Definition of Drought
Drought is defined as a period in which a region has a deficit in its water supply. Drought is a normal feature of climate which happens in all climate zones from time to time.
Usually, drought is talked about in one of two perspectives- meteorological and hydrological. A drought in terms of meteorology takes into account deficiencies in measured precipitation. Each year's measurements are then compared to what is determined as a "normal" amount of precipitation and drought is determined from there. For hydrologists, droughts are monitored by checking stream flow and lake, reservoir, and aquifer water levels. Precipitation is also considered here as it contributes to the water levels.
In addition, there are agricultural droughts that can impact crop production and cause changes to the natural distribution of various species. The farms themselves can also cause droughts to happen as soil is depleted and therefore cannot absorb as much water, but they can be impacted by natural droughts as well.
Causes of Drought
Because drought is defined as a deficit in water supply, it can be caused by a number of factors. The most important one though relates to the amount of water vapor in the atmosphere as this is what creates precipitation. More rain, sleet, hail, and snow can occur where there are moist, low pressure air systems. If there is an above average presence of dry, high pressure air systems instead, less moisture is available to produce precipitation (because these systems cannot hold as much water vapor). This results in a deficit of water for the areas over which they move.
The same can also happen when winds shift air masses and warm, dry, continental air moves over an area as opposed to cooler, moist, oceanic air masses. El Nino, which affects the ocean's water temperature, also has an impact on precipitation levels because in years when the temperature cycle is present, it can shift the air masses above the ocean, often making wet places dry (drought prone) and dry places wet.
Finally, deforestation for agriculture and/or building combined with the resultant erosion can also cause drought to begin because as soil is moved away from an area it is less able to absorb moisture when it falls.
Stages of Drought
Since many areas, regardless of their climatic region, are prone to drought, different definitions of the stages of drought have developed. They are all somewhat similar however, usually ranging from a drought warning or watch, which is the least severe. This stage is declared when a drought could be approaching. The next stages are mostly called drought emergency, disaster, or critical drought stage. This final stage begins after a drought has occurred for a long period and water sources begin to be depleted. During this stage, public water use is limited and oftentimes drought disaster plans are put into place.