometers across the entire
country. The Division of Forestry then uses the more
traditional rain gauge measurements to adjust the radar
data, making sure that what the radar "sees"
agrees with what is happening on the ground.
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