How Many? A Dictionary of Units of Measurement
© Russ Rowlett and the University of North Carolina at Chapel Hill

Table of Contents
About the Dictionary
Using the Dictionary

Drought Severity Classification

Source: U.S. National Drought Mitigation Center.

Drought Severity Classification

Category Description
Possible Impacts
Palmer Drought Index
CPC Soil Moisture Model (Percentiles)
USGS Weekly Streamflow (Percentiles)
Percent of Normal Precip
Standardized Precipitation Index (SPI)
Satellite Vegetation Health Index
D0 Abnormally Dry
Going into drought: short-term dryness slowing planting, growth of crops or pastures; fire risk above average. Coming out of drought: some lingering water deficits; pastures or crops not fully recovered.
-1.0 to
21-30 21-30
<75% for 3 months
-0.5 to
D1 Moderate Drought
Some damage to crops, pastures; fire risk high; streams, reservoirs, or wells low, some water shortages developing or imminent, voluntary water use restrictions requested
-2.0 to
11-20 11-20
<70% for 3 months
-0.8 to
D2 Severe Drought
Crop or pasture losses likely; fire risk very high; water shortages common; water restrictions imposed
-3.0 to
6-10 6-10
<65% for 6 months
-1.3 to
D3 Extreme Drought
Major crop/pasture losses; extreme fire danger; widespread water shortages or restrictions
-4.0 to
3-5 3-5
<60% for 6 months
-1.6 to
D4 Exceptional Drought
Exceptional and widespread crop/pasture losses; exceptional fire risk; shortages of water in reservoirs, streams, and wells, creating water emergencies
-5.0 or less 0-2 0-2
<65% for 12 months
-2.0 or less 1-5
Additional indices used, mainly during the growing season, include the USDA/NASS Topsoil Moisture, Crop Moisture Index (CMI), and Keetch Byram Drought Index (KBDI). Indices used primarily during the snow season and in the West include the River Basin Snow Water Content, River Basin Average Precipitation, and the Surface Water Supply Index (SWSI).

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January 29, 2002