The percent of normal precipitation, as presented in the Oklahoma
Rainfall Update, is one of the simplest and most useful
indicators of drought. It may be calculated over a variety of
time periods -- for a single month, a growing season, or an
annual or water year. The homepage of the Oklahoma
Climatological Survey provides a wide array of this and
related information garnered primarily from the Oklahoma
Mesonet, a network of 115 automated stations located throughout
the state.
The Palmer
Drought Severity Index uses precipitation, air temperature,
soil moisture, evapotranspiration and previous indices to generate
a positive
or negative number, with a value of 0 being normal, -4 and
below an extremely dry condition, and 4 and above an extremely
wet condition. The PDSI is most effective in measuring impacts
sensitive to soil moisture conditions, such as agriculture.
However, the PDSI is slow to respond to rainfall events and
subsequent runoff and it may underestimate or overestimate the
severity of ongoing dry periods. The PDSI is updated weekly.
The Standardized
Precipitation Index, more sensitive than the PDSI, provides
a comparison of precipitation over a specified period with precipitation
totals from that same period for all years included in the historical
record. The 3-month SPI provides a seasonal estimation of precipitation
which, primarily in agricultural regions, might be more applicable
in highlighting available moisture conditions than the PDSI.
The 6-month SPI can be very effective in showing precipitation
over distinct seasons. The SPI is updated around the 15th of
each month.
The Keetch-Byram Drought Index, an excellent tool to monitor
drought-related fire conditions, is a soil/duff index that ranges
from 0 (no drought) to 800 (extreme drought) and is based on
a soil capacity of 8 inches of water. The KBDI is calculated
11 times each day as an integral product of the Oklahoma
Fire Danger Model.
The Crop
Moisture Index uses a meteorological approach to monitor
crop conditions from week to week. Unlike the Palmer Drought
Severity Index, which monitors long-term wet and dry spells,
the CMI is designed to evaluate short-term moisture conditions
in major crop-producing regions, such as Oklahoma. The CMI is
issued weekly. A representation of crop
moisture by climate division is available.
rules 
