Dozens of definitions are used around the world that defined lack of rain over various time periods, or measured impacts such as reservoir levels or crop losses. Thus, because drought is a complex phenomena which is difficult to define, the climatological community has defined four types of drought: 
Meteorological drought happens when dry weather patterns dominate an area. Meteorological drought is usually based on long-term precipitation departures from normal, but there is no consensus regarding the threshold of the deficit or the minimum duration of the lack of precipitation that make a dry spell an official drought.
Hydrological drought refers to deficiencies in surface and subsurface water supplies. It’s measured as stream flow, and as lake, reservoir, and ground water levels. Agricultural drought occurs when there is insufficient soil moisture to meet the needs of a particular crop at a particular time. A deficit of rainfall over cropped areas during critical periods of the growth cycle can result in destroyed or underdeveloped crops with greatly depleted yields.
Agricultural drought happens when crops become affected. Agricultural drought is typically evident after meteorological drought but before a hydrological drought.
Socioeconomic droughts occur when physical water shortage begins to affect the population, individually and collectively. Most socioeconomic definitions of drought associate it with supply, demand, and economic good.
- NOAA – National Climatic Data Center – Definition of Drought: https://www.ncdc.noaa.gov/monitoring-references/dyk/drought-definition
- NASA Earth Observatory – Drought: The Creeping Disaster: http://earthobservatory.nasa.gov/Features/DroughtFacts/
- Image Source: http://www.climate.gov/news-features/featured-images/extreme-drought-south-shrinks-water-supplies [Accessed 25 January, 2014]
- Image Source: http://www.climate.gov/news-features/featured-images/brazos-river-runs-dry-during-texas-drought [Accessed 25 January, 2014]
- Image Source: http://www.climate.gov/sites/default/files/crop_in_drought_0.jpg [Accessed 25 January, 2014]
- Image Source: http://www.climate.gov/climate-and-energy-topics/ecosystem-changes-0 [Accessed 25 January, 2014]