Agricultural Drought Indices - US Department of Agriculture

Figure 3. Daily gridded SPI by region with county overlay for the High Plains region of the United
States.
There are different ways to render this type of index. Commonly, a modeled approach is used. In
the case of the indicators explained below, we look at a percentile ranking approach as the
backbone of our composite efforts in the United States. The method is completely transferable
and can be easily modified to fit those indicators and indices that are readily available around the
world.
The U.S. and North American Drought Monitors
To build as comprehensive and flexible a drought early warning system (DEWS) as possible, it is
important to monitor drought across the many sectors mentioned earlier. A single index will rarely
work for all places at all times and for all types of droughts. Most coordinated monitoring efforts at
the national level are going to need to track all types of droughts. In cases such as these, it is
important to utilize and incorporate a consolidation of indices and indicators into one
comprehensive “composite indicator.” A composite (hybrid) indicator approach allows for the most
robust way of detecting and determining the magnitude (duration + intensity) of droughts as they
occur. Through a convergence-of-evidence approach, one can best determine (for a particular
state, province, country, or region for a particular time of the year) which indices and indicators do
the best job of depicting and tracking various types of droughts. The users can then determine
which indicators to use and how much weight to give each indicator/index in a “blended approach”
that incorporates a multiple parameter and weighting scheme. Such approaches have been used
in the U.S. Drought Monitor (USDM) and North American Drought Monitor (NADM) as described
below and as part of a series of Objective Drought Indicator Blend (OBDI) products, which are
produced weekly for integration into the USDM process. It is, in fact, the process of the USDM that
makes it work, the collaborative nature and integration of the latest indicators coupled with expert
field input from experts around the country.
The U.S. Drought Monitor (USDM): Created in 1999, the weekly U.S. Drought Monitor (USDM)
(Figure 4) was one of the first, if not the first, to use a composite indicator approach (Svoboda et al.
2002). The USDM is not a forecast, but rather an assessment, or snapshot, of current drought
conditions. The product is not an index in and of itself, but rather a combination of indicators and
116