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Drought         (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 intensity.
        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 drought.
        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
        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
        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 those needs).

Hydrological
        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
        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.

 
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