Frequently Asked Questions - The Hydrology of Drought
"Drought is a condition of moisture deficit sufficient to have an
adverse effect on vegetation, animals, and man over a sizeable area."
The following questions and answers provide background on some of the scientific issues regarding drought. For questions on regulatory issues, such as water-use restrictions in states and localities, please refer to the appropriate authority in your jurisdiction.
Questions and answers are original compositions or are compiled from any available sources and credit is given where appropriate. New material will be added as needed. Contributions are welcome.
- Weather and Drought
- What causes drought?
- When does a drought begin?
- Does a shortage of rain mean a drought will occur?
- Why doesn't a drought go away when it rains?
- What is the Palmer Index?
Weather and Drought
A drought is a period of drier-than-normal conditions that results in water-related problems. Precipitation (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 Pacific Northwest is more than 150 inches.
The amount of rain and snow also varies with the seasons. In some areas, most of the yearly precipitation falls in the early spring. In the Southeast, most of the yearly precipitation falls during the hurricane season in late summer and fall. Even if the total amount of rainfall for a year is about average, rainfall shortages can occur during a period when moisture is critically needed for plant growth.
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, and 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.
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.
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.
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 groundwater, 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 in 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 groundwater to also return to normal.
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, or longer--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. It is designed to indicate normal conditions at the beginning and end of the growing season, and 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.
Effects of Drought on Water Use
All of the water that we use in our homes comes from either a groundwater source, such as a well, or from a surface-water source, such a river, lake, or reservoir. Precipitation falls on the Earth's surface and eventually adds water (recharge) into an aquifer. This water may be pumped into your home from a well that taps into the aquifer. If your water source is a reservoir, precipitation and other surface water collects in the reservoir. This water is piped to homes from a public supplier.
If you are on a public water supply, additional information on your local drinking water system is available from the U.S. Environmental Protection Agency.
Many local jurisdictions offer tips on how to conserve water. You should contact your local water provider or water regulatory agency for tips that may be appropriate for your area. General tips on conserving water are available for water users from the U.S. Environmental Protection Agency. Agricultural water users should discuss conservation options with their local Cooperative Extension Service agent.
Effects of Drought on Groundwater Resources
Groundwater, which is found in aquifers below the surface of the Earth, is one of the Nation's most important natural resources. Groundwater is the source of about 38 percent of the water that county and city water departments supply to households and businesses (public supply). It provides drinking water for more than 97 percent of the rural population who do not get their water delivered to them from a county/city water department or private water company.
The water level in the aquifer that supplies a well does not always stay the same. Droughts, seasonal variations in rainfall, and pumping affect the height of the underGroundwater levels. If a well is pumped at a faster rate than the aquifer around it is recharged by precipitation or other underground flow, then water levels in the well can be lowered. This can happen during drought, due to the extreme deficit of rain. The water level in a well can also be lowered if other wells near it are withdrawing too much water.
A well is said to have gone dry when water levels drop below a pump intake. This does not mean your well will never have water in it again, as the water level may come back through time as recharge increases. The water level in your 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.
Wells screened in unconfined water-table aquifers are more directly influenced by the lack of rain than those screened in deeper confined aquifers. This means that it may be more likely for the water level in wells screened in the water table to drop below the pump level and prevent water from being obtained. This does not mean that wells in a confined aquifer will not go dry, as they are also influenced by pumping rates and lack of recharge.
If you own a water-table well and you pump excessive amounts of water from your well, there is a danger of your well going dry as consumption continues and groundwater levels fall. Since aquifers can be quite extensive, the usage of your well can influence other people miles away. Groundwater that supplies your well also feeds streams during periods of low flow, so pumping from your well may also cause the water levels in streams to be lower.
Moreland, J.A., 1993 Drought: U.S. Geological Survey Water Fact Sheet, Open-File Report 93-642, 2