Dark red indicates the most severe drought, light yellow areas are normal, and green areas have denser than normal vegetation. However, dozens of more specific drought definitions are used around the world that are defined according to the lack of rain over various time periods, or measured impacts such as reservoir levels or crop losses. Because of the various ways drought is measured, an objective drought definition has yet to be produced upon which everyone can agree. Drought can be defined according to meteorological, hydrological, or agricultural criteria. 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 is typically evident after meteorological drought but before a hydrological drought. For the purposes of this Web site, our definition most closely resembles that of an agricultural drought.