The Palmer Index was developed by Wayne Palmer in the 1960s and uses temperature and rainfall information in a formula to determine dryness. The Palmer Index is most effective in determining long term drought—a matter of several months—and is not as good with short-term forecasts (a matter of weeks). It uses a 0 as normal, and drought is shown in terms of minus numbers; for example, minus 2 is moderate drought, minus 3 is severe drought, and minus 4 is extreme drought.
The Palmer Index can also reflect excess rain using a corresponding level reflected by plus figures; i.e., 0 is normal, plus 2 is moderate rainfall, etc. The advantage of the Palmer Index is that it is standardized to local climate, so it can be applied to any part of the country to demonstrate relative drought or rainfall conditions. The negative is that it is not as good for short term forecasts, and is not particularly useful in calculating supplies of water locked up in snow, so it works best east of the Continental Divide.
The Keetch-Byram Drought Index (KBDI), created by John Keetch and George Byram in 1968 for the United States Department of Agriculture's Forest Service, was devised based on mathematical models for predicting the likelihood of wildfire based on soil moisture and other conditions related to drought.
KBDI is a soil/duff drought index that ranges from 0 (no drought) to 800 (extreme drought) and is based on the soil capacity in 8 inches (200 mm) of water. The depth of soil required to hold 8 inches of moisture varies. A prolonged drought (high KBDI) influences fire intensity largely because fuels have a lower moisture content.
The KBDI is a measure of meteorological drought; it reflects water gain or loss within the soil. It does not measure fuel moisture levels in the 1 to 10 hour fuel classes, which must be measured by other means for an accurate assessment of fuel moisture, regardless of the drought index readings.