As stated by Dr. Michael J. Hayes:
"The Crop Moisture Index (CMI) uses a meteorological approach to monitor week-to-week crop conditions. It was
developed by Palmer (1968) from procedures within the calculation of the PDSI. Whereas the PDSI monitors
long-term meteorological wet and dry spells, the CMI was designed to evaluate short-term moisture conditions
across major crop-producing regions. It is based on the mean temperature and total precipitation for each week
within a climate division, as well as the CMI value from the previous week. The CMI responds rapidly to
changing conditions, and it is weighted by location and time so that maps, which commonly display the weekly
CMI across the United States, can be used to compare moisture conditions at different locations. Weekly maps
of the CMI are available as part of the USDA/JAWF Weekly Weather and Crop Bulletin".
"Because it is designed to monitor short-term moisture conditions affecting a
developing crop, the CMI is not a good long-term drought monitoring tool. The
CMI’s rapid response to changing short-term conditions may provide misleading
information about long-term conditions. For example, a beneficial rainfall
during a drought may allow the CMI value to indicate adequate moisture
conditions, while the long-term drought at that location persists. Another
characteristic of the CMI that limits its use as a long-term drought monitoring
tool is that the CMI typically begins and ends each growing season near zero.
This limitation prevents the CMI from being used to monitor moisture conditions
outside the general growing season, especially in droughts that extend over
several years. The CMI also may not be applicable during seed germination at the
beginning of a specific crop’s growing season."
The Crop Moisture Index (CMI) is classified by the following scale:
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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.