Sunday, November 23, 2014

U.S. Drought Monitor Classification Scheme

What You See

D0-D4: The Drought Monitor summary map identifies general drought areas, labelling droughts by intensity, with D1 being the least intense and D4 being the most intense. D0, drought watch areas, are either drying out and possibly heading for drought, or are recovering from drought but not yet back to normal, suffering long-term impacts such as low reservoir levels.

S and L: Since "drought" means a moisture deficit bad enough to have social, environmental or economic effects, we generally include a description of what the primary physical effects are: 

S = Short-Term, typically less than 6 months (e.g. agriculture, grasslands)
L = Long-Term, typically more than 6 months (e.g. hydrology, ecology) 

The Thinking Behind the Map

Drought intensity categories are based on five key indicators, numerous supplementary indicators including drought impacts, and local reports from more than 350 expert observers around the country. The accompanying drought severity classification table shows the ranges for each indicator for each dryness level. Because the ranges of the various indicators often don't coincide, the final drought category tends to be based on what the majority of the indicators show and on local observations. The analysts producing the map also weigh the indices according to how well they perform in various parts of the country and at different times of the year. Additional indicators are often needed in the West, where winter snowfall in the mountains has a strong bearing on water supplies. It is this combination of the best available data, local observations and experts’ best judgment that makes the U.S. Drought Monitor more versatile than other drought indicators.

Drought Severity Classification

 

Ranges

Category

Description

Possible Impacts

Palmer Drought Index

CPC Soil
Moisture Model
(Percentiles)

USGS Weekly Streamflow
(Percentiles)

Standardized Precipitation Index (SPI)

Objective Short and Long-term Drought Indicator Blends (Percentiles)

D0

Abnormally
Dry

Going into drought: short-term dryness slowing planting, growth of crops or pastures. Coming out of drought: some lingering water deficits;  pastures or crops not fully recovered

-1.0 to -1.9

21-30

21-30

-0.5 to -0.7

21-30

D1

Moderate Drought

Some damage to crops, pastures; streams, reservoirs, or wells low, some water shortages developing or imminent; voluntary water-use restrictions requested

-2.0 to -2.9

11-20

11-20

-0.8 to -1.2

11-20

D2

Severe
Drought

Crop or pasture losses likely;  water shortages common; water restrictions imposed

-3.0 to -3.9

6-10

6-10

-1.3 to -1.5

6-10

D3

Extreme Drought

Major crop/pasture losses;  widespread water shortages or restrictions

-4.0 to -4.9

3-5

3-5

-1.6 to -1.9

3-5

D4

Exceptional Drought

Exceptional and widespread crop/pasture losses; shortages of water in reservoirs, streams, and wells creating water emergencies

-5.0 or less

0-2

0-2

-2.0 or less

0-2

Short-term drought indicator blends focus on 1-3 month precipitation. Long-term blends focus on 6-60 months. Additional indices used, mainly during the growing season, include the USDA/NASS Topsoil Moisture, Keetch-Byram Drought Index (KBDI), and NOAA/NESDIS satellite Vegetation Health Indices.  Indices used primarily during the snow season and in the West include snow water content, river basin precipitation, and the Surface Water Supply Index (SWSI). Other indicators include groundwater levels, reservoir storage, and pasture/range conditions. 

The National Drought Mitigation Center | 3310 Holdrege Street | P.O. Box 830988 | Lincoln, NE 68583–0988
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