FAQ's about Drought

A drought is a period of drier-than-normal conditions that results in water-related problems. Precipitation (either rain or snow) falls in uneven patterns across the country. The amount of precipitation at a particular location varies from year to year, but over a period of years, the average amount is fairly constant. In the deserts of the Southwest, the average precipitation is less than 3 inches per year. In contrast, the average yearly precipitation in the Northwest is more than 150 inches.

When no rain or only a very small amount of rain falls, soils can dry out and plants can die. When rainfall is less than normal for several weeks, months, or years, the flow of streams and rivers declines, water levels in lakes and reservoirs fall; also the depth to water in wells increases. If dry weather persists and water-supply problems develop, the dry period can become a drought.

Reference: Moreland, 1993, Drought: U.S. Geological Survey Water Fact Sheet, Open-File Report 93-642

The current El Niño will probably surpass the greatest El Niño of century, that of 1982-83. During the past 40 years, nine El Niños have affected the western coasts of North and South America. Most of them raised water temperatures along 5000 miles of coast. The weaker events raised sea temperatures only a few degrees Fahrenheit and caused mild changes in weather. But the strong ones, like the El Niño of 1982-83, left a climatic imprint that was global in extent.

El Niño recurs irregularly, from two years to a decade, and no two events are exactly alike. Before the 1982-83 El Niño event, scientists did not collect detailed information on El Niños, so information is scanty for making high-quality predictions about the effects of the current El Niño of 1997-98.

The impacts of El Niños can be devastating, as illustrated by some of the effects of the unusually strong El Niño of 1982-83:

- Drought (sometimes with associated wildfires) in many nations (particularly in the western Pacific Rim, southern and northern Africa, southern Asia, southern Europe, and parts of South and Central America);- Severe cyclones that damaged island communities in the Pacific;- Flooding over wide areas of South America, western Europe, and the Gulf Coastal states; - Severe storms in the western and northeastern United States.

Water levels in wells are constantly changing both in the short term and over the long term. Some wells even have a seasonal change. In the short term, water levels can be lowered just by pumping water out of the well for use. Also, a well may be pumped so much as to cause the water level in nearby wells to be lowered, too. It all depends on how fast the aquifer that the well uses is resaturated with water from the surface or from the area surrounding it (recharge). In some places people have withdrawn water faster than water replenishes the aquifer, and the wells have stopped producing water.

Sometimes this is a long-term problem occurring over a very large area. If it takes a long time to replenish the aquifer, maybe because the aquifer is composed of rock that only allows water to move through it very slowly, a field of wells may stop producing. Users will have to wait until the aquifer becomes more saturated again before turning the pumps back on. Also, an aquifer can only contain water if there is water coming into it, usually from rainwater seeping down from the surface. In a severe drought water levels in wells can significantly decline.

The U.S. Geological Survey (USGS) stream-gaging program provides streamflow data for a variety of purposes that range from current needs, such as flood forecasting, to future or long-term needs, such as detection of changes in streamflow due to human activities or global warming. The development of data on the flow of the Nation's rivers mirrors the development of the country. From the establishment of the first stream-gaging station operated by the USGS in 1889, this program has grown to include 7,292 stations in operation as of 1994. Data from the active stations, as well as from discontinued stations, are stored in a computer data base that currently holds mean daily-discharge data for about 18,500 locations and more than 400,000 station-years of record. The stream-discharge data base is an ever-growing resource for water-resources planning and design, hydrologic research, and operation of water-resources projects.

The USGS stream-gaging program provides hydrologic information needed to help define, use, and manage the Nation's water resources. The program provides a continuous, well-documented, well-archived, unbiased, and broad-based source of reliable and consistent water data. Because of the nationally consistent, prescribed standards by which the data are collected and processed, the data from individual stations are commonly used for purposes beyond the original purpose for an individual station. Those possible uses include the following:

• Enhancing the public safety by providing data for forecasting and managing floods
• Characterizing current water-quality conditions
•  Determining input rates of various pollutants into lakes, reservoirs, or estuaries
• Computing the loads of sediment and chemical constituents
• Understanding the biological effects of contamination
• Delineating and managing flood plains
• Setting permit requirements for discharge of treated wastewater
• Designing highway bridges and culverts
• Setting minimum flow requirements for meeting aquatic life goals
• Monitoring compliance with minimum flow requirements
• Developing or operating recreation facilities
• Scheduling power production
• Designing, operating, and maintaining navigation facilities
• Allocating water for municipal, industrial, and irrigation uses
• Administering compacts or resolving conflicts on interstate rivers
• Defining and apportioning the water resources at our international borders
• Evaluating surface- and ground-water interaction
• Undertaking scientific studies of long-term changes in the hydrologic cycle

