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U.S. Drought Monitor Indicators

Using Precipitation Indices to Determine Severity, Long-term Impact

Nov 28, 2007

Although drought is difficult to define or forecast, climatologists can determine the impact and intensity of droughts using drought indicators and precipitation indices.

Droughts As Natural Hazards

Droughts, although less apparent than other natural hazards, result in billions of dollars of damage and destruction in the United States. Although the immediate affects may not be as drastic or spectacular as other weather-related disasters such as tornadoes or hurricanes, droughts affect more people altogether than any other natural hazard, according to the National Climate Data Center (NCDC). No region in North America is unsusceptible to droughts. El Nino, specifically La Nina has been known to cause extensive drought conditions.

Drought By Definition

The definition of drought is not a clear and simple one. Drought is not determined by the number of dry days but by the amount of moisture before and after a dry period. It is dependent on specific conditions and the region. A drought in the desert is not the same as a drought in a rain forest. The National Climate Data Center defines drought as "a significant deficit in moisture availability due to lower than normal rainfall". A drought is defined by rainfall, crop productivity, soil moisture, reservoir levels and other factors. However, since rainfall records have only been kept in the last century, it is difficult to determine the severity or to compare droughts over long periods of time. Paleoclimatology is used to study the past climate, including analyzing tree rings, sediment, archaeology and historical documents in order to determine drought severity and prediction.

Drought Indicators

Drought are monitored by the National Climate Data Center, which contains the largest active archive of weather data in America. The U.S. Drought Monitor provides updates and predictions of drought impact on agricultural and hydrological areas. Drought are monitored by

Dryness Categories
Drought Intensity
Drought or Dryness Types

Climatologists use two main indices to determine drought severity and impact.

Palmer Drought Severity Index (PDSI) - measures drought intensity and duration by measuring precipitation, air temperature and local soil moisture. The index uses a scale to determine severity, ranging from - 4.0 for extreme drought to +4.0 meaning extremely moist conditions.
Standard Precipitation Index (SPI) - This index relies on precipitation amounts alone. It can determine short-term and long-term applications. The index displays data over 1, 3, 6, 9, and 12 month durations.

1930s Dust Bowl Drought

The Dust Bowl of the 1930s is considered to be the major drought of the 20th century. This devastating drought lasted seven years, happening in three waves: 1934, 1936 and 1939-40. The Dust Bowl drought was so severe and widespread, covering over half of the United States, that it is considered a natural disaster. The sustained drought conditions left the soil depleted of moisture. The soil lifted into the air by the wind, formed huge dust clouds, called "black blizzards". These dust clouds killed agriculture, resulting in a great migration of the region's population from the Great Plains to the West Coast.

The copyright of the article U.S. Drought Monitor Indicators in Climatology is owned by Aurae Beidler. Permission to republish U.S. Drought Monitor Indicators in print or online must be granted by the author in writing.

Drought Leaves Land Barren

Comments

Dec 4, 2007 3:57 PM

Aurae Beidler :

Which areas of the U.S. or Canada are currently experiencing drought? Do you see these ending anytime soon? Do you think the winter will end these droughts?

Dec 13, 2007 8:14 AM

Alicia King :

I live in Atlanta Georgia, and the US Southeast is getting hit pretty hard with the drought (GA, FL, TN are the ones I hear about most in the news).

We've been getting a little bit of rain, but by no means has it been enough to make up for the shortfall. I'm guessing it won't be made up until springtime.

Meanwhile, we are having 70-degree days and mild enough weather that our landscaping is still begging for water when it does rain.

One of the big controversies here is that Atlanta's reserves at Lake Lanier were pumped down to Florida to keep a certain National swampland nice and swampy. I'm on the fence about this, I can see how the environmental concerns are very real and the conservation efforts are necessary, but at the same time I honestly doubt this is the first drought that swamp has survived.

Meanwhile, we discuss strange things now at our Christmas parties. I've had chats about grey water conservation for pouring on our compost piles (because we're not supposed to use disposals, but compost needs to stay damp to "cook" properly!) and the question of weighing out water conservation over recycling concerns (e.g. plastic cups save dishwashing, but you have to rinse to recycle anyway, or you don't recycle at all...?)

Dec 13, 2007 1:26 PM

Aurae Beidler :

That's not good to hear. Droughts can be so devastating! If you look at the U.S. Drought Monitor http://drought.unl.edu/dm/monitor.html it shows a severe or extreme drought in progress for your area. Looks like we're heading for an "all-time driest year on record" for the SE U.S.
But remember that weather data has only been kept for about 100 years, so it's hard to tell if this is the worst drought or not.
I wish I could send over some of our Oregon rain but actually it's been dry this month.
Take care and I hope relief is coming your way soon!

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