Moisture / Drought Maps
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Forecast
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Keetch-Byram Drought Index
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US48 | AK
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US48
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Keetch and Byram (1968) designed a
drought index specifically for fire potential assessment. It is a
number representing the net effect of evapotranspiration and
precipitation in producing cumulative moisture deficiency in deep
duff and upper soil layers. It is a continuous index, relating to
the flammability of organic material in the ground.
The KBDI attempts to measure
the amount of precipitation necessary to return the soil to full
field capacity. It is a closed system ranging from 0 to 800 units
and represents a moisture regime from 0 to 8 inches of water
through the soil layer. At 8 inches of water, the KBDI assumes
saturation. Zero is the point of no moisture deficiency and 800 is
the maximum drought that is possible. At any point along the scale,
the index number indicates the amount of net rainfall that is
required to reduce the index to zero, or saturation.
The inputs for KBDI are weather station latitude, mean annual
precipitation, maximum dry bulb temperature, and the last 24 hours
of rainfall. Reduction in drought occurs only when rainfall exceeds
0.20 inch (called net rainfall). The computational steps involve
reducing the drought index by the net rain amount and increasing
the drought index by a drought factor.
- KBDI = 0 - 200: Soil moisture and large class fuel moistures
are high and do not contribute much to fire intensity. Typical of
spring dormant season following winter precipitation.
- KBDI = 200 - 400: Typical of late spring, early growing season.
Lower litter and duff layers are drying and beginning to contribute
to fire intensity.
- KBDI = 400 - 600: Typical of late summer, early fall. Lower
litter and duff layers actively contribute to fire intensity and
will burn actively.
- KBDI = 600 - 800: Often associated with more severe drought
with increased wildfire occurrence. Intense, deep burning fires
with significant downwind spotting can be expected. Live fuels can
also be expected to burn actively at these levels.