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What are the causes of drought?

A drought is a prolonged period of water deficit, and usually occurs when an area does not receive significant precipitation for a sustained period of time, say several months (Chen et al., 2009; Linsely et al., 1959). Droughts is extreme climate event that percentage-wise is likely to change more rapidly than the mean climate (Trenberth et al. 2003). It is critical to monitor and analyze it, also understand and perhaps predict its variability. The potential for large increases in the extreme climate events under global warming is of particular concern (Trenberth et al. 2004). However, the precise quantification of droughts is difficult because there are many different definitions for these extreme events (e.g., meteorological, hydrological, and agricultural droughts; see Wilhite 2000 and Keyantash and Dracup 2002) and the criteria for determining the start and end of a drought also vary. Droughts can be classified into four categories as meteorological, hydrological, agricultural and socio-economic (White and Walcott, 2009). The preparedness and planning for a drought depend on the information about its areal extent, severity and duration (Mishra and Singh, 2011).

Drought has been defined by the international meteorological community in general terms as a "prolonged absence or marked deficiency of precipitation," a "deficiency of precipitation that results in water shortage for some activity or for some group," or a "period of abnormally dry weather sufficiently prolonged for the lack of precipitation to cause a serious hydrological imbalance" (World Meteorological Organization 1992; American Meteorological Society 1997).

A drought is caused by drier than normal conditions that can eventually lead to water supply issues. Hot temperatures can make a drought event worse by causing moisture to evaporate from the soil. Just because a region is hot and dry doesn't necessarily mean it is going through a drought. Droughts only occur when an area is abnormally dry, for example the people who lives in England they also see drought events. Drought is a type of natural catastrophe that involves below-average precipitation or a severe water supply shortage over a sustained period of time.

If we want to count the causes of drought we should believe that it is not exactly easy. Drought is a vague and mysterious phenomena, and if we want to knowing it deeply and precisely it is not easy. We can say that drought has many causes and there are various causes of drought. There are different papers and literatures that all of them have tried to recognize the causes of drought. You can achieve that there are common items in all of them. Here, Agrimetsoft research team has attempted to gather them, as follows:

Precipitation deficits over an extended period can be devastating to human life and health, water resources, and economies and are commonly described as meteorological droughts (Yu et al., 2013). Meteorological drought is characterized by lack of precipitation over weeks, months, or years (Heim, 2002; Potop et al., 2014; Salehnia et al., 2017). So, the first or mainstay causes of drought: little or "no precipitation" is one of the major causes of a drought. Lower than average levels of rainfall over a sustained period of time can dry the soil and lead to crop failures. Meteorological disturbances like extremely high temperatures and changes in wind patterns can lead to lower than normal rainfall in an area.

The second causes of drought is "fluctuating ocean and land temperatures". Ocean temperatures largely dictate global weather patterns, including dry and wet conditions on land, and even tiny temperature fluctuations can have huge ripple effects on climate systems. The third causes of drought is "air circulation patterns in the atmosphere". As hot air rises and expands, it creates a contrasting flow of air from cooler areas where air condenses and sinks. This gives rise to air currents that move moisture around the atmosphere and result in different patterns of rainfall in different regions. In this regard, key events like El Nino or La Nina help contribute to drought in areas. All the water we ever had we have today and it is stored in the air or on land. Weather patterns move the water in the air around. This is constantly changing.

The fourth cause of drought is "the quantity of moisture in the soil layer" or in the other hand reduced soil moisture. When soil moisture is depleted there is less evaporation of water to create clouds. Surface temperatures rise, more water is needed and less is available which contributes to a more severe drought.

The fifth cause of drought is "Global warming induced climate change" is believed to be one of the more recent causes of drought. While climate change can bring more precipitation to some areas due to the melting of glaciers and higher rates of evaporation from water bodies, it will lead to droughts in other areas where higher temperatures will dry up the remaining water bodies. In addition, climate change alters large-scale atmospheric circulation patterns, which can shift storm tracks off their typical paths. This, in turn, can magnify weather extremes such as drought events.

At the end the sixth causes of drought is "Hhuman being activities". Besides meteorological factors that cause drought, human activity can also be a cause. Human activity has reduced the amount of rainfall in many regions of the world. Sometimes how much water humans consume, and the timing of that consumption, factors into how much water is available at a later date for people, plants and animals. Thus, drought can also be viewed as an imbalance between supply and demand. One study estimates that from 1960 to 2010, the human consumption of water increased the frequency of drought in North America by 25 percent. Human's activities push and trigger "causes of drought", and by a glance search you can find out several of them, such as deforestation and soil degradation, more and wrong agriculture methods, create more industries and make air pollutions, high number of populations especially in under-developed countries, and several other facts.

In sum, we can summarize the above discussion of "What are the causes of drought?" as: When a particular area gets less rain than usual, the soil gets much less moisture, too. The soil starts drying out and plants die. When this pattern continues for several weeks, months or years, the flow of streams and rivers decreases and water levels in lakes, reservoirs and wells fall. Eventually, the unusual dry weather causes water supply issues, and the dry period becomes a drought.

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