导航|地址|刷新
How are the Drought Categories Assigned?…

National Drought Mitigation Center

Home

Education
Drought Basics

Drought In-depth
What is Drought?

Types of Drought

What is Climatology?

Drought for Kids
What is Drought?

How do People Study Drought?

How Does Drought Affect Our Lives?

Can We Protect Ourselves From Drought?

Glossary

Dust Bowl

Tutorials
U.S. Drought Monitor Tutorial
Who Makes the Map?

What Does the Map Show?

What Data are Used to Make the Map?

How are the Drought Categories Assigned?

What is the Timeline for Production?

Where can I Find the U.S. Drought Monitor?

How Can I Contribute?

Who uses the U.S. Drought Monitor?

Grass-Cast Tutorial

Drought Games

Planning
Plan for Drought

Planning Processes
Drought-ready Communities

Planning and Drought

Drought Plans
International Plans

State Drought Contacts

StatePlanning

State Plans

Tribal Plans

Drought Mitigation Database

Ranch Planning

Monitoring
U.S Drought Monitor

Drought Monitoring Tools

Climographs
Legacy

Historic PDSI

River Basin PDSI

Standardized Precipitation Index
Map Interpretation

Maps

SPI Program

OWWLS

Publications
News
News Archive

DroughtScape

Dry Horizons

Drought Summaries
Quarterly Summary Archive

Monthly Summary Archive

Annual Reports

Fact Sheets

Social Media Resources

Scholarly Publications

Drought Network News

Subscribe

Our Work
Global

Projects
Northeast U.S.

USDM Northwest

Prob Exceedance

International Resources
Project Detail

About Us
Who We Are

Contact Us

Events

Employment

Support NDMC

Drought Headlines
Drought Headlines Archive

Web Policy

Update Bookmarks

Page Not Found

How are the Drought Categories Assigned?

SECTIONS

+
What is the U.S. Drought Monitor?

Overview of the U.S. Drought Monitor

The Beginning of the U.S. Drought Monitor

The Evolution of the U.S. Drought Monitor

Who Makes the Map?

+
What Does the Map Show?

Drought Intensity Scale

Timescales of Impacts

Summary and Disclaimer

What Data are Used to Make the Map?

+
How are the Drought Categories Assigned?

Percentile Method

Convergence of Evidence

What is the Timeline for Production?

Where can I Find the U.S. Drought Monitor?

+
How Can I Contribute?

Contributing Your Data and Information

Importance of Drought Impacts

+
Who uses the U.S. Drought Monitor?

Uses of the U.S. Drought Monitor

The U.S. Drought Monitor’s Role in Policy

Brochure

Percentile Method

Each category on the U.S. Drought Monitor has a percentile range associated with it. For example, D4 is in the first‐to‐second percentile range. Percentiles help to place the drought into a historical context.

Let’s look at this another way using 100 years of precipitation data from a station in Nebraska. Percentiles are determined by ranking the data from largest to smallest. In this example, 2007 had the highest precipitation with just over 39 inches of rainfall and 2012 had the least with about 11‐and‐a‐half inches. Additional years falling in the top 5 wettest and driest are indicated by raindrops.

Let’s rank the precipitation amounts in order, from the year with the most precipitation to the year with the least precipitation. The average, or mean, precipitation for this location is 24.21. This exact amount only occurred in 2 years out of 100, 1962 and 1995.

Now, let’s apply the percentile categories for the U.S. Drought Monitor. Exceptional drought, D4, corresponds to the lowest 2 precipitation values. This is the most severe drought, with the worst conditions on record. It would only be expected to occur once or twice within a 100‐year period.

Extreme drought, D3, occupies positions 3 through 5. These conditions are still among the worst on record and would be expected to occur once every 20 to 50 years.

Severe drought, D2, occupies positions 6 through 10. This type of drought would be expected to occur once every 10 to 20 years.

Moderate drought, D1, occupies positions 11 through 20. This type of drought would be expected to occur about once every 5 to 10 years.

Abnormally Dry conditions, D0, occupy positions 21 through 30. We would expect these types of conditions once every 3 to 5 years.

Rainfall that falls within the top 70 positions do not get a designation on the U.S. Drought Monitor and show up as white areas on the map. As you can see, drought is truly a rare event and extreme and exceptional droughts are even rarer.

Convergence of Evidence

The authors of the U.S. Drought Monitor use Geographic Information System software, called ArcMap, which lets the authors overlay percentile maps for multiple types of data at multiple time scales to determine where the data is converging ‐‐ that is, what is the D‐x level of dryness or drought that most of the data indicate. A unique thing about the U.S. Drought Monitor is that over 400 experts around the country provide feedback and ground truth for the map. This provides for a built‐in validation step, which ties the indicators to the reality of local conditions and impacts that are occurring.

What do percentiles and the convergence of evidence mean for the map? A location that is designated with Exceptional Drought (D4) is experiencing a drought that would only be expected to occur once or twice within a 100‐year period AND that is evident through a strong consensus across multiple types of indicators.

These figures shows some sample data, put into percentiles and colored with the appropriate U.S. Drought Monitor categories. The solid lines on each figure are an overlay of the previous week’s drought categories. Keep in mind that this is only a small subset of the several dozen maps that the authors look at each week. Notice that the indicators do not line up nicely and tell the story as to what drought category the location is in. In cases such as this, the author’s must use a combination of the physical data, feedback from the local experts, impact reports, and experience to determine how the evidence converges to a final drought category.
PrevNext

Contact

------

National Drought Mitigation Center
University of Nebraska-Lincoln

3310 Holdrege Street, Lincoln, 68583-0988

P.O. Box 830988, Lincoln, 68583-0988

(402) 472–6707

(402) 472-2946

ndmc@unl.edu

------

More Contact Info

Web Policy

News

------

Brief Mentions July 2026

July 29, 2026

------

Greenberg expands on meteorology, GIS knowledge in graduate program

July 28, 2026

------

NDMC graduate program experiences prepare Winkler for environmental role in Hawaii

July 24, 2026

------

More News

Follow Us

------

Follow the NDMC on social media to receive the latest information and updates about our work.

X

Mastodon

Instagram

Facebook

YouTube

©2026 - National Drought Mitigation Center

------
导航页打开网址