Frequently Asked Questions
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.
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
When does a
Does a shortage of
rain mean a drought will occur?
Why doesn't a
drought go away when it rains?
What is the Palmer
- Effects of Drought on Water Use
How does water
reach my home?
What can I do to
help conserve water?
- Effects of Drought on Ground-Water Resources
How important is
How does the water
level in my well change?
What determines if
a well will go dry?
How do I find out
if my well will go dry?
I paid to have my
own private well installed, so why can't I use the water any way that I
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 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.
does a drought begin?
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.
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.
a shortage of rain mean that a drought will occur?
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.
doesn't a drought go away when it rains?
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.
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.
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.
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.
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.
is the Palmer Index?
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.
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.
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.
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.
Moisture Index (CMI) is also a formula that was also developed by Wayne
Palmer subsequent to his development of the Palmer Drought Index.
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.
designed to indicate normal conditions at the beginning and end of the
growing season; it uses the same levels as the Palmer Drought Index.
from the Palmer Index in that the formula places less weight on the data
from previous weeks and more weight on the recent week.
of Drought on Water Use
does water reach my home?
All of the
water that we use in our homes comes from either a ground-water 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.
can I do to help conserve water?
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 municipal, commercial,
industrial, and residential
water users from the U.S.
Environmental Protection Agency. Agricultural water users should discuss
conservation options with their local Cooperative Extension Service agent.
of Drought on Ground-Water Resources
information on ground water can be found in the
Ground Water Fact Sheet.
important is ground water?
which is found in aquifers below the surface of the Earth, is one of the
Nation's most important natural resources. Ground water 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
does the water level in my well change?
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 underground water 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.
determines if a well will go dry?
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.
do I find out if my well will go dry?
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.
paid to have my own private water well installed, so why can't I use the
water any way I want to?
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
ground-water levels fall. Since aquifers can be quite extensive, the usage
of your well can influence other people miles away. Ground water 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. You
can view a map
of the regional aquifer systems from the USGS
Ground Water Atlas of the United States. More information on aquifers
may be found in the USGS
Ground Water Fact Sheet.
- Moreland, J.A.,
Geological Survey Water Fact Sheet, Open-File Report 93-642, 2 p.
Wednesday, June 03, 2009
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