Defining Drought

For the purposes of the ADEMRMP, 'Drought' is defined as an extended period of below-normal precipitation resulting in decreased soil and subsoil moisture levels and diminished surface water supplies affecting crop growth, livestock water and irrigation water. There is however, no one definition that works in all circumstances. The Concise Oxford Dictionary says drought is "the continuous absence of rain: dry weather: or lack of moisture."

The National Drought Mitigation Center in Lincoln, Nebraska says there are actually two main kinds of drought definitions: conceptual and operational. Conceptual definitions of drought, such as that in the dictionary help people understand the "idea" of a drought. Operational definitions define the onset, severity and the end of a drought. No single operational definition can be developed. For example, parts of the Sahara Desert must have less than 2 millimetres of water a year and parts of the Amazon Rain Forest must have less than 2,000 millimetres per year before a drought situation would arise. It is difficult to develop an operational definition of drought for Alberta. The Westlock area usually has more precipitation than the Medicine Hat area. Allowances must also be made for summer evaporation which increases from north to south.

To complicate matters more, there are four types of drought: meteorological, hydrological, agricultural and socioeconomic. Meteorological drought usually specifies the degree of departure from the average over a period of time (eg. 75 percent of average rainfall over a 30-year period). Hydrological drought refers to the decline in surface and subsurface water. Agricultural drought occurs when there is not enough water available for a crop to grow. When the physical water shortage begins to affect people, then it becomes a socioeconomic drought.