(Information courtesy of National Drought Mitigation Center)
Many people consider droughts to be a rare event when in fact they are a
normal and recurrent feature of the climate. When asked to define
drought, there isn't just one right answer. Many different definitions
exist for this word. Drought implies a lack of moisture for an
extended period of time which in turns causes a deficit of moisture in the
soil. This can mean different things for different areas. For
areas which receive high amounts of precipitation, a condition of drought
can develop more rapidly than in an area which doesn't receive high amounts
of precipitation. Many problems can arise due to droughts, including
crop damage and water supply shortage. How severe a drought is depends
mostly on the degree of the deficiency, the time period, and the size of the
area affected. The timing is also a significant factor with the
onset/duration of droughts. The primary season in which it occurs,
delays in the beginning of the normal rainy time periods, and rain events
occurring relative to the growth stage of crops are examples of this timing.
Every region has been through a period in which a condition of drought
existed. In some areas, these conditions can be influenced greatly by
certain mechanisms. These mechanisms include an increase in area and
persistence of sub-tropical high pressure cells, changes in summer monsoonal
circulations, lower ocean temperatures, and the displacement of the mid
latitude storm track. Low relative humidity, high temperatures, and
high winds are also factors which can greatly impact a drought and its
Two different types of drought definitions exist. The first, known as
a conceptual definition, is put into general terms. This makes it
easier for people to understand the concept of drought. Conceptual
definitions are also important in establishing drought policies.
Science-based assessments are what help determine declarations of extreme
the second type is known as an operational definition. This type aids
in helping people understand things like the beginning, end, and severity of
droughts. When the beginning of a drought is determined, the
operational definitions help in specifying the degree of departure from
average of precipitation over a time period. This can be done by
comparing the current conditions with historical climatic data, usually over
a 30-year period of data. This definition can also be used when
dealing with agriculture, to tell things such as the impact the drought is
having on crops an the rate of soil depletion. Drought severity,
frequency, and duration for a historical period can be established
operationally, but requires weather data from various time scales (such as
hourly, daily, monthly, etc). This information can be extremely useful
in preparing for possible future droughts.
There are four perspectives on drought: meteorological, agricultural,
hydrological, and socioeconomic. What follows is a description of each
of these four perspectives.
Meteorological drought is usually defined by the measure of the departure of
precipitation from the normal and the duration of the dry period. As
mentioned before, the area of concern must be taken into consideration with
this definition. Atmospheric conditions that cause the deficiencies of
moisture vary greatly from region to region. Some definitions identify
droughts based on the number of days an area goes with precipitation that is
lower than a specified level. This is only applicable for regions in
which a characteristic is a year-round precipitation period, such as
tropical rainforests and humid subtropical and mid-latitude climates.
In areas which are characterized by seasonal precipitation periods, such as
the central United States, extended periods without rainfall is a common
occurrence. In these cases, determining drought based on
non-precipitation days is unrealistic.
Agricultural definitions refer to situations in which the moisture in the
soil is no longer sufficient to meet the needs of the crops growing in the
area. Focus is placed on precipitation shortages, reduced ground
water/reservoir levels, differences between actual and potential
evapotranspiration, and so on. Good definitions of agricultural
drought will account for susceptibility of crops during different stages in
its development. When soil moisture is lacking, this may hinder its
proper development, leading to low plant numbers and eventually lower final
yield. The water demand a crop has depends on weather conditions (such
as temperature, relative humidity), its biological make-up, what stage of
growth the crop is in, and the physical/chemical make-up of the soil.
If soil moisture is high enough to allow for proper early development, later
lacking moisture may not deplete final yield if the moisture can be replaced
as the growing season goes on (irrigation, or sufficient rainfall meets
Hydrological drought deals with surface and subsurface water supplies (such
as stream flow, reservoir/lake levels, ground water). Extended periods
of lacking precipitation cause these water supplies to drop below normal.
This drought is no different than the others in regard to the fact it is
caused by a lack of moisture, but is different than the others in one
significant way. Hydrological droughts are usually not occurring at
the same time as the others, instead lags behind. This drought deals
more with effects the lack of moisture has on the hydrological system as a
whole. It takes longer periods of time for the lack of moisture to
show up in places such as the ground water, reservoir, and lake levels.
When the flow in these places is affected significantly enough, this can
have economic effects on the area on things such as hydroelectric power
plants and recreational areas.
Though the climate/weather is the main contributor to hydrological drought,
things such as changes in landscape, land use, and construction of dams also
have significant impacts on the drought. Such changes may not have a
great effect on the immediate region, it is a sure thing that it will impact
the region downstream from the moisture. This is also true with
meteorological drought. An example of this type of thing occurring
would be in the case of a drought in the Northern Great Plains. Since
the Missouri River flows to the south, the lack of moisture to the north
will also impact the area downstream from the drought inflicted area.
The changes in land/water use in the Great Plains will alter the
hydrological characteristics such as the flow and runoff rates, which in
turn could cause a drought in the area downstream from the original area to
the north. This shows how land use changes/human alterations can
alter the frequency of water shortages even when no meteorological drought
is being observed.
Socioeconomic drought refers to the situation that occurs when water
shortages begin to effect people and their lives. It associates
economic good with the elements of meteorological, agricultural, and
hydrological drought. It is different than the other definitions in
the fact that this drought is based on the process of supply and demand.
Many economic goods (for example: water, food grains, fish, hydroelectric
power), have their supplies greatly dependent on the weather. Due to
natural variations in climate, some years have high supplies of water, but
other years the supply is very low. A socioeconomic drought takes
place when the supply of an economic good cannot meet the demand for that
product, and the cause of this short-fall is weather-related (water supply).
For most cases, demand for goods increase due to population increases and
consumption. Improved production, technology, and construction on
reservoirs for water supplies may increase the supply for goods. If
both are increasing, the rate of this change is crucial. If demand is
increasing faster than the supply, the impact of a drought will be much more
significant on the area it affects.