Data for one or more of these purposes are needed at some point in time on virtually every stream in the country, and a data-collection system must be in place to provide the required information. The general objective of the stream-gaging program is to provide information on or to develop estimates of flow characteristics at any point on any stream. Streamflow data are needed for immediate decision making and future planning and project design. Data, such as that needed to issue and update flood forecasts, are referred to as "data for current needs." Other data, such as that needed for the design of a future, but currently unplanned, bridge or reservoir or development of basinwide pollution control plans, are referred to as "data for future or long-term needs." Some data, of course, fit into both classifications; for example, a station that supplies data for flood forecasting and also provides data to define long-term trends. Reference: Wahl, K.L., Thomas, W.O., Jr., and Hirsch, R.M., 1995The stream-gaging program of the U.S. Geological Survey: U.S. Geological Survey Circular 1123, 22 p.

For more information on the National Streamflow Information Network (NSIP) go to: http://water.usgs.gov/nsip/

A period of below-normal rainfall does not necessarily result in drought conditions. Some rain returns to the air as water vapor when water evaporates from water surfaces and from moist soil. Plant roots draw some of the moisture from the soil and return it to the air through a process called transpiration. The total amount of water returned to the air by these processes is called evapotranspiration. Sunlight, humidity, temperature, and wind affect the rate of evapotranspiration. When evapotranspiration rates are large, soils can lose moisture and dry conditions can develop. During cool, cloudy weather, evapotranspiration rates may be small enough to offset periods of below-normal precipitation and a drought may be less severe or may not develop at all.

Reference: Moreland, 1993, Drought: U.S. Geological Survey Water Fact Sheet, Open-File Report 93-642

A well is said to have gone dry when water levels drop below a pump intake. This does not mean that a dry well will never have water in it again, as the water level may come back through time as recharge increases. The water level in a well depends on a number of things, such as the depth of the well, the type (confined or unconfined) of aquifer the well taps, the amount of pumping that occurs in this aquifer, and the amount of recharge occurring. Wells screened in unconfined water table aquifers are more directly influenced by the lack of rain than those screened in deeper confined aquifers. A deep well in a confined aquifer in an area with minimal pumping is less likely to go dry than a shallow, water-table well

The beginning of a drought is difficult to determine. Several weeks, months, or even years may pass before people know that a drought is occurring. The end of a drought can occur as gradually as it began. Dry periods can last for 10 years or more. During the 1930's, most of the United States was much drier than normal. In California, the drought extended from 1928 to 1937. In Missouri, the drought lasted from 1930 to 1941. That extended dry period produced the "Dust Bowl" of the 1930's when dust storms destroyed crops and farms.

The first evidence of drought usually is seen in records of rainfall. Within a short period of time, the amount of moisture in soils can begin to decrease. The effects of a drought on flow in streams and reservoirs may not be noticed for several weeks or months. Water levels in wells may not reflect a shortage of rainfall for a year or more after a drought begins.

Reference: Moreland, 1993, Drought: U.S. Geological Survey Water Fact Sheet, Open-File Report 93-642

The Palmer Index (more properly called the Palmer Drought Severity Index) was developed by Wayne Palmer of the U.S. Weather Bureau (now the National Weather Service) in the 1960's and uses temperature and rainfall information in a formula to determine dryness. It has become the semi-official drought index.

The Palmer Index is most effective in determining long term drought (a matter of several months) and is not as good with short-term forecasts (a matter of weeks). It uses a 0 as normal, and drought is shown in terms of minus numbers; for example, minus 2 is moderate drought, minus 3 is severe drought, and minus 4 is extreme drought.

The Palmer Index can also reflect excess rain using a corresponding level reflected by plus figures; i.e., 0 is normal, plus 2 is moderate rainfall, etc.

The advantage of the Palmer Index is that it is standardized to local climate, so it can be applied to any part of the country to demonstrate relative drought or rainfall conditions. The negative is that it is not as good for short term forecasts, and is not particularly useful in calculating supplies of water locked up in snow, so it works best east of the Continental Divide.

The Crop Moisture Index (CMI) is also a formula that was also developed by Wayne Palmer subsequent to his development of the Palmer Drought Index.

The CMI responds more rapidly than the Palmer Index and can change considerably from week to week, so it is more effective in calculating short-term abnormal dryness or wetness affecting agriculture.

CMI is designed to indicate normal conditions at the beginning and end of the growing season; it uses the same levels as the Palmer Drought Index.

It differs from the Palmer Index in that the formula places less weight on the data from previous weeks and more weight on the recent week.

Based on http://www.drought.noaa.gov/palmer.html

Rainfall in any form will provide some drought relief. A good analogy might be how medicine and illness relate to each other. A single dose of medicine can alleviate symptoms of illness, but it usually takes a sustained program of medication to cure an illness. Likewise, a single rainstorm will not break the drought, but it may provide temporary relief.

