National Drought Mitigation Center

Measuring Drought

There is no single definition for drought. When a drought begins and ends is difficult to determine. Rainfall data alone won't tell you if you are in a drought, how severe your drought may be, or how long you have been in drought.

We can, however, identify various indicators of drought, such as rainfall, snowpack, streamflow, and more, and track these indicators to monitor drought.

Researchers have developed a number of tools to help define the onset, severity, and end of droughts. Drought indices take thousands of bits of data on rainfall, snowpack, streamflow, etc., analyze the data over various time frames, and turn the data into a comprehensible big picture.

A drought index value is typically a single number, which is interpreted on a scale of abnormally wet, average, and abnormally dry.

Drought Measurements

The Standard Precipitation Index (SPI)

The Standard Precipitation Index (SPI) shows the actual precipitation compared to the probability of precipitation for various time frames.

The SPI is an index based on precipitation only. It can be used on a variety of time scales, which allows it to be useful for both short-term agricultural and long-term hydrological applications.

A drought event occurs any time the SPI is continuously negative and reaches an intensity of -1.0 or less. The event ends when the SPI becomes positive.

Each drought event, therefore, has a duration defined by its beginning and end, and an intensity for each month that the event continues. The positive sum of the SPI for all the months within a drought event can be termed the drought’s “magnitude”.

Sample SPI map
SPI Values
2.0+ extremely wet
  1.5 to 1.99 very wet
1.0 to 1.49 moderately wet

-.99 to .99

near normal
-1.0 to -1.49 moderately dry
-1.5 to -1.99 severely dry
-2 and less extremely dry

The Palmer Drought Severity Index (PDSI)

The Palmer Drought Severity Index (PDSI) has been used the longest for monitoring drought.

The PDSI allows for a categorization of various levels of wetness and dryness that are prominent over an area. The PDSI is calculated based on precipitation and temperature data, as well as the local Available Water Content (AWC) of the soil.Learn more

Palmer values may lag emerging droughts by several months; are less well suited for mountainous land or areas of frequent climatic extremes; and are complex—has an unspecified, built-in time scale that can be misleading.

Sample PDSI map
PDSI Classification
4.0 or more

extremely wet

3.0 to 3.99

very wet

2.0 to 2.99 moderately wet
1.0 to 1.99 slightly wet
0.5 to 0.99 incipient wet spell
0.49 to -0.49 near normal
-0.5 to -0.99 incipient dry spell
 -1.0 to -1.99 mild drought

-2.0 to -2.99

moderate drought

-3.0 to -3.99

severe drought
-4.0 or less  extreme drought

Crop Moisture Index (CMI)

A derivative of the PDSI is the Crop Moisture Index (CMI) which looks at moisture supply in the short term for crop producing regions. The CMI monitors week-to-week crop conditions. 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.

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.

Weekly maps of the CMI are available as part of the USDA/JAWF Weekly Weather and Crop Bulletin.

Sample CMI map