A light to moderate shower will probably only provide cosmetic relief. It might make folks feel better for awhile, provide cooling, and make the vegetation perk up. During the growing season, most of the rain that falls will be quickly evaporated or used by plants. Its impact is short term.

A thunderstorm will provide some of the same benefits as the shower, but it also may cause loss of life and property if it is severe. Thunderstorms often produce large amounts of precipitation in a very short time, and most of the rain will run off into drainage channels and streams rather than soak into the ground. If the rain happens to fall upstream of a reservoir, much of the runoff will be captured by the reservoir and add to the available water supply. No matter where the rain falls, stream levels will rise quickly and flooding may result. Also, because the rainfall and runoff can be intense, the resulting runoff can carry significant loads of sediment and pollutants that are washed from the land surface.

Soaking rains are the best medicine to alleviate drought. Water that enters the soil recharges ground water, which in turn sustains vegetation and feeds streams during periods when it is not raining. A single soaking rain will provide lasting relief from drought conditions, but multiple such rains over several months may be required to break a drought and return conditions to within the normal range.

Tropical storm rains are usually of the soaking variety, although they may also be intense such as during a thunderstorm and lead to some of the same problems. Tropical storms often produce more total rainfall than a "regular" soaking rain and can provide longer relief than a single soaking rain. However, tropical rains may also be of such intensity that they exceed the capacity of soil to absorb water and often result in significant runoff and flooding. Tropical rains can help to fill water-supply reservoirs and provide long-term drought insurance. However, the path of a tropical storm is very important in determining its impacts. For example, tropical storms are for the most part a near-coast phenomena whereas water-supply reservoirs may be inland, such as is the case for the Washington, D.C, water supply. If significant rainfall does not occur upstream of reservoirs, the drought relief aspects of tropical storms may be of only little consequence. All things considered, a single tropical storm at the right place, at the right time, and with the right amount of rainfall can break a drought.

Considering all of the above, even when a drought has been broken it may not be truly over. The benefits of substantial rainfall such as from a tropical storm may last for months, but a return to normal rainfall patterns and amounts is necessary for conditions in streams, reservoirs, and ground water to also return to normal.

Real-time streamflow data available on USGS pages are PROVISIONAL data that have not been reviewed or edited. These data may be subject to significant change and are not citable until reviewed and approved by the U.S. Geological Survey. Real-time streamflow data may be changed after review because the stage-discharge relationship may have been affected by:

• backwater from ice or debris such as log jams
• algae and aquatic growth in the stream
• sediment movement
• malfunction of recording equipment

Data are reviewed periodically to ensure accuracy. Each station record is considered PROVISIONAL until the data are published. The data are usually published within 6 months of the end of the water year.

Data users are cautioned to consider carefully the provisional nature of the information before using it for decisions that concern personal or public safety or the conduct of business that involves substantial monetary or operational consequences.

To view the USGS streamflow information on drought go to Drought Watch. This map shows below normal 7-day average streamflow compared to historical streamflow for the day of the year (United States).

The term "acid rain" is commonly used to mean the deposition of acidic components in rain, snow, fog, dew, or dry particles. The more accurate term is "acid precipitation." Distilled water, which contains no carbon dioxide, has a neutral pH of 7. Liquids with a pH less than 7 are acid, and those with a pH greater than 7 are alkaline (or basic). "Clean" or unpolluted rain has a slightly acidic pH of 5.6, because carbon dioxide and water in the air react together to form carbonic acid, a weak acid. Around Washington, D.C., however, the average rain pH is between 4.2 and 4.4.

The extra acidity in rain comes from the reaction of air pollutants, primarily sulfur oxides and nitrogen oxides, with water in the air to form strong acids (like sulfuric and nitric acid). The main sources of these pollutants are vehicles and industrial and power-generating plants. In Washington, the main local sources are cars, trucks, and buses.

The USGS provides access to water-resources data collected at approximately 1.5 million sites in all 50 States, the District of Columbia, and Puerto Rico. Online access to this data is organized around the categories listed to the left.

The USGS investigates the occurrence, quantity, quality, distribution, and movement of surface and underground waters and disseminates the data to the public, State and local governments, public and private utilities, and other Federal agencies involved with managing our water resources.

This information is available at: http://waterdata.usgs.gov/nwis

Go to the USGS National Water Information System Web site (NWISWeb) for a tutorial on how to use NWISWeb.

Water is vital to our Nation and the U.S. Geological Survey plays an important role in the tracking and mapping our water resources. The National Hydrography Dataset component of The National Map supports this mission and is widely used in the study of hydrology, natural resources, and pollution control. Users of USGS geospatial data discuss the role of the National Hydrography Dataset in water rights management in California, fisheries management in Michigan, and drinking water threat analysis nationwide.

For a multimedia presentation on this subject go to:  http://gallery.usgs.gov/videos